Physics I Test 2 Review

In which of these processes is the total energy of the system conserved? A. A rock (the system) falls through air and lands with a thud on the ground. B. A car (the system) slams on its brakes and comes screeching to a stop. C. A train car (the system) is pulled up a hill by a locomotive. D. Two balls of putty (the system) collide in midair and stick together.

:"D. Two balls of putty (the system) collide in midair and stick together "As long as the system has no total work done on it by external forces, the total energy will always be conserved. The balls of putty exert non-conservative forces on each other, meaning the mechanical energy of the system will not be conserved, but even non-conservative forces still conserve energy. When a rock (the system) falls through air and lands with a thud on the ground, there is total work done on the rock by Earth's gravity, by air resistance, and by the impact force from the ground. Therefore the rock's total energy is not conserved. When a car (the system) slams on its brakes and comes screeching to a stop, there is total work done on the car by friction with the road. Therefore the car's total energy is not conserved. When a train car (the system) is pulled up a hill by a locomotive, there is total work done on the train car by Earth's gravity and by the locomotive (and perhaps by other forces as well). Therefore the train car's total energy is not conserved."

The mechanical energy of a system is conserved during a certain process only if __________. A. non-conservative forces do zero total work on the system during that process B. non-conservative forces are not exerted on the system during that process C. no external forces are exerted on the system at all during that process D. the mechanical energy of each part of the system stays exactly the same during that process

:A. non-conservative forces do zero total work on the system during that process "The mechanical energy of a system is defined as the sum of kinetic energy and potential energy of the system. Non-conservative forces (such as a normal force) can be exerted on the system, but as long as they don't do any total work on the system the mechanical energy will still be conserved. Non-conservative forces can be exerted on the system during the process without changing the mechanical energy of the system, so long as they do zero total work. Internal conservative forces between parts of the system cannot change the system's mechanical energy"

Playing in the rain, a little girl tackles a little boy. Just before she tackles him, the boy is running east at 3 m/s and the girl is running south at 4 m/s. After the tackle, the girl holds on and the two slide on the wet grass. Our system consists of just the two children. Before the tackle, in roughly what direction does the total momentum of the system point? A. Somewhere south of east, but we can't be any more specific than that. B. Generally south of east, and specifically more south than east C. Generally south of east, and specifically more east than south D. Directly east E. Directly south

A. Somewhere south of east, but we can't be any more specific than that. "The momenta of the two children are directed east and south, meaning the total momentum will definitely be somewhere south of east. However, we do not have the mass of either child so we don't know which one has the larger momentum before the tackle."

Playing in the rain, a little girl tackles a little boy. Just before she tackles him, the boy is running east at 3 m/s and the girl is running south at 4 m/s. After the tackle, the girl holds on and the two slide on the wet grass. Our system consists of just the two children. Immediately after the tackle, what can we say about how the momenta of each child compare? A. The momentum of both children will be in the same direction. B. The momentum of both children will be the same. C. The momentum of both children will be zero. D. The total momentum of the system will be zero, but the momentum of each child will not be zero.

A. The momentum of both children will be in the same direction. "Because the girl holds on after the tackle the two children will be moving in the same direction at the same speed. Thus we know they will each have momentum in the same direction. Unless the two children happen to have exactly the same mass, they will not have equal momenta. Since the system had a non-zero momentum just before the collision, the system cannot have zero momentum after the collision. That would violate the conservation of momentum."

What is true about the work done by a conservative force? A. The work done by a conservative force is always path independent. B. The work done by a conservative force is always path dependent. C. The work done by a conservative force will change the total mechanical energy of a system. D. The work done by a conservative force will increase the internal energy of a system.

A. The work done by a conservative force is always path independent. "This is the definition of a conservative force. The work done by a conservative force acting on an object as the object moves from point A to point B will be the same, no matter what path the object follows to get from point A to point B. The total mechanical energy of a system, which includes kinetic and potential energies, is never changed by a conservative force."

Two cars experience a collision on a city road. Both drivers are using their brakes when the cars hit. Which statement is true? A. Using the impulse approximation, conservation of momentum can be applied to the very brief time period of the collision itself (giving approximate results). B. Conservation of momentum cannot be applied at all. C. Using the momentum approximation, conservation of momentum can be applied to the time period leading up to the collision (giving approximate results) but not to the collision itself. D. Friction from the road does not change the fact that the momentum of the two cars will be perfectly conserved during the collision.

A. Using the impulse approximation, conservation of momentum can be applied to the very brief time period of the collision itself (giving approximate results). "The collision between the cars involves brief forces that are much stronger than the forces of friction exerted on the cars by the road. Thus if we apply conservation of momentum to a very thin "slice" of time surrounding the collision, the total momentum of the two cars will not change very much and will be approximately conserved. Conservation of momentum is only strictly true when there is exactly zero net external force exerted on the system. However, the impulse approximation allows us to get approximate answers by applying conservation of momentum in situations where there are external forces that are much smaller than the internal forces of the interaction we are interested in. The "momentum approximation" is not a real term used in physics, it was just made up for this question. More specifically, the time immediately before the collision is when the forces of friction exerted on the cars will be changing the car's velocity the most, so that is not a time period in which conservation of momentum can be applied at all."

