Physics Concept Questions Chapter 4-6
6.1 Two masses m1 and m2, with m1 < m2, have equal kinetic energy. How do the magnitudes of their momenta compare? a) not enough info b) p1 < p2 c) p1 = p2 d) p1 > p2
b
6.6 In a perfectly inelastic one-dimensional collision between two objects, what initial condition alone is necessary so that all of the original kinetic energy of the system is gone after the collisions? a) the objects must have momenta with the same magnitude but opposite directions b) the objects must have the same mass c) the objects must have the same velocity d) the objects must have the same speed, with velocity vectors in opposite directions.
a
6.4 An object of mass m moves to the right with a speed v. It collides head-on with an object of mass 3m moving with speed v/3 in the opposite direction. If the two objects stick together, what is the speed of the combined object or mass 4m, after the collision? a) 0 b) v/2 c) v d) 2v
a because they are moving in opposite directions, the two initial m and v combine to make zero. 1mv - 1mv = 0
5.2 A block slides at constant speed down a ramp while acted on by three forces: its weight, the normal force, and kinetic friction. Respond to each statement, true or false. a) the combined net work done by all three forces on the block equals zero b) each force does zero work on the block as it slides c) each force does negative work on the block as it slides
a) true b) false c) false
An exoplanet has twice the mass and half the radius of the earth. Find the acceleration due to gravity on its surface, in terms of g., the acceleration of the earths surface a) g b) 0.5 g c) 2 g d) 4 g e) 8 g
e; the radius is divided by 2 and squared so you get 1/4. then you have to flip it and multiple by the 2 on top to get 8g
5.5 Calculate the elastic potential energy of a spring with spring constant k = 225 N/m that is a) compressed b) stretched by 1.00 x 10^-2 m
(.5)(225)(1.00 x 10^-2)^2 = 0.0113 a. 0.0113 J b. 0.0113 J the PE is the same compressed and stretched
Identify the action-reaction pairs in the following situations:(a) a man take a step, (b) a snowball hits a girl in the back, (c) a baseball player catches a ball, (d) a gust of wind strikes a window.
(a) As the man takes a step, the action is the force his foot exerts on Earth; the reaction is the force exerted by Earth on his foot. (b) Here, the action is the force exerted by the snowball on the girl's back; the reaction is the force exerted by the girl's back on the snowball. (c) This action is the force exerted by the glove on the ball; the reaction is the force exerted by the ball on the glove. (d) This action is the force exerted by the air molecules on the window; the reaction is the force exerted by the window on the air molecules.
(a) Can the kinetic energy of a system be negative? (b) Can the gravitational potential energy of a system be negative? Explain.
(a) Kinetic energy is always positive. Mass and speed squared are both positive. (b) Gravitational potential energy can be negative when the object is lower than the chosen reference level.
(a) If two automobiles collide, they usually do not stick together. Does this mean the collision is elastic? (b) Explain why a head-on collision is likely to be more dangerous than other types of collisions.
(a) No. In most collisions, there is some loss of kinetic energy(meaning that collision is not elastic) but the objects fail to stick together as they would in a completely inelastic collision(a collision in which the maximum possible KE energy loss, consistent with the conservation of momentum, occurs). (b) A greater portion of the incident KE is transformed to other forms of energy in a head-on collision than in a glancing collision. Thus, the expectation of damage to passengers is greatest in head-on collisions.
During a stress test of the cardiovascular system, a patient walks and runs on a treadmill. (a) Is the energy expended by the patient equivalent to the energy of walking and running on the ground? Explain. (b) What effect, if any, does tiling the treadmill upward have? Discuss.
(a) The effects are the same except for such features as possibly having to overcome air resistance if the wind is blowing outside. (b) The person must lift his body slightly with each step on the tilted treadmill. Thus, the effect is that of running uphill.
Choose the best answer. A car traveling at constant speed has a net force of zero acting on it. a) true b) false c) the answer depends on motion
A
A large bed-sheet is held vertically by two students. A third student, who happens to be the star pitcher on the baseball team, throws a raw egg at the sheet. Explain why the egg doesn't break when it hits the bed-sheet, regardless of its initial speed.
A certain impulse is required to stop the egg. But, if the time during which the momentum change of the egg occurs is increased, the resulting force on the egg is reduced. The time is increased when the sheet billows out as the egg is brought to a stop. The force is reduced low enough so that the egg will not break.
5.3 Three identical balls are thrown from the top of the building, all with the same initial speed. The first ball is thrown horizontally, the second at some angle above the horizontal, and the third at some angle below the horizontal. Neglecting air resistance, rank the speeds of the balls as they reach the ground, from fastest to slowest.
All three balls strike the ground at the same speed
An Earth satellite is in a circular orbit at an altitude of 500 km. Explain why the work done by the gravitational force acting on the satellite is zero. Using the work energy theorem, what can you say about the speed of the satellite?
