PHYS 221 CONCEPTS

When an object is under the influence of both conservative and non conservative forces, the sum of its kineti. And potential energies does not change

False

You are seated in a bus and notice that a hand strap that is hanging from the ceiling hangs away from the vertical in the backward direction. From this observation, you can conclude that

You cannot conclude anything about the direction of the velocity of the bus.

A 615 N student standing on a scale in an elevator notices that the scale reads 645 N. From this information, the student knows that the elevator must be moving

You cannot tell if it is moving upward or downward.

When can we be certain that the average velocity of an object is always equal to its instantaneous velocity?

only when the velocity is constant

A rock is under water in a shallow lake. As the rock sinks deeper and deeper into water, the buoyant force on it

remains constant

Suppose the force of the air drag on an object is proportional to the speed of the object and in the direction opposite the object's velocity. If you throw an object upward, the magnitude of its acceleration is greatest

right after the object is released.

A stalled car is being pushed up a hill at constant velocity by three people. The net force on the car is

0

A stock person at the local grocery store has a job consisting of the following five segments: (1) picking up boxes of tomatoes from the stockroom floor (2) accelerating to a comfortable speed (3) carrying the boxes to the tomato display at constant speed (4) decelerating to a stop (5) lowering the boxes slowly to the floor. During which of the five segments of the job does the stock person do positive work on the boxes?

1 and 2

Two identical balls are thrown directly up are, ball A at speed v and ball B at 2v. Which are true

1. The acceleration of each ball is same during entire motion 2. Ball B will go 4x as high as A because it had 4x kinetic E

Under what condition is |A⃗ − B⃗ |=A+B?

1. The magnitude of vector B⃗ is zero. 2. Vectors A⃗ and B⃗ are in opposite directions.

A shell that is initially at rest explodes into two fragments, one fragment 25 times heavier than the other. If any gas from the explosion has negligible mass, then

1. the momentum change of the lighter fragment is exactly the same as the momentum change of the heavier fragment. 2. the kinetic energy change of the lighter fragment is 25 times as great as the kinetic energy change of the heavier fragment.

If the mass of the earth and all objects on it were suddenly doubled, but the size remained the same, the acceleration due to gravity at the surface would become

2 times what it now is.

Two objects having masses m1 and m2 are connected to each other as shown in the figure and are released from rest. There is no friction on the table surface or in the pulley. The masses of the pulley and the string connecting the objects are completely negligible. What must be true about the tension T in the string just after the objects are released?

2<m2G

A satellite in a circular orbit of radius R around planet X has an orbital period T. If Planet X had one-fourth as much mass, the orbital period of this satellite in an orbit of the same radius would be

2T

A 4.0 kg object is moving with speed of 2.0m/s. A 2.0 kg object is moving with speed 4.0 m/s. Both encounter same braking force. Which travels farthest?

2kg object

Planet Z-34 has a mass equal to 1/3 that of Earth, a radius equal to 1/3 that of Earth, and an axial spin rate 1/2 that of Earth. With g representing, as usual, the acceleration due to gravity on the surface of Earth, the acceleration due to gravity on the surface of Z-34 is

3g

Alice and Tom dive from an overhang into the lake below. Tom simply drops straight down from the edge, but Alice takes a running start and jumps with an initial horizontal velocity of 25 m/s. Neither person experiences any significant air resistance. Compare the time it takes each of them to reach the lake below.

Alice and Tom will reach the surface of the lake at the same time.

Two stones, one of mass m and the other of mass 2m, are thrown directly upward with the same velocity at the same time from ground level and feel no air resistance. Which statement about these stones is true?

At its highest point, the heavier stone will have twice as much gravitational potential energy as the lighter one because it is twice as heavy.

A 4.0-kg object is moving with speed 2.0 m/s. A 1.0-kg object is moving with speed 4.0 m/s. Both objects encounter the same constant braking force, and are brought to rest. Which object travels the greater distance before stopping?

Both objects travel the same distance.

Two identical balls are thrown directly upward, ball A at speed v and ball B at speed 2v, and they feel no air resistance. Which statement about these balls is correct?

Ball B will go four times as high as ball A because it had four times the initial kinetic energy.

Block 1 and block 2 have the same mass, m, and are released from the top of two inclined planes of the same height making 30° and 60° angles with the horizontal direction, respectively. If the coefficient of friction is the same in both cases, which of the blocks is going faster when it reaches the bottom of its respective incline?

Block 2 is faster.