A variety of forces are applied to an object such that the net force does positive work on that object. What can you conclude about the speed of the object? A. When the total work done on the object is positive, the object's speed will increase. B. When the total work done on the object is positive, the object's speed will decrease. C. When the total work done on the object is positive, the object's speed will be constant. D. When the total work done on the object is positive, the object will stop.

A. When the total work done on the object is positive, the object's speed will increase. "This is because the object's speed increases if its kinetic energy increases, which must happen when the total work done is positive. When the total work done on an object is positive, the object's kinetic energy and speed will never decrease or remain constant."

If some quantity, let's call it "Z," is conserved for a certain system, during a certain process, that means __________. A. Z for that system does not change in any way during that process B. the value of Z for each part of the system does not change during that process C. any change in Z for that system during that process is offset by an equal but opposite change for the Z of the surroundings D. the value of Z for each part of the system must change in the same way during that process

A. Z for that system does not change in any way during that process "A conserved quantity's total value for a system does not change at all. It is possible for separate parts of the system to change their Z, as long as those changes are offset by other parts of the system, meaning the total value for the system doesn't change. If separate parts of the system change their Z in the same way, the changes will not cancel out and the total value of Z will change, meaning it will not be conserved. Whether the value of Z for the surroundings changes or not doesn't matter. If the value of Z changes for the system, then Z was not conserved in that process."

If two cars have the same mass and speed they __________. A. might have the same momentum but might not B. must have the same momentum C. cannot have the same momentum

A. might have the same momentum but might not "Since momentum is a vector the two cars would also have to have the same direction of motion in order for them to have the same momentum. If they have the same mass and speed but different directions, they would not have the same momentum."

The change in an object's momentum during a certain time interval is equal to __________. A. the net force exerted on the object times the duration of the time interval B. the net force exerted on the object times the distance the object travels in that time C. the change in the speed of the object times the mass of the object D. the change in the net force exerted on the object times the duration of the time interval

A. the net force exerted on the object times the duration of the time interval "Impulse is defined as a force times a duration that the force is applied. The net impulse is found using the net force on the object times a certain time duration. The impulse-momentum theorem shows that the net impulse on an object equals the change in that object's momentum during that time interval. Force and distance can be used to calculate work but not impulse, and net work equals the change in kinetic energy. Change in speed is not the same as change in velocity, so it cannot be used to find the change in momentum. The change in the net force is not what determines impulse. It is the net force during that time interval that matters"

The momentum of an object is defined as __________. A. the object's mass times its velocity B. the object's mass times its speed C. the object's weight times its velocity D. the object's weight times its speed

A. the object's mass times its velocity "Momentum is a vector quantity, so mass times speed is insufficient to define the momentum. Mass and weight are different physical quantities. Weight is not part of the definition of momentum."

Which of the following forces is considered a conservative force? A. The force of tension from a rope B. Gravity C. The force of friction due to a surface D. The force of someone pushing an object

B. Gravity "The work done by the force of gravity is path independent. The forces of tension, friction, and a person pushing can change the total mechanical energy of a system and the work done by these forces is path dependent, so they are non-conservative forces."

A block slides along a rough surface and comes to a stop. What can you conclude about the frictional force exerted on the block? A. The frictional force does positive work on the block and decreases its kinetic energy. B. The frictional force does negative work on the block and decreases its kinetic energy. C. The frictional force does negative work on the block and increases its kinetic energy. D. The frictional force does positive work on the block and increases its kinetic energy. E. The frictional force does no work on the block and doesn't change its kinetic energy.

B. The frictional force does negative work on the block and decreases its kinetic energy. "The frictional force acting on the block points in the opposite direction to the displacement of the block, so by definition the work is negative. And, according to the work-energy theorem, the work done is equal to the change in kinetic energy. A negative change in kinetic energy means that the kinetic energy decreases and thus stops the block"

In both figures, a particle of mass m, is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest and slides down a ramp with a rough surface. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom? (a) 90 degree angle incline (b) Slanted incline A. The particle in figure (b) will have more KE than the particle in figure (a) at the bottom B. The particle in figure (a) will have more KE than the particle in figure (b) at the bottom C. Both particles will have the same KE at the bottom D. The particles will have the same KE, but the particle in figure (a) will be moving faster at the bottom E. The particles will have the same KE, but the particle in figure (b) will be moving faster at the bottom

B. The particle in figure (a) will have more kinetic energy than the particle in figure (b) at the bottom. "In figure (a), there are no non-conservative forces doing work so the mechanical energy is conserved. The kinetic energy of the particle when it gets to the bottom will be equal to the gravitational potential energy it started with: *mgh*. But in figure (b), friction—a non-conservative force—acts on the particle and decreases its total mechanical energy, converting some mechanical energy into thermal energy. As a result, this particle has less mechanical energy and therefore less kinetic energy than the particle in figure (a) when it gets to the bottom of the ramp."