As the satellite moves in a circular orbit about the earth, its displacement during any smaller time interval is perpendicular to the gravitational force, which always acts toward the center of the earth. Therefore, the work done by the gravitational force during any displacement is zero. Because the work-energy theorem says that the net work done on an object during any displacement is equal to the change in its kinetic energy, and the work done in this case is zero, the change in the satellite's KE is zero, hence, its speed remains constant gravity is trying to move it towards the earth but we are only moving horizontally along the earths axis. constantly accelerating. W = Fd ( d is a parallel distance) if you are sliding something across the table, the frictional force does work because it takes energy out of your system since it is going in an opposite direction of the displacement being slid... but the normal force does no work to the system because its not helping change the energy in the direction that it is going.
A more ordinary example of conservation of momentum than a rocket ship occurs in a kitchen dish-washing machine. In this device, water at high pressure is forced out of small holes on the spray arms. Use conservation of momentum to explain why the arms rotate, directing water to all dishes.
As the water is forced out of hole in the arm, the arm imparts a horizontal impulse to the water. The water then exerts an equal and opposite impulse on the spray arm, causing the spray arm to rotate in the direction opposite that of the spray.
Consider a tug-of-war , in which two teams pulling on a rope are evenly matched so that no motion takes place. Is work done on the rope? On the pullers? On the ground? Is work done on anything?
Because no motion takes place, the rope undergoes no displacement and no work is done on it. For the same reason, no work is being done on the pullers or the ground. Work is being done only within the bodies of the pullers; heart applying forces on blood to move throughout the body.
When a punter kicks a football, is he doing any work on the ball while the toe of his foot is in contact with it? Is he doing any work on the ball after it loses contact with his toe? Are any forces doing work on the ball while it is in flight?
During the time that the toe is in contact with the ball, the punter is doing work on the ball. After the ball loses contact with the toe, only the gravitational force and the retarding force due to air resistance continue to do work on the ball throughout its flight.
In perfectly inelastic collisions between two objects, there are events in which all of the original kinetic energy is transformed to forms other than kinetic. Give an examples of such an event.
If all the kinetic energy disappears, there must be no motion of either of the objects after the collision. If neither is moving, the final momentum of the system is zero, and the initial momentum of the system must also have been zero. A situation in which this could be true would be the head-on collision of two objects having momenta of equal momentum but opposite directions.
A space explorer is moving through space far from any planet or star. He notices a large rock, taken as a specimen from an alien planet, floating around the cabin of the ship. Should he push it gently, or should he kick it toward the storage compartment? Explain.
If it has a large mass, it will take a large force to alter its motion even when floating in space. Thus, to avoid injuring himself, he should push it gently toward the storage compartment.
If only one force acts on an object, can it be in equilibrium? Explain.
In order for an object to be in equilibrium, the resultant force acting on it must be zero. Thus, it is not possible for an object to be in equilibrium when a single force of non-zero magnitude acts on it.
A ball of clay of mass m is thrown with a speed v against a brick wall. The clay sticks to the wall and stops. Is the principle of conservation of momentum violated in this example?
Initially, the clay had momentum directed toward the wall. When it collides and sticks to the wall, neither the clay nor the wall appears to have any momentum. It is therefore tempting to conclude that momentum is not conserved. The "lost" momentum, however, it actually imparted to the wall and the Earth, causing both to move. Because of the Earths enormous mass, its recoil speed is too small to detect.
Your PE teacher throws you a tennis ball at a certain velocity and you catch it. You are now given the following choice: The teacher can throw you a medicine ball with the same velocity, the same momentum, or the same kinetic energy as the tennis ball. Which option would you choose in order to make to easiest catch, and why?
It will be easiest to catch the medicine ball when its speed(and KE) is lowest. Throwing the ball at the same velocity will be the most difficult, because the speed will not be reduced at all. Throwing the ball at the same momentum will reduce the velocity by the ratio of the masses. Throwing the ball with the same KE will also reduce the velocity but only by the square root of the ratio of the masses. throwing the medicine ball with the same momentum is the best
An open box slides across a friction-less, icy surface of a frozen lake. What happens to the speed of the box as water from a rain shower falls vertically downward into the box? Explain.
Its speed decreases as its mass increases. No external horizontal forces act on the box-rainwater system, to its horizontal momentum cannot change as the box moves along the surface. Because the product mVx must be constant, and because the mass of the box m is increasing as it slowing fills with water, the horizontal speed of the box must decrease.
Does a larger net force exerted on an object always produce a larger change in the momentum of the object, compared to a smaller net force. Explain.
No. Impulse (Fdeltatime) depends on the force and the time interval during which it is applied.
Does a larger net force always produce a larger change in kinetic energy than a smaller net force? Explain.
No. The change in kinetic energy of an object is equal to the net work done on it. This net work is the product of the net force acting on the object and the displacement in the direction of the force. Thus, a small magnitude force acting through a large distance may do more work(and hence produce a greater change in KE) than a large force acting through a small distance.