You swing a bat and hit a heavy box with force 1500N. The force the box exerts is

Exactly 1500 N whether or not the box moves

If A⃗ −B⃗ =0, then the vectors A⃗ and B⃗ have equal magnitudes and are directed in the opposite directions from each other.

F

If all the components of a vector are equal to 1, then that vector is a unit vector.

F

If the acceleration of an object is negative, the object must be slowing down.

F

If two nonzero vectors point in the same direction, their dot product must be zero.

F

If you swing a bucket of water fast enough in a vertical circle, at the highest point the water does not spill out because an outward force balances the pull of gravity on the water.

F

A restoring force of magnitude F acts on a system with a displacement of magnitude x. In which of the following cases will the system undergo simple harmonic motion?

F is directly proportional to x

A dumbbell-shaped object is composed by two equal masses, m, connected by a rod of negligible mass and length r. If I1 is the moment of inertia of this object with respect to an axis passing through the center of the rod and perpendicular to it and I2 is the moment of inertia with respect to an axis passing through one of the masses, it follows that

I2 > I1.

Suppose that an object is moving with constant nonzero acceleration. Which of the following is an accurate statement concerning its motion?

In equal times its velocity changes by equal amounts.

Suppose that a car traveling to the west (the -x direction) begins to slow down as it approaches a traffic light. Which statement concerning its acceleration in the x direction is correct?

Its acceleration is positive but its velocity is negative.

Two bodies P and Q on a smooth horizontal surface are connected by a light cord. The mass of P is greater than that of Q. A horizontal forceF⃗ (of magnitude F) is applied to Q as shown in the figure, accelerating the bodies to the right. The magnitude of the force exerted by the connecting cord on body P will be

Less than F but not Zero

Two men, Joel and Jerry, push against a wall. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do.

Neither of them does any work.

A woman is straining to lift a large crate, without success because it is too heavy. We denote the forces on the crate as follows: P is the upward force the woman exerts on the crate, C is the vertical contact force exerted on the crate by the floor, and W is the weight of the crate. How are the magnitudes of these forces related while the woman is trying unsuccessfully to lift the crate?

P + C = W

Two objects are dropped from a bridge, an interval of 1.0 s apart, and experience no appreciable air resistance. As time progresses, the DIFFERENCE in their speeds

Remains constant

A mass M is attached to an ideal massless spring. When this system is set in motion with amplitude A, it has a period T. What is the period if the amplitude of the motion is increased to 2A?

T

A satellite is orbiting the earth. If a payload of material is added until it doubles the satellite's mass, the earth's pull of gravity on this satellite will double but the satellite's orbit will not be affected.

T

A satellite of mass m has an orbital period T when it is in a circular orbit of radius R around the earth. If the satellite instead had mass 4m, its orbital period would be

T

If a force always acts perpendicular to an object's direction of motion, that force cannot change the object's kinetic energy.

T

If the dot product of two nonzero vectors is zero, the vectors must be perpendicular to each other.

T

If the graph of the position as a function of time for an object is a horizontal line, that object cannot be accelerating.

T

The magnitude of a vector can never be less than the magnitude of one of its components.

T

When an object is solely under the influence of conservative forces, the sum of its kinetic and potential energies does not change.

T

A box of mass m is pulled with a constant acceleration a along a horizontal frictionless floor by a wire that makes an angle of 15° above the horizontal. If T is the tension in this wire, then

T>Ma

An object is executing simple harmonic motion. What is true about the acceleration of this object? (There may be more than one correct choice.)

The acceleration is a maximum when the displacement of the object is a maximum,The acceleration is zero when the speed of the object is a maximum,The acceleration is a maximum when the object is instantaneously at rest.

A ball is thrown directly upward and experiences no air resistance. Which one of the following statements about its motion is correct?

The acceleration is downward during the entire time the ball is in the air.

Jacques and George meet in the middle of a lake while paddling in their canoes. They come to a complete stop and talk for a while. When they are ready to leave, Jacques pushes George's canoe with a force F⃗ to separate the two canoes. What is correct to say about the final momentum and kinetic energy of the system if we can neglect any resistance due to the water?

The final momentum is zero but the final kinetic energy is positive.

A baseball is located at the surface of the earth. Which statements about it are correct? (There may be more than one correct choice.)

The gravitational force on the ball due to the earth is exactly the same as the gravitational force on the earth due to the ball.

If the eastward component of vector A⃗ is equal to the westward component of vector B⃗ and their northward components are equal. Which one of the following statements about these two vectors is correct?