In figure (b), a particle of mass m is released from rest at the top of a frictionless ramp of height h and in figure (c), a particle of mass 2m is released from rest at the top of the same frictionless ramp. Which particle, the one in figure (b) or (c), will have more kinetic energy at the bottom? A. Both particles will have the same kinetic energy at the bottom. B. The particle in figure (c) will have more kinetic energy than the particle in figure (b) at the bottom. C. The particle in figure (b) will have more kinetic energy than the particle in figure (c) at the bottom. D. The particles will have the same kinetic energy, but the particle in figure (b) will be moving faster at the bottom. E. The particles will have the same kinetic energy, but the particle in figure (c) will be moving faster at the bottom.

B. The particle in figure (c) will have more kinetic energy than the particle in figure (b) at the bottom. "Since there are no non-conservative forces doing work to change the mechanical energy of the system in either figure, the mechanical energy is conserved. This means that the gravitational potential energy that each particle starts with will convert completely into kinetic energy at the bottom. Both particles start at the same height but the particle in figure (c) is twice as large as the particle in figure (b), so the particle in figure (c) will have more gravitational potential energy. This will result in the particle in figure (c) having more kinetic energy at the bottom than the particle in figure (b). Kinetic energy is defined as ½mv2, where v is the speed of the object. For the particle in figure (b) ½mv2 = mg and for the particle in figure (c) ½(2m)v2= 2mg; since the mass cancels out, they have the same speed."

You observe two identical balls of putty headed directly toward each other at the same speed; what can you say about their total momentum? A. They have twice the momentum of either ball by itself. B. They have zero total momentum. C. They have some total amount of momentum, with a magnitude between zero and double the momentum of either ball by itself. D. Nothing. The momentum of separate objects cannot be combined.

B. They have zero total momentum. "They have the same mass, equal speeds, and are moving in opposite directions. Adding their momentum vectors will give a total of exactly zero. Since they are headed in opposite directions their momenta can't be added as if they were scalars. They will combine to give zero. If they were moving in arbitrary directions it would be correct to say that they have some total amount of momentum, with a magnitude between zero and double the momentum of either ball by itself. Since we know their directions of motion we can be more specific. Momentum is a vector quantity that can indeed be combined for different objects in order to obtain the total momentum of a system."

The impulse approximation is used in situations where __________. A. internal forces between parts of the system are small enough not to affect the momentum of any part of the system B. external forces on the system are present, but they are small compared to the large, brief internal forces between parts of the system C. no net external force is exerted on the system D. no external forces are exerted on the system at all

B. external forces on the system are present, but they are small compared to the large, brief internal forces between parts of the system "Because the external forces cause so little net impulse on the system during the collision, it is useful to find an approximate result as if the external forces were not present. Internal forces between parts of the system (such as between colliding rugby players) are generally the process we are interested in understanding. If they were too small to affect any part of the system, nothing would be happening. Having no net external force exerted on the system would mean momentum was exactly conserved and no approximation would be needed. Having no external forces exerted on the system at all would mean momentum was exactly conserved and no approximation would be needed."

Momentum __________. A. can be positive or negative but is not a vector B. is a vector, with a magnitude and a direction C. is a vector but can only have a magnitude and not a direction D. is a scalar, with a magnitude and direction

B. is a vector, with a magnitude and a direction "When working with one-dimensional vectors the direction is sometimes indicated using (+) or (-) signs, but that doesn't apply to 2D or 3D vectors. All vectors have both magnitude and direction. Scalar quantities have only a numerical value (though they can have positive or negative values)."

If Quantity X is conserved for a certain system and during a certain process, that means__________. A. the value of Quantity X for each part of the system does not change during that process B. quantity X for that system does not change in any way during that process C. any change in Quantity X for that system during that process is offset by an equal but opposite change for the Quantity X of the surroundings D. the value of Quantity X for each part of the system must change in the same way during that process

B. quantity X for that system does not change in any way during that process "A conserved quantity's total value for a system does not change at all. It is possible for separate parts of the system to change their Quantity X, as long as those changes are offset by other parts of the system, meaning the total value for the system doesn't change. If separate parts of the system change their Quantity X in the same way, the changes will not cancel out and the total value of Quantity X will change, meaning it will not be conserved. Whether the value of Quantity X for the surroundings changes or not doesn't matter. If the value of Quantity X changes for the system, then Quantity X was not conserved in that process."