A skater is standing still on a friction-less ice rink. Her friend throws a Frisbee straight at her. IN which of the following cases is the largest momentum transferred to the skater? (a) The skater catches the Frisbee and holds onto it. (b) The skater catches the Frisbee momentarily, but then drops it vertically downward. (c) The skater catches to Frisbee, holds it momentarily, and throws it back to her friend.
Since the total momentum of the skater-Frisbee system is conserved, the momentum transferred to the skater equals the magnitude of the change in the Frisbee's momentum. This is greatest when the skater thrown the Frisbee back after catching it.
If you push on a heavy box that is at rest, you must exert some force to start its motion. Once the box is sliding, why does a smaller force maintain its motion?
The coefficient of static friction is large than that of kinetic/dynamic friction. To start the box moving, you must counterbalance the maximum static friction force. This force exceeds the kinetic friction force that you must counterbalance to maintain constant velocity of the box once it starts moving.
(a) What force causes an automobile to move? (b) A propeller-driven airplane? (c) A rowboat?
The force causing an automobile to move is the friction between the tires and the roadway as the automobile attempts to push the roadway backward. The force driving a propeller airplane forward is the reaction force exerted by the air on the propeller as the rotating propeller pushes the air backward(the action). In a rowboat, the rower pushes the water backward with the oars(the action). The water pushes forward on the oars and hence the boat(the reaction)
One commentator remarked that the force of the explosion at the Twin Towers of the World Trade Center was strong enough to blow glass and parts of the steel structure to small fragments. Yet the television coverage showed thousands of sheets of paper floating down, many still intact. Explain how that could be.
The glass, concrete, and steel were part of the rigid structure that shattered upon impact of the airplanes with the towers and upon collapse of the buildings as the steel support structures weakened due to high temperatures of the burning fuel. The sheets of paper floating down were probably not in the vicinity of the direct impact, where they would have burned after being exposed to very high temperatures. The papers were most likely situated on desktops or open file cabinets and were blown out of the buildings as they collapsed.
A passenger sitting in the rear of a bus claims that she was injured as the driver slammed on the brakes, causing a suit-case to come flying toward her from the front of the bus. If you were the judge in this case, what disposition would you make? Explain.
The inertia of the suitcase would keep it moving forward as the bus stops. There would be no tendency for the suitcase to be thrown backward toward the passenger. The case should be dismissed
Analyze the motion of a rock dropped in water in terms of its speed and acceleration as it falls. Assume a resistive force is acting on the rock that increases as the velocity of the rock increases.
The net force acting on the object decreases as the resistive force increases. Eventually, the resistive force become equal to the weight of the object, and the net force goes to zero. In this condition, the object stops accelerating, and the velocity stays constant. The rock has reached its terminal velocity.
An air bag inflates when a collision occurs, protecting a passenger from serious injury. Why does the air bag soften the blow? Discuss the physics involved in this dramatic photograph.
The passenger must undergo a certain momentum change in the collision. This means that an impulse must be imparted the passenger by the steering wheel, the window, and air bad, or something. By increasing the time during which this momentum change occurs, the resulting force on the passenger can be decreased.
In a tug-of-war between two athletes, each pulls on the rope with a force of 200N. What is the tension in the rope? If the rope doesn't move, what horizontal force does each athlete exert against the ground?
The tension in the rope is the maximum force that occurs in both directions. In this case then, since both are pulling with a force of magnitude 200 N, the tension is 200N. If the rope does not move, the the force on each athlete must equal zero. Therefore, each athlete exerts 200N against the ground.
A woman is standing on the earth. In terms of magnitude, is her gravitational force on the earth a) equal to b) less than c) greater than the earths gravitational force on her?
a
If the speed of a particle is doubled, what happens to its kinetic energy? a) it becomes four times larger b) it becomes two times larger c) it because the square root of 2 times larger d) it is unchanged e) it becomes half as large
a
Two stones, one with twice the mass of the other, are thrown straight up and rise to the same heigh h. Compare their changes in GPE a) they rise to the same height so the stone with twice the mass has twice the change in GPE b) they rise to the same height, so they have the same change in GPE c) the answer depends on their speeds at height h
a
Two toboggans (with riders) of the same mass are at rest on top of a steep hill. As they slide down the hill, toboggan A takes a straight path down the slope while toboggan B winds back and forth along a more gently sloping path. Toboggan A makes the trip in half the time of toboggan B. a) compare the work done by gravity on each toboggan b) compare the average power delivered by gravity on the two toboggans
a. Gravity does equal work on each toboggan b. because gravity does equal work on each toboggan and A makes the trip in half the time of B, the average power delivered by gravity to A is twice that delivered to B.