The magnitude of vector A⃗ is equal to the magnitude of vector B⃗ .

An object is moving forward with a constant velocity. Which statement about this object MUST be true?

The net force on the object is zero.

The figure shows the position of an object (moving along a straight line) as a function of time. Assume two significant figures in each number. Which of the following statements about this object is true over the interval shown?

The object is accelerating to the left.

Two objects have masses m and 5m, respectively. They both are placed side by side on a frictionless inclined ramp and allowed to slide down from rest without any air resistance. Which one of the following statements about these objects is correct?

The objects reach the bottom of the incline at the same time

A small car has a head-on collision with a large truck. Which of the following statements concerning the magnitude of the average force due to the collision is correct?

The small car and the truck experience the same average force.

A girl throws a stone from a bridge. Consider the following ways she might throw the stone. The speed of the stone as it leaves her hand is the same in each case, and air resistance is negligible. Case A: Thrown straight up. Case B: Thrown straight down. Case C: Thrown out at an angle of 45° above horizontal. Case D: Thrown straight out horizontally. In which case will the speed of the stone be greatest when it hits the water below?

The speed will be the same in all cases.

The motions of a car and a truck along a straight road are represented by the velocity-time graphs in the figure. The two vehicles are initially alongside each other at time t = 0. At time T, what is true about these two vehicles since time t = 0?

The truck will have traveled further than the car.

Two objects have masses m and 5m, respectively. They both are placed side by side on a frictionless inclined plane and allowed to slide down from rest.

The two objects reach the bottom of the incline at the same time.

Three cars (car F, car G, and car H) are moving with the same velocity when the driver suddenly slams on the brakes, locking the wheels. The most massive car is car F, the least massive is car H, and all three cars have identical tires. (a) Which car travels the longest distance to skid to a stop?

They all travel the same distance in stopping.

Two objects are thrown from the top of a tall building and experience no appreciable air resistance. One is thrown up, and the other is thrown down, both with the same initial speed. What are their speeds when they hit the street?

They are traveling at the same speed.

Jan and Len throw identical rocks off a tall building at the same time. The ground near the building is flat. Jan throws her rock straight downward. Len throws his rock downward and outward such that the angle between the initial velocity of the rock and the horizon is 30°. Len throws the rock with a speed twice that of Jan's rock. If air resistance is negligible, which rock hits the ground first?

They hit at the same time

A certain planet has an escape speed V. If another planet has twice size and twice the mass of the first planet, its escape speed will be

V

If A>B, under what condition is |A⃗ − B⃗ |=A−B?

Vectors A⃗ and B⃗ are in the same direction.

Is it possible for a system to have negative potential energy?

Yes, since the choice of the zero of potential energy is arbitrary.

When a rigid body rotates about a fixed axis, all the points in the body have the same

When a rigid body rotates about a fixed axis, all the points in the body have the same

Which, if any, of the following three statements concerning the work done by a conservative force is NOT true? Select option "All of the above statements are true." if all three statements are true.

When the starting and ending points are the same, the total work is zero. It can always be expressed as the difference between the initial and final values of a potential energy function. It is independent of the path of the body and depends only on the starting and ending points.

If you set the cruise control of your car to a certain speed and take a turn, the speed of the car will remain the same. Is the car accelerating?

Y

An object is moving with constant non-zero acceleration along the +x-axis. A graph of the velocity in the x direction as a function of time for this object is

a straight line making an angle with the time axis.

A monkey is sitting at the top of a tree 20 m above ground level. A person standing on the ground wants to feed the monkey. He uses a bow and arrow to launch the food to the monkey. If the person knows that the monkey is going to drop from the tree at the same instant that the person launches the food, how should the person aim the arrow containing the food? Air resistance is small enough to be ignored.

aim at the monkey

A pilot drops a package from a plane flying horizontally at a constant speed. Neglecting air resistance, when the package hits the ground the horizontal location of the plane will

be over the package.

In a perfectly ELASTIC collision between two perfectly rigid objects

both the momentum and the kinetic energy of the system are conserved.

You are standing on a skateboard, initially at rest. A friend throws a very heavy ball towards you. You can either catch the object or deflect the object back towards your friend (such that it moves away from you with the same speed as it was originally thrown). What should you do in order to MINIMIZE your speed on the skateboard?

catch the ball

The motion of a particle is described in the velocity versus time graph shown in the figure. We can say that its speed

decreases and then increases.