During a large, multi-object collision that involves object C, the change in the momentum of object C is equal to __________. A. the impulse exerted on object C by the last object to touch it B. the net impulse exerted on object C C. the impulse exerted on other objects by object C D. the net impulse exerted on object C during the second half of the collision

B. the net impulse exerted on object C "The impulse-momentum theorem says that any change in an object's momentum always occurs due to a net impulse exerted on that object. Any single impulse or the net impulse from only half of the collision will not equal the net impulse exerted on object C and will not equal the change in object C's momentum. The impulse exerted on other objects by object C will be equal and opposite to the net impulse exerted on object C (Newton's third Law). Thus the impulse exerted on other object's by object C will be equal in magnitude but opposite to the change in the momentum of object C."

A weightlifter brings a 400-N barbell upward from his shoulders to a point 50 cm higher at a steady speed. During this process, what is the total work done on the barbell? A. 200 joules B. 400 joules C. 0 joules D. -200 joules E. -400 joules

C. 0 joules "According to the work-energy theorem the total work on an object is equal to the change in that object's kinetic energy. Since the barbell is moving at a constant speed its kinetic energy does not change during that process and the total work on it must be zero. In other words, the work done by the weightlifter (+200 J) and the work done by gravity (-200 J) cancel each other out, causing no change in the barbell's kinetic energy. 200 J is the amount of work done by the weightlifter, but it is not the totalwork done on the barbell. -200 J is the amount of work done by Earth's gravity, but it is not the totalwork done on the barbell. Since the work done by the weightlifter and Earth's gravity have opposite signs they cannot be added to get a total of 400 J or -400 J."

Can an object have more than one momentum at a given moment? A. Yes, each velocity the object has is associated with a particular momentum at that moment. B. Yes, each mass the object has is associated with a particular momentum at that moment. C. No, an object always has one unique velocity and one unique mass at any given moment. D. No, much like its mass, the momentum of an object stays the same until the object is physically changed in some way.

C. No, an object always has one unique velocity and one unique mass at any given moment. "The momentum of an object is not a basic property of the object. It changes depending on the velocity of the object as well as its mass."

A block of mass "m" is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction to compress the spring and then release the block, what happens to the energy in the system immediately after the block is released? A. The elastic potential energy in the spring increases while the kinetic energy of the block decreases. B. The elastic potential energy in the spring decreases while the gravitational potential energy and kinetic energy of the block both increase. C. The elastic potential energy in the spring decreases while the kinetic energy of the block increases. D. The elastic potential energy in the spring and the gravitational potential energy of the block both decrease while the kinetic energy of the block increases.

C. The elastic potential energy in the spring decreases while the kinetic energy of the block increases. "Since there are no non-conservative forces doing work on the system, the mechanical energy of the system is conserved. This means that the elastic potential energy that is stored in the system when the spring is compressed will convert into kinetic energy of the block as the spring decompresses. The gravitational potential energy of the block does not change during this horizontal motion because the block does not experience a change in height at all. The only types of mechanical energy that change during this motion are kinetic energy and elastic potential energy."

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction and compress the spring, what is true about the work done by the spring on the block during this motion? A. The spring does positive work on the block because the potential energy in the spring increases. B. The spring does positive work on the block because the spring force is in the same direction as the block's displacement. C. The spring does negative work on the block because the spring force is in the opposite direction of the block's displacement. D. The spring does negative work on the block because the potential energy in the spring decreases. E. The spring does no work on the block during this motion.

C. The spring does negative work on the block because the spring force is in the opposite direction of the block's displacement. "When a spring is compressed, the spring force pushes in the opposite direction, tending to restore the spring back to its equilibrium position. In this case, the spring force would point in the positive x-direction with the displacement of the block in the negative x-direction, resulting in a negative amount of work done by the spring force. As the spring is compressed, the potential energy stored in the spring increases. The potential energy stored in the spring is a function of the deformation of the spring, x, and increases more and more as the spring is compressed."

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. The block can be pushed in the negative x-direction to compress the spring or pulled in the positive x-direction to stretch the spring. Where along the x-axis does the block have to be for the spring to have zero potential energy? A. The spring has zero potential energy when the block is at x = -x_max, where the spring is compressed to its maximum value. B. The spring has zero potential energy when the block is at x = +x_max, where the spring is stretched to its maximum value. C. The spring has zero potential energy when the block is at x = 0, where the spring is neither stretched nor compressed. D. The spring has zero potential energy when the block is at either x = ±x_max, where the spring is stretched/compressed to its maximum value. E. The spring always has non-zero potential energy no matter where the block is located.

C. The spring has zero potential energy when the block is at x = 0, where the spring is neither stretched nor compressed. "The elastic potential energy stored in a spring is a function of the deformation in the spring, x, or how much it has been stretched or compressed from equilibrium: ½kx2. At equilibrium, x = 0, so the elastic potential energy is zero. When the spring is stretched or compressed to its maximum value, the elastic potential energy stored in the spring is at maximum. The elastic potential energy then decreases as the spring relaxes back to its equilibrium position.