6.5 A skater is using very low-friction rollerblades. A friend throws a Frisbee to her, on the straight line along which she is coasting. Describe each of the following events as an elastic, an inelastic or a perfectly inelastic collision between the skater and the Frisbee. a) she catches the frisbee and holds it b) she tries to catch the frisbee, but it bounces off her hands and fall in the ground in front of her c) she catches the Frisbee and immediately throws it back with the same speed (relative tot he ground)
a. perfectly inelastic b. inelastic c. inelastic
A certain truck has twice the mass of a car. Both are moving at the same speed. If the kinetic energy of the truck is K, what is the kinetic energy of the car? a) K/4 b) K/2 c) 0.71 K d) K e) 2K
b
Two particles of different mass start from rest. The same net force acts on both of them as they move over equal distances. How do their final kinetic energies compare? a) the particle of larger masses has more kinetic energy b) the particle of smaller mass has more kinetic energy c) the particles have equal kinetic energies d) either particle might have more kinetic energy
b
Which of the following statements are true? a) an astronaut's weight is the same on the moon as on earth b) an astronauts mass is the same on the international space station as it is on earth c) earths gravity has no effect on astronauts inside the international space station d) an astronaut's mass is greater on earth than on the moon e) none of the baove
b
A truck loaded with sand accelerates along a highway. The driving force on the truck remains constant. What happens to the acceleration of the truck as its trailer leaks sand at a constant rate through a hole in the bottom? a) it decreases at a steady rate b) it increases at a steady rate c) it increases and then decreases d) it decreases and then increases e) it remains constant
b F=ma the mass decreases and so the acceleration increases to keep the F the same
***A car accelerates uniformly from rest. Ignoring air friction when does the car requires the greatest power? a) when the car first accelerates from rest b) just as the car reaches its maximum speed c) when the car reaches half its maximum speed d) the question is misleading because the power required is constant e) more information is needed
b (at a greater velocity and when force is constant, there is a larger power. also you move more quickly in a shorter amount of time which increases power since they are inversely proportionate)
****Mark and David are loading identical cement blocks onto Davids pickup truck. Mark lifts his block straight up from the ground to the truck, whereas David slides his block up a ramp on massless, frictionless rollers. Which statement is true? a) mark does more work than david b) mark and david do the same amount of work c) david does more work than mark d) none of these are true because the angle of the incline is unknown. e) none are true because we don't know the mass.
b they are the same because we are assuming they are at constant velocities because they have a certain amount of potential energy
5.8 A book of mass m is projected with a speed v across a horizontal surface. The book slides until it stops due to the friction between the book and the surface. The surface is now tilted 30 degrees and the book is projected up the surface with the same initial speed v. When the book has come to rest, how does the decrease in mechanical energy of the book - Earth system compare with that when the book slid over the horizontal surface? a) it's the same b) it's larger on the tilted surface c) it's smaller on the tilted surface d) more information is needed
c there is still energy left over. It is converted into PE. vs. when its on a horizontal surface, it is all converted into the work done by friction and no energy is left over.
If the net work done on a particle is zero, which of the following statements must be true? a) the velocity is zero b) the velocity is decreased c) the velocity is unchanged d) the speed is unchanged e) more information is needed
d a) when you let ur foot off of the brake you are coasting and so velocity is not zero but the net force is zero because acceleration is zero b) cannot make the net work zero when the velocity is decreasing because there is a change in acceleration. c) in the example of the satellite, you have no net work being done while there is still accelerating happening which means there is a change in velocity (it changes in direction not magnitude) d) you are covering the same distance as you were before... because of that your net work is zero. if your next force is zero, you cannot change speed but you can change direction of velocity.
If an object is in equilibrium, which of the following statements is not true? a) the speed of the object remains constant b)the acceleration of the object is zero c)the net force acting on the object is zero d)the object must be at rest e)the velocity is constant
d - equilibrium means that speed IS constant, acceleration is ZERO, net force is ZERO and velocity IS constant.
Two identical ice hockey pucks labeled A and B are sliding towards each other at speed v. Which one of the following statements is true concerning their momenta and kinetic energies? a) Pa = Pb and KEa = KEb b) Pa = - Pb and KEa = -KEb c) Pa = -Pb and KEa = KEb d) PA = Pb and KEa = KEb
direction doesn't matter for energy direction matters for momentum c kinetic energy cannot be negative
5.6 True or False: The elastic potential energy of a stretched or compressed spring is always positive?
True
4.5 If you press a book flat against a vertical wall with your hand, in what direction is the friction force exerted by the wall on the book? a) downward b) upward c) out from the wall d) into the wall
b
A batter bunts a pitched baseball, blocking the ball without swinging. (a) Can the baseball deliver more kinetic energy to the bat and batter than the ball carries initially? (b) Can the baseball deliver more momentum to the bat and batter than the ball carries initially? Explain.