As you are leaving a building, the door opens outward. If the hinges on the door are on your right, what is the direction of the angular velocity of the door as you open it?

down

You swing a bat and hit a heavy box with a force of 1500 N. The force the box exerts on the bat is

exactly 1500 N whether or not the box moves.

The reason an astronaut in an earth satellite feels weightless is that

he astronaut is falling.

A 50-cm3 block of wood is floating partially submerged in water, and a 50-cm3 block of iron is totally submerged in water. Which block has the greater buoyant force on it?

iron

A very small round ball is located near a large solid sphere of uniform density. The force that the large sphere exerts on the ball

is exactly the same as it would be if all the mass of the sphere were concentrated at the center of the sphere.

For general projectile motion, when the projectile is at the highest point of its trajectory

its velocity is perpendicular to the acceleration.

Salt water is more dense than fresh water. A ship floats in both fresh water and salt water. Compared to the fresh water, the volume of salt water displaced by the ship is

less than the volume of fresh water.

A brick is resting on a rough incline as shown in the figure. The friction force acting on the brick, along the incline, is

less than the weight of the brick.

On a smooth horizontal floor, an object slides into a spring which is attached to another mass that is initially stationary. When the spring is most compressed, both objects are moving at the same speed. Ignoring friction, what is conserved during this interaction?

momentum and mechanical energy

A small glider is coasting horizontally when suddenly a very heavy piece of cargo falls out of the bottom of the plane. You can neglect air resistance. Just after the cargo has fallen out

neither the cargo nor the plane change speed.

If you double the pressure on the surface of a can of water, the buoyant force on a stone placed in that water will

not change

Suppose you are playing hockey on a new-age ice surface for which there is no friction between the ice and the hockey puck. You wind up and hit the puck as hard as you can. After the puck loses contact with your stick, the puck will

not slow down or speed up.

A mass M is attached to an ideal massless spring. When this system is set in motion, it has a period T. What is the period if the mass is doubled to 2M?

sqrt(2T)

A string is attached to the rear-view mirror of a car. A ball is hanging at the other end of the string. The car is driving around in a circle, at a constant speed. Which of the following lists gives all of the forces directly acting on the ball?

tension and gravity

At a certain depth in the ocean, the absolute pressure is p. If you go to twice that depth (treating the water as incompressible)

the absolute pressure will be less than 2p.

When a parachutist jumps from an airplane, he eventually reaches a constant speed, called the terminal speed. Once he has reached terminal speed

the force of air drag on him is equal to his weight.

Two objects, one of mass m and the other of mass 2m, are dropped from the top of a building. When they hit the ground

the heavier one will have twice the kinetic energy of the lighter one.

While an object is in projectile motion (with upward being positive) with no air resistance

the horizontal component of its velocity remains constant and the vertical component of its acceleration is equal to -g.

In simple harmonic motion, the speed is greatest at that point in the cycle when

the magnitude of the acceleration is a minimum.

In an INELASTIC collision between two objects

the momentum of the system is conserved but the kinetic energy of the system is not conserved.

Consider a plot of the displacement (x) as a function of the applied force (F) for an ideal elastic spring. The slope of the curve would be

the reciprocal of the spring constant

A ball is tossed vertically upward. When it reaches its highest point (before falling back downward)

the velocity is zero, the acceleration is directed downward, and the force of gravity acting on the ball is directed downward.

A cup of water containing an ice cube at 0°C is filled to the brim. The tip of the ice cube sticks out of the surface. As the ice melts, you observe that

the water level remains the same

A 3.00-kg ball swings rapidly in a complete vertical circle of radius 2.00 m by a light string that is fixed at one end. The ball moves so fast that the string is always taut and perpendicular to the velocity of the ball. As the ball swings from its lowest point to its highest point

the work done on it by gravity is -118 J and the work done on it by the tension in the string is zero.

When you ride a bicycle, in what direction is the angular velocity of the wheels?

to your left

Swimmers at a water park have a choice of two frictionless water slides as shown in the figure. Although both slides drop over the same height, h, slide 1 is straight while slide 2 is curved, dropping quickly at first and then leveling out. How does the speed v1 of a swimmer reaching the end of slide 1 compares with v2, the speed of a swimmer reaching the end of slide 2?

v1 = v2

The position x of an object varies with time t. For which of the following equations relating x and t is the motion of the object simple harmonic motion? (There may be more than one correct choice.)

x = 8 cos 3t,x = 5 sin 3t,) x = 2 cos(3t - 1)