What is true about the work done by a non-conservative force? A. The work done by a non-conservative force will always increase the internal energy of a system. B. The work done by a non-conservative force will always increase the internal energy of a system. C. The work done by a non-conservative force will always change the total mechanical energy of a system. D. The work done by a non-conservative force will always increase the mechanical energy of a system. E. The work done by a non-conservative force will always decrease the mechanical energy of a system.

C. The work done by a non-conservative force will always change the total mechanical energy of a system. "When the total work done on a system is due to non-conservative forces, that total work done will change the kinetic energy of the system, according to the work-energy theorem: . Friction is an example of a non-conservative force that can increase the internal energy of a system but not all non-conservative forces will do this. When non-conservative forces do work, they will sometimes increase the total mechanical energy of a system and other times they might decrease the total mechanical energy."

When you lift a book upward off of a table, what is true about the work done on the book by the force of gravity? A. The work done by the force of gravity is positive and proportional to the upward displacement of the book. B. The work done by the force of gravity is negative and constant, regardless of the magnitude of the upward displacement. C. The work done by the force of gravity is negative and proportional to the upward displacement of the book. D. The work done by the force of gravity is positive and does not depend on the magnitude of the upward displacement.

C. The work done by the force of gravity is negative and proportional to the upward displacement of the book. "The work done by gravity is negative because the force of gravity, which points downward, is in the opposite direction of the displacement, which is upward. The work done by the force of gravity is proportional to the displacement of the book because the force of gravity is approximately constant everywhere near the surface of Earth. The work will not be constant but will depend on the displacement of the book. More negative work will be done by gravity as you lift the book higher and higher."

You observe two identical balls of putty heading directly toward each other at equal speeds. What can you say about their total kinetic energy? A. They have zero total kinetic energy. B. They have some total amount of kinetic energy, with a magnitude between zero and double the kinetic energy of either ball by itself. C. They have twice the kinetic energy of either ball by itself. D. Nothing. The kinetic energy of separate objects cannot be combined.

C. They have twice the kinetic energy of either ball by itself. "They have the same mass and equal speeds so they have equal amounts of kinetic energy. Since kinetic energy is a scalar (not a vector), the direction that they are moving doesn't matter. Kinetic energy is a scalar quantity that depends on the mass, which is always positive, and the squared magnitude of the velocity, which is also always positive, so the kinetic energy cannot be negative and it is not possible to have a total kinetic energy of zero."

The units for momentum are __________. A. N*m B. kg*s/m C. kg*m/s D. N

C. kg*m/s "which we can see from the definition of momentum, p = mv. N*m are the units of work (and also of torque) but are not the units for momentum. Kg*s/m is close to correct, but the units for velocity have been inverted. N is the unit for force, not momentum."

The image shows two balls (1 and 2) that are going to collide as they approach the point marked 0. Ball 1 has a mass of 2m and speed v, while ball 2 has mass m and speed 2v. Both balls are moving at 45º angles with respect to the vertical. Our system consists of just the two balls and is isolated from external forces. Just *before* the collision __________. A. the total momentum of the system is zero B. the horizontal components of the momentum of the two balls are the same C. the total horizontal component of the momentum of the system is zero D. the momentum of the two balls is the same

C. the total horizontal component of the momentum of the system is zero "The magnitude of each ball's momentum is the same and since they have symmetric angles relative to the vertical direction, they will both have the same magnitude x-component, given by . Since their horizontal components of momentum are oppositely directed, they will cancel out and the total horizontal component of the momentum of the system is zero. Neither the horizontal components nor the total momentum of the two balls are the same because of the different directions of their horizontal components. The vertical components of the two balls do not cancel out, so the total momentum of the system is not zero."

The image shows two identical balls about have a head-on collision. Our system consists of just the two balls, both of which have the same speed. Just before the collision __________. A. the momentum of ball 1 equals the momentum of ball 2 B. the total momentum of the system is equal to twice the momentum of either ball by itself C. the total momentum of the system is zero D. the two balls have the same speed component of momentum, but not the same momentum

C. the total momentum of the system is zero "Identical balls have the same mass, and since they have equal speeds the two balls have momenta with equal magnitude but opposite directions. Thus the total momentum of the system is zero. Because momentum is a vector, two momentums that point in different directions can never be considered equal. The term "speed component of momentum" isn't a term that's actually used anywhere. That answer could be modified to be correct if it stated that they have the same magnitude of momentum, but not the same momentum."

The image shows two balls about have a head-on collision. Ball A has a mass m and speed of 2v, whereas ball B has mass 2m and speed v. Our system consists of just the two balls and is isolated from external forces. Just after the collision __________. A. either ball A or ball B must be at rest B. both ball A and ball B must be at rest C. the total momentum of the system must be zero D. both ball A and ball B must rebound with the same speed they had before the collision

C. the total momentum of the system must be zero "Because of their initial masses, speeds, and directions, the two balls end up having initial momenta that have equal magnitudes but opposite directions. Thus the total momentum of the system before the collision is zero. Since the system is isolated from external forces, we know the momentum must be conserved, meaning the total momentum of the system after the collision will also be zero. However, we don't know anything about how that total momentum will be arranged. If only one of the balls ends up at rest, the total final momentum would not be zero, and conservation of momentum would be violated. If both ball A and ball B end up at rest, they would indeed have a total momentum of zero. But that isn't the only way to end up with a zero total momentum, so it isn't accurate to say that this must be what happens. If both ball A and ball B rebound with the same speed they had before the collision, they would indeed have a total momentum of zero. But that isn't the only way to end up with a zero total momentum, so it isn't accurate to say that this must be what happens."