(a) No. It cannot carry more kinetic energy than it possesses. That would violate the law of energy conservation. (b) Yes. By bouncing from the object it strikes, it can deliver more momentum in a collision than it possess in its flight. Momentum is still conserved but because the ball is moving with the same magnitude but in an opposite direction it adds to the initial momentum of the ball equalling the new momenutm of the bat.
If two objects collide and one is initially at rest, (a) is it possible for both to be at rest after the collision? (b) Is it possible for only one to be at rest after the collision? Explain.
(a) No. One of the objects was in motion before collision, so the system consisting of the two particles had a nonzero momentum before impact. Since momentum is always conserved in collisions, the system must have nonzero momentum after impact, meaning that at least on of the particles must be in motion. (b) Yes. It is possible for one of the particles to be at rest after collision, provided the other particle leaves the collision with a momentum equal to the total momentum of the two-particle system before impact.
A weight lifter stands on a bathroom scale. (a) As she pumps a barbell up and down, what happens to the reading on the scale? (b) Suppose she is strong enough to actually throw the barbell upward. How does the reading on the scale vary now?
(a) The barbell always exerts a downward force on the lifter equal in magnitude to the upward force that she exerts on the barbell. Since the lifter is in equilibrium, the magnitude of the upward force exerted on her by the scale(that is, the scale is reading) equals the sum of her weight and the downward force exerted by the barbell. (pushing the barbell up will accelerate her motion thus increasing the weight on the scale - lowering it down decelerates thus decreasing the reading on the scale) As the barbell goes through the bottom of the cycle and is being lifted upward, the scale reading exceeds the combined weights of the lifter and the barbell. At the top of the motion and as the barbell is allowed to move back downward, the scale reading is less than the combined weight. (b) If the barbell is moving upward, the lifter can declare she has thrown it just by letting go of it for a moment. Thus, the case is included in the previous answer.
Discuss whether any work is being done by each of the following agents and, if so, whether the work is positive or negative: (a) a chicken scratching the ground, (b) a person studying, (c) a crane lifting a bucket of concrete, (d) the force of gravity on the bucket in part (c), (e) the leg muscles of a person in the act of sitting down.
(a) The chicken does positive work on the ground (the force that the chicken exerts is parallel to the dirts displacement) (b) No work is done. (c) The crane does positive work on the bucket. (d) The force of gravity does negative work on the bucket. (e) The leg muscles do negative work on the individual. if force and displacement are the same direction that is positive
In golf, novice players are often advised to be sure to "follow through" with their swing. (a) Why does this make the ball travel a longer distance? (b) If a shot is taken near the green, very little follow-through is required. Why?
(a) The follow-through keep the club in contact with the ball as long as possible, maximizing the impulse. Thus, the ball accrues a larger change in momentum than without the follow-through, and it leaves the club with a higher velocity and travels farther. (b) With a short shot to the green, the primary factor is control, not distance. Hence, there is little of no follow-through, allowing the golfer to have a better feel for how hard he/she is striking the ball.
As a simple pendulum swings back and forth, the forces acting on the suspended object are the force of gravity, the tension in the supporting cord, and air resistance. (a) Which of these forces, if any, does no work on the pendulum? (b) Which of these forces does negative work at all time during the pendulum's motion? (c) Describe the work done by the force of gravity while the pendulum is swinging.
(a) The tension in the supporting cord does no work, because the motion of the pendulum is always perpendicular to the cord and therefore to the tension force. (the pathway of the pendulum is tangent, it is going in a semi circle, the tension is perpendicular to the tangent, thus it does work on the pendulum) (b) The air resistance does negative work at all times, because the air resistance is always acting in the direction opposite that of the motion. (c) The force of gravity always acts downwards; therefore, the work done by gravity is positive on the downswing and negative on the upswing.
A ball is held in a person's hand. (a) Identify all the external forces acting on the ball and the reaction to each. (b) If the ball is dropped, what force is exerted on it while it is falling? Identify the reaction force in this case. (Neglect air resistance)
(a) Two external forces act on the ball(i) One is downward gravitational force extended by Earth.(ii) The second force on the ball is an upward normal force exerted by the hand. The reactions to these forces are (i) an upward gravitational force exerted by the ball on the hand. (ii) a downward force exerted by the ball on the hand. (b) After the ball leaved the had, the only external force acting on the ball is the gravitational force exerted by Earth. The reaction is an upward gravitational force exerted by the ball on Earth.
A weight is connected to a spring that is suspended vertically from the ceiling. If the weight is displaced downward from its equilibrium position and released, it will oscillate up and down. (a) If air resistance is neglected, will the total mechanical energy of the system(weight plus Earth plus spring) be conserved? (b) How many forms of potential energy are there for this situation?
(a) Yes, the total mechanical energy of the system is conserved because the only forces acting are conservative: the force of gravity and the spring force. (b) There are two forms of potential energy and elastic potential energy in this case: gravitational potential energy and elastic potential energy stored in the spring.