In which of these processes is the momentum of the object conserved? A. A car (the object) pulls away from a stop sign. B. An asteroid (the object) accelerates steadily toward the moon. C. A cyclist (the object) collides with a pedestrian. D. A skydiver (the object) with her parachute deployed falls toward the ground at a steady speed. E. A train (the object) goes around a curve at a steady speed.

D. A skydiver (the object) with her parachute deployed falls toward the ground at a steady speed. "Having a quantity be conserved means that it doesn't change. In order for momentum to be conserved it must have an unchanging mass and velocity (including both speed and direction). When a cyclist collides with a pedestrian it is reasonable to talk about the momentum of the system (both the cyclist and pedestrian) as being conserved, but the cyclist will change speed and probably direction, so the momentum of just the cyclist will not be conserved. As a car pulls away from a stop sign, it is speeding up, increasing the magnitude of its momentum. As an asteroid accelerates toward the moon, it is speeding up, increasing the magnitude of its momentum. As a train goes around a curve at a steady speed, it is changing the direction of its momentum."

During a complex collision between many objects, including object A and object C, the impulse exerted on object A by object C is equal to __________. A. the impulse exerted on object C by object A B. the change in the momentum of object C C. the change in the momentum of object A D. None of the listed responses is correct.

D. None of the listed responses is correct. "Impulse is a vector quantity. Because of Newton's third Law the two objects must exert equal and opposite forces on each other. Since they will do so for equal amounts of time they will exert impulses with equal magnitudes but opposite directions on each other. The change in momentum of an object equals the net impulse exerted on it during the collision, which is not the same as any single impulse it experiences."

Which of the following forces is considered a conservative force? A. The force of tension from a rope B. The force of friction due to a surface C. The force of someone pushing an object D. The force due to a spring

D. The force due to a spring "The work done by the force due to a spring is path independent. The forces of tension, friction, and a person pushing can change the mechanical energy of a system and the work done by these forces is path dependent, so they are non-conservative forces."

The graph shows the x-component of a force applied to an object versus the position of that object in the x-direction. How is the work done by this force determined from the data in this graph? A. The work done by the force is equal to the average slope of the force versus position function. B. The work done by the force is equal to the maximum value of the force. C. The work done by the force is equal to the maximum value of the force multiplied by the object's position. D. The work done by the force is equal to the area under the force versus position function.

D. The work done by the force is equal to the area under the force versus position function. "Work is defined as F_xDeltaX, or the x-component of the force multiplied by the displacement in the x-direction when the force is constant. If the force is varying then one needs to consider infinitesimal segments where the force is approximately constant and add them together. This sum of small rectangular areas is equivalent to the area under the curve when calculating the area under the force versus position graph."

If some quantity, let's call it "Z," is conserved for a certain system, during a certain process, that must mean __________. A. the value of Z for each part of the system cannot change during that process B. individual parts of the system may all experience an increase or decrease in Z during that process, but it cannot happen that some parts increase while others decrease C. the value of Z for each part of the system must be the same as the value of Z for the entire system D. any change in Z for parts of the system during the process is offset by an equal but opposite change for the Z of other parts of the system

D. any change in Z for parts of the system during the process is offset by an equal but opposite change for the Z of other parts of the system "In order for the total value of Z to stay constant the internal changes in Z must cancel out. The value of Z for individual parts of the system can change, as long as some other part of the system experiences an equal but opposite change. For example, some parts might experience an increase in Z while other parts experience a decrease by the same amount. Generally, the only way for each part of the system to have the same value of Z as the entire system is for Z to be zero for all parts of the system. Having zero Z is not what it means for Z to be conserved."

The momentum of a system is conserved during a certain process only if __________. A. no external forces are exerted on the system at all during that process B. no internal forces between parts of the system are exerted at all during that process C. the momentum of each part of the system stays exactly the same during that process D. no net external force is exerted on the system during that process

D. no net external force is exerted on the system during that process "The total momentum of a system can only be changed by a net force acting from outside the system. If there is no such net force on the system, the total momentum of the system cannot change, meaning it is conserved. If there are no external forces exerted on the system at all during that process then momentum will indeed be conserved, but this is not the only way for momentum to be conserved. There can be external forces on the system, just as long as they add up to a zero net force exerted on the system. If the momentum of each part of the system stays exactly the same during that process then momentum will indeed be conserved (because nothing is happening), but this is not the only way for momentum to be conserved. Internal forces between parts of the system cannot change the system's total momentum because they always add up to zero due to Newton's third law. So they can exist or not without affecting the possibility that the total momentum will be conserved."

Two asteroids are flying through space, each with an unknown momentum, p_1 and p_2. Which of these is a true statement about the x-component of their total momentum, p_x? A. p_x must have an angle between 0 and 90 degrees. B. p_x must be positive. C. p_x must equal p_y. D. p_x must have a magnitude between zero and the sum of the magnitudes of their individual momenta. Meaning 0 <= |p_x| <= (|p_1|+|p_2|).

D. p_x must have a magnitude between zero and the sum of the magnitudes of their individual momenta. Meaning 0 <= |p_x| <= (|p_1|+|p_2|). "The x-component of a vector cannot "have an angle." It always points in either the (+) or (-) x-direction. Without more detail about the two asteroids we cannot tell whether p_x must be positive or must equal p_y.

The image shows two balls (1 and 2) that are going to collide as they approach the point marked 0. Ball 1 has a mass of 2m and speed v, while ball 2 has mass m and speed 2v. Both balls are moving at 45º angles with respect to the vertical. Our system consists of just the two balls and is isolated from external forces. Just *after* the collision __________. A. the total vertical component of the momentum of the system must be zero B. the total momentum of the system must be zero C. the horizontal component of each ball must be the same D. the total horizontal component of the momentum of the system must be zero

D. the total horizontal component of the momentum of the system must be zero "Before the collision the magnitude of each ball's momentum is the same and since they have symmetric angles relative to the vertical direction, they will both have the same magnitude x-component, given by |p_x| = p * cos(45). Since their horizontal components of momentum are oppositely directed, they will cancel out and the total horizontal component of the momentum of the system is zero. Since the system is isolated from external forces, we know the total momentum of the system will be conserved. So the total horizontal component of the momentum of the system must still be zero after the collision as well. If the horizontal component of each ball were the same the total horizontal component of the momentum of the system would not be zero, violating the conservation of momentum. The total vertical component of the momentum of the system was not zero before the collision, so it cannot be zero after the collision without violating the conservation of momentum. The total momentum of the system was not zero before the collision, so it cannot be zero after the collision without violating the conservation of momentum."

When helping a friend move into a new home, you push a chair across the room. What do you know about the force that you exert on the chair? A. The force you exert on the chair contribute to the overall change in KE of the system (the chair) with a positive amount and therefore does positive work on the system. B. The force you exert on the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does negative work on the system. C. The force you exert on the chair does not contribute to the overall change in kinetic energy of the system (the chair) in any way and therefore does no work on the system. D. The force you exert on the chair contributes to the overall change in kinetic energy of the system (the chair) with a positive amount and therefore does negative work on the system. E. The force you apply to the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does positive work on the system.

A. The force you exert on the chair contributes to the overall change in kinetic energy of the system (the chair) with a positive amount and therefore does positive work on the system. "We can think of work as the transfer of energy to a body by the application of a force. Since the force you apply to the chair is in the same direction as the displacement of the chair, then the work done by that force is positive and helps to increase the chair's kinetic energy."

Two hockey players collide on the ice. Which statement reflects how we can apply conservation of momentum to this situation? A. During the very brief collision the momentum of each player is a conserved quantity. B. During the very brief collision the total momentum of the two players (the system) is a conserved quantity. C. From several seconds before the collision until several seconds after the collision the total momentum of the two players (the system) is a conserved quantity. D. From several seconds before the collision until several seconds after the collision the momentum of each player is a conserved quantity.

B. During the very brief collision the total momentum of the two players (the system) is a conserved quantity. "The momentum of each player is not itself a conserved quantity since they both experience a net force during the collision. It is the total momentum of the system (the two players, in this case) that can be considered to be conserved since the system experiences no net external force. However, we can only consider it to be conserved if we restrict ourselves to just the brief collision between the players. If we extend our time to include time before and after the collision, the players might use their skates to change speed or direction (using external forces) and the total momentum of the two skaters would no longer be conserved."

The image shows two balls about have a head-on collision. Ball A has a mass m and speed of 2v, whereas ball B has mass 2m and speed v. Our system consists of just the two balls and is isolated from external forces. Just before the collision __________. A. the momentum of ball A equals the momentum of ball B B. the total momentum of the system is zero C. the total momentum of the system is equal to twice the momentum of either ball by itself

B. the total momentum of the system is zero "Because of their masses, speeds, and directions, the two balls have momenta that have equal magnitudes but opposite directions. Thus the total momentum of the system is zero. Because momentum is a vector, two momentums that point in different directions can never be considered equal."

A weightlifter exerts an upward force on a 1000-N barbell and holds it at a height of 1 meter for 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time? A. 500 watts B. 1000 watts C. 2000 watts D. 0 watts

D. 0 watts "Power is the rate at which work is done, or . Since the barbell is not moving, the weightlifter is not doing work on the barbell. Therefore, if the work done is zero, then the power is also zero."