(a) If gold were sold by weight, would you rather buy it in Denver or in Death Valley? (b) If it were sold by mass, which of the two locations would you prefer to buy it? Why?
(a)w=mg and g decreases with altitude. Thus, to get a good buy, purchase it in Denver. (b) If gold were sold by mass, it wouldn't matter where you bought it
Roads going up mountains are formed into switchbacks, with the road weaving back and forth along the face of the slope such that there is only a gentle rise on any portion of the roadway. Does the configuration require any less work to be done by an automobile climbing the mountain, compared with one traveling on a roadway that is straight up the slope? Why are switchbacks used?
If we ignore any effects due to rolling friction on the tires of the car, we find that the same amount of work would be done in driving up the switchback and in driving straight up the mountain because the weight of the car is moved upwards against gravity by the same vertical distance in each case. If we included friction, there is more work done in driving the switchback because the distance over which the friction force acts is much longer. So why do we use switchbacks? The answer lies in the force required to drive straight up the hill. To negotiate roadways running straight uphill, engines would have to be redesigned to enable them to apply much larger forces.
A sharpshooter fires a rifle while standing with the butt of the gun against his shoulder. If the forward momentum of a bullet is the same as the backward momentum of the gun, why isn't it as dangerous to be hit by the gun as by the bullet?
It is the product mv that is the same for both the bullet and the gun. The bullet has a larger velocity and a small mass, while the gun had a small velocity and a large mass. Furthermore, the bullet carries much more KE than the gun.
The driver of a car slams on her brakes to avoid colliding with a deer crossing the highway. What happened to the car's kinetic energy as it comes to rest?
The kinetic energy is converted into internal energy(heat, friction) within the brake pads of the car, the roadway, and the tires. break loses heat to the atmosphere
Suppose you are driving a car at a high speed. Why should you avoid slamming on your brakes when you want to stop in the shortest possible distance?(Newer cars have anti-lock brakes that avoid this problem.)
When a tire is rolling, the point on the tire in contact with the ground is momentarily at rest relative to the ground. Thus, static friction exists between the tire and the ground under these conditions. When the brake lock, the tires begin to skid over the ground and kinetic friction now exists between the tires and the ground. Since the kinetic friction force is less than the maximum static friction force, the friction force tending to slow the car is less with the brakes locked than while the tires continue to roll. the kinetic coefficient is smaller so you have a weaker force to stop the car once you are sliding.
In the motion picture It Happened One Night, Clark Gable is standing inside a stationary bus in front of Claudette Colbert, who is seated. The bus suddenly starts moving forward and Clark falls into Claudette's lap. Why did this happen?
When the bus starts moving, Claudette's mass is accelerated by the force exerted by the back of the seat on her body. Clark is standing, however, and the only force acting on him is the friction between his shoes and the floor of the bus. Thus, when the bus starts moving, his feet accelerate forward, but the rest of his body experiences almost no accelerating force(only that due to his being attached to his accelerating feet). As a consequence, his body tends to stay almost at rest, according to Newton's first law, relative to the ground. Relative to Claudette, however, he is moving toward her and falls into her lap.
Two carts move in the same direction along a frictionless air track, each acted on by the same constant force for a time interval t. Cart 2 has twice the mass of cart 1. Which one of the following statements is true? a) Each cart has the same change in momentum b) Cart 1 has the greater change in momentum c) Cart 2 has the greater change in momentum d) the changes in momenta depend on the initial velocities
a m2= 2 m1 = 1 mass 1 = F=ma so, a = F/m mass 2 = F=2ma so, a= F/2m vf = vi + at 1) vf = (F/m) t mv; so, vf = 1 2) vf = (F/2m) t 2mv; so, vf = 1
Two carts on air track have the same mass and speed and are traveling towards each other. If they collide and stick together, find (a) the total momentum and (b) total kinetic energy of the system. (c) Describe a different colliding system energy of the system with this same final momentum and kinetic energy.
a) 0 (1mv - 1mv = 0) b) 0 (.5mv^2 - .5mv^2 = 0) c) Answers vary, but any perfectly inelastic collision with zero initial momentum will have zero final kinetic energy
5.7 Elastic potential energy depends on the spring constant and the distance the spring is stretched or compressed. By what factor does the elastic potential energy change if the spring's stretch is (a) doubled or (b) tripled?
a) 4 b) 9
4.4 A small sports car collides head-on with a massive truck. The greater impact force (in magnitude) acts on a) the car b) the truck c) neither, the force is the same on both Which vehicle undergoes the greater magnitude acceleration? d) the car e) the truck f) the acceleration are the same
c & d
A bowling ball is suspended from the ceiling of a lecture hall by a strong cord. The ball is drawn away from its equilibrium position and released from rest at the tip of the demonstrator's nose. (a) If the demonstrator remains stationary, explain why the ball does not strike her on its return swing. (b) Would this demonstrator be safe if the ball were given a push from its starting position at her nose?
a) The ball initially has gravitational potential energy mgh, and zero kinetic energy. Because a small amount of energy is always spent overcoming air resistance and friction in the support as the ball swings, it will come to rest on the return swing at a level slightly lower than its initial position. (b) If anyone gives the ball a forward push anywhere along its path, positive work is done on the ball and it will tend to reach a level higher than its initial position before coming to rest again. In this case, the demonstrator may have to move out of the way to avoid being hit.