A weightlifter exerts an upward force on a 1000-N barbell and lifts the barbell 1 meter upward in 2 seconds. Approximately how much power does the weightlifter exert on the barbell during this time? A. 500 watts B. 1000 watts C. 2000 watts D. 0 watts

A. 500 watts "Power is the rate at which work is done or W/difference in t. If we assume the weightlifter applies a force that is almost equal to tge weight of the barbell, then he does 1000 J of work. 1000 J / 2 s = 500 watts"

The standard units of momentum are kg*m/s. Which of these is an alternate way to express these units? A. S/(kg*m) B. N*m C. N/s D. N*s

D. N*s "We can see this most directly from the impulse-momentum theorem. We can also see this by expressing N as kg*m/s2 and seeing that if we multiply by s we are back to kg*m/s. S/(kg*m) is the units of momentum inverted, which is not the same. N*m are the units of work (and also of torque) but are not the units for momentum. N/s would give kg*m/s3, which are not the units for momentum."

When helping a friend move into a new home, you push a chair across the room. What do you know about the force of friction applied to the chair by the floor? A. The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a positive amount and therefore does positive work on the system. B. The frictional force applied to the chair does not contribute to the overall change in kinetic energy of the system (the chair) in any way and therefore does no work on the system. C. The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a positive amount and therefore does negative work on the system. D. The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does negative work on the system. E. The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does positive work on the system.

D. The frictional force applied to the chair contributes to the overall change in kinetic energy of the system (the chair) with a negative amount and therefore does negative work on the system. "We can think of work as the transfer of energy to or from a body by the application of a force. Since the frictional force applied to the chair is in the opposite direction to the displacement of the chair, then the work done by that force is negative and helps to decrease the chair's kinetic energy."

In both figures, a particle of mass m is released from rest at a height, h. In figure (a), the particle is dropped straight downward and in figure (b) the particle is released from rest down a *frictionless* ramp. Which particle, the one in figure (a) or (b), will have more kinetic energy at the bottom? A. The particle in figure (a) will have more kinetic energy than the particle in figure (b) at the bottom. B. The particle in figure (b) will have more kinetic energy than the particle in figure (a) at the bottom. C. The particle in figure (b) will have more kinetic energy than the particle in figure (a) at the bottom. D. The particles will have the same kinetic energy, but the particle in figure (b) will be moving faster at the bottom. E. Both particles will have the same kinetic energy at the bottom.

E. Both particles will have the same kinetic energy at the bottom. "Since there are no non-conservative forces doing work to change the mechanical energy of the system in either figure, the mechanical energy is conserved. This means that the gravitational potential energy that each particle starts with will convert completely into kinetic energy at the bottom. Both particles start at the same height so they have the same amount of gravitational potential energy. Therefore, they will gain the same amount of kinetic energy upon reaching the bottom of the motion. Kinetic energy is defined as ½mv2, where v is the speed of the object. Since both particles have the same kinetic energy at the bottom, they must also have the same speed."

When helping a friend move into a new home, you push a chair across the room. What do you know about the force of gravity applied to the chair? A. The force of gravity applied to the chair decreases the energy of the system (the chair) and therefore does negative work on the system. B. The force of gravity applied to the chair increases the energy of the system (the chair) and therefore does positive work on the system. C. The force of gravity applied to the chair increases the energy of the system (the chair) and therefore does negative work on the system. D. The force of gravity applied to the chair decreases the energy of the system (the chair) and therefore does positive work on the system. E. The force of gravity applied to the chair does not change the energy of the system (the chair) and therefore does no work on the system.

E. The force of gravity applied to the chair does not change the energy of the system (the chair) and therefore does no work on the system. "This is because the gravitational force on the chair is perpendicular to the displacement of the chair. The work done by a force that is perpendicular to the displacement will always be zero. With zero work done by the force of gravity, this force does not change the energy of the chair."

A block of mass m is attached to a horizontal spring and rests on a flat, smooth surface as seen in the figure. If you push on the block in the negative x-direction and compress the spring, what is true about the potential energy stored in the spring during this motion? A. The potential energy in the spring increases because the spring does positive work on the block. B. The potential energy in the spring decreases because the spring does negative work on the block. C. The potential energy in the spring decreases because the spring does positive work on the block. D. The potential energy in the spring is zero. E. The potential energy in the spring increases because the spring does negative work on the block.

E. The potential energy in the spring increases because the spring does negative work on the block "By definition, the change in potential energy is equal to the negative work done by the force: delta U = -W. Since the work done by the spring force is negative because the force points in the opposite direction of the displacement, this makes the potential energy increase: + delta U = - (-W) Whenever the spring is compressed or stretched, it will store non-zero potential energy. The potential energy stored in the spring is a function of the deformation of the spring, x, and increases more and more as the spring is compressed."