(a) If the height of a playground slide is kept constant, will the length of the slide or whether it has bumps make any difference in the final speed of children playing on it? Assume that the slide is slick enough to be considered frictionless. (b) Repeat part (a), assume that the slide is not frictionless
a) When the slide is frictionless, changing the length or shape of the slide will not make any difference in the final speed of the child, as long as the difference in the heights of the upper and lower ends of the slide is kept constant. (b) If friction must be considered, the path length along which the friction force does negative work will be greater when the slide is longer or given bumps. Thus, the child will arrive at the lower end with less kinetic energy(less speed).
4.1 Which of the following statements are true? a) An object can move even when no force acts on it b) if an object isn't moving, no external forces act on it c) if a single force acts on an object, the object accelerates d) if an object accelerates, at least one force is acting on it e) if an object isn't accelerating, no external force is acting on it. f) if the net force acting on an object is in the positive x direction, the object moves only in the positive x direction
a, c, d
****As a mass tied to the end of a string swings from its highest point down to its lowest point, it is acted on by three forces: gravity, tension, and air resistance. Which force does: a) positive work b) negative work c) zero work
a. gravity b. air resistance c. tension
****For each of the situations given, state whether frictional forces do positive, negative or zero work on the italicized object: a) a plate slides across the table and is brought to rest by friction b) a person pushes a chair at constant speed across a rough, horizontal surface c) a box is set down on a stationary conveyor belt. The conveyor is turned on and the box begins to move d) a person exerts a horizontal force on a banana at rest on a counter top. the banana remains at rest In most situations we have encountered in this chapter, frictional forces tend to reduce the kinetic energy of an object. However, frictional forces can sometimes increase an object's kinetic energy. Describe a few situations in which friction causes an increase in kinetic energy.
a) a negative work is being done on the plate b) positive energy is being done to the chair c) zero work d) zero work. it didn't move because the force of friction was greater If a crate is located on the bed of a truck, and the truck accelerates, the friction force extended on the crate causes it to undergo the same acceleration as the truck, assuming that the crate doesn't slip. Another example is a car that accelerates because of the frictional forces between the road surface and its tires. This force is in the direction of the motion of the car and produces an increase in the cars kinetic energy.
4.3 Respond to each statement, true or false: a) no force of gravity acts on an astronaut in an orbiting space station b) at three earth radii from the center of earth, the acceleration of gravity is one-ninth its surface value c) if two identical planets, each with surface gravity g and volume V, coalesce into one planet with volume 2V, the surface gravity of the new planet is 2g d) one kg of gold would have greater value on earth than on the moon
a) false b) true (f=ma and f=m(MG/r^2) c) false (if you double M mMG/r^2, then you get m2MG/2r^2) d) false; no the mass stays the same so it has the same value...
A baseball is thrown from the outfield toward home plate. a) True or False: Neglecting air resistance, the momentum of the baseball is conserved during its flight. b) True or False: Neglecting air resistance, the momentum of the baseball - Earth system is conserved during the baseball's flight.
a) false b) true
Suppose you are reshelving books in a library. As you lift a book from the floor and place it on the top shelf, two forces act on the book: your upward lifting force and the downward gravity force a) is the mechanical work done by your lifting force, positive, zero, or negative? b) is the work done by the gravity force positive, zero, or negative? c) is the sum of the works done by both forces positive, zero, or negative?
a) positive b) negative c) zero
4.6 A crate is sitting in the center of a flatbed truck. As the truck accelerates to the east, the crate moves with it, and doesn't slide on the bed of the truck. In what direction is the friction force exerted by the bed of the truck on the crate? a. To the west b. to the east c. there is no friction force because the crate isn't sliding
b if the truck was on a bed of ice it would move westward which means that the force of friction of the truck is moving eastward.
For the situation described in the previous question, which cart experiences the greater change in kinetic energy? a) Each cart has the same change in kinetic energy b) Cart 1 c) Cart 2 d) it is impossible to tell without knowing the initial velocities
b m1v1 = 2m2v2 cancel the mass and you get v1 = 2v2 because cart 1 has double the velocity, you will have a greater change in kinetic energy v1 = 2v2
4.2 Which has greater value, a newton of gold on earth or a newton of gold on the moon? a) the newton of gold on the earth b) the newton of hold on the moon c) the value is the same, regardless
b they have the same newtons. because the gravity constant is less on the moon, it has a greater mass to have the same N
4.7 Suppose your friend is sitting on a sled and asks you to move her across a flat, horizontal field. You have a choice of (a) pushing her from behind by applying a force downward on her shoulders at 30 degrees below the horizontal or (b) attaching a rope to the front of the sled and pulling with a force at 30 degrees above the horizontal. Which option would be easier and why?
b. pulling the sled is easier because you subtract that force from the normal force. By exerting an upward force component on the sled, you reduce the normal force on the ground and so reduce the force of kinetic friction
In figure 4.4, a locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall? a) the force exerted by the locomotive on the wall was larger than the force the wall could exert on the locomotive b) the force exerted by the locomotive on the wall was the same in magnitude as the force exerted by the wall on the locomotive c) the force exerted by the locomotive on the wall was less than the force exerted by the wall on the locomotive. d) the wall cannot be said to exert a force; after all, it broke.
b; newtons third law says it is equal and opposite but the structural integrity is what makes the wall break. But the force always pushes back equal and oppositely
At a bowling alley, two players each score a spare when their bowling balls make head-on approximately elastic collisions at the same speed with identical pins. After the collisions, the pin hit by ball A moves much more quickly than the pin hit by ball B. Which ball has more mass?
ball A
4.8 Consider the two situations shown in 4.3, in which there is no acceleration. In both cases the men pull with a force of magnitude F. Is the reading on the scale in part (i) of the figure (a) greater than, (b) less than, or (c) equal to the reading in part (ii)?
c
4.9 For the woman being pulled forward on the toboggan, is the magnitude of the normal force exerted by the ground on the toboggan (a) equal to the total weight of the woman plus the toboggan (b) greater than the total weight, (c) less than the total weight, or (d) possibly greater than or less than the total weight, depending on the size of the weight relative to the tension in the rope?
c
5.4 Bob, of mass m drops from a tree limb at the same time that Esther, also of mass m, begins her descent down a frictionless slide. If they both start at the same height above the ground, which of the following is true about their kinetic energies as they reach the ground? a) bob's kinetic energy is greater than ester b) Esther's KE is greater than bob's c) they have the same KE d) the answer is dependent on the slide
c
6.3 A car and a large truck traveling at the same speed collide head-on and stick together. Which vehicle undergoes the larger change in the magnitude of its momentum? a) the car b) the truck c) the change in magnitude of momentum is the same for both d) impossible to determine
c
A large crate of mass m is placed on the back of a truck but not tied down. As the truck accelerates forward with an acceleration, the crate remains at rest relative to the truck. What force causes the crate to accelerate forward? a) the normal force b) the force of gravity c) the force of friction between the crate and the floor of the truck d) the ma force e) none
c
If two particles have equal momenta, are their kinetic energies equal? a) yes, always b) no, never c) no, except when their masses are equal d) no, except when their speeds are the same e) yes, as long as they move along parallel lines
c
6.2 A boy standing at one end of a floating raft that is stationary relative to the shore walks to the opposite end of the raft, away from the shore. As a consequence, the raft a) remains stationary b) moves away from the shore c) moves toward the shore
c he has a forward momentum and the raft has a backward momentum
As a block slides down a frictionless incline, which of the following statements is true? a) both its speed and acceleration increase b) its speed and acceleration remain constant c) its speed increases and its acceleration remains constant d) both its speed and acceleration decrease e) its speed increases and its acceleration decreases
c because there is one NET constant force acting on the object... gravity... the acceleration is constant. BUT because there is an acceleration (although it is constant) there is a change in speed. In this case the speed is increasing.
6.7 A bowling ball onboard a space station is floating at rest relative to the station and an astronaut nudges a Ping-Pong ball toward it at speed v, initiating a perfectly elastic head-on collision. Which answer is closest to the Ping-Pong ball's speed after the collision? a) 0 b) v/2 c) v d) 2v e) 3v
c speed is going to be the same that was given in. the speed of the pingpong is going to be equal to that of the speed that it hit the bowling ball at. v
4.3 A small sports car collides head-on with a massive truck. The greater impact force (in magnitude) acts on a)the car b) the truck c) neither, the force is the same on both. Which vehicle undergoes the greater magnitude of acceleration? d) the car e) the truck f) accelerations are the same
c & d
5.1 In figure a-d, a block moves to the right in the positive x direction through the displacement x, while under the influence of a force with the same magnitude F. Which of the following is the correct order of the amount of work done by the force F, from most positive to the most negative?
c, a, d, b force and displacement going in the same direction will make positive work, force and displacement going in the opposite direction will make negative work.
A crate remains stationary after it is has been placed on a ramp inclined at an angle with the horizontal. Which of the following statements must be true about the magnitude of the frictional force that acts on the crate? a) it is larger than the weight of the crate b) it is at least equal tot he weight of the crate c) it is equal to u,n d) it is greater than the component of the gravitational force acting down on the ramp e) it is equal to the component of the gravitational force acting down on the ramp.
d
