PHY 121: RQ 06 - 09

A box of mass m is pressed against (but is not attached to) an ideal spring of force constant k and negligible mass, compressing the spring a distance x. After it is released, the box slides up a frictionless incline as shown in the figure and eventually stops. If we repeat this experiment with a box of mass 2m

the lighter box will go twice as high up the incline as the heavier box.

Two identical cars are driving in opposite directions at the same speed. Their kinetic energies have ____.

the same magnitude and sign

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

Which, if any, of the following statements concerning the work done by a conservative force is NOT true? It is independent of the path of the body and depends only on the starting and ending points. 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. All of the above statements are true. None of the above statements are true.

All of the above statements are true.

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.

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.

Earth's orbit around the Sun is slightly elliptical. Thus, Earth actually gets closer to the Sun during part of the year. What happens to Earth's orbital speed when it is closer to the Sun?

Earth's orbital speed is greater when it is closer to the Sun than when it is farther from the Sun.

You are standing in a moving bus, facing forward, and you suddenly fall forward as the bus comes to an immediate stop. The force acting on you that causes you to fall forward is

No forces were acting on you to cause you to fall.

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

Person B does twice the work of person A, and in one-half of the time . How does the power output of person B compare to person A?

Person B has four times the power output of person A

Two planets having equal masses are in circular orbit around a star. Planet A has a smaller orbital radius than planet B. Which statement is true?

Planet A has more kinetic energy, less potential energy, and less mechanical energy (potential plus kinetic) than planet B.

Two satellites of equal mass are in circular orbit around Earth. Satellite A has a smaller orbital radius than Satellite B. Which statement is true? Satellite A has less kinetic energy than Satellite B. Satellite A has more kinetic energy than Satellite B. Satellite A has less potential energy than Satellite B. Satellite A has more potential energy than Satellite B

Satellite A has more kinetic energy than Satellite B. Satellite A has less potential energy than Satellite B.

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 hanging by a string from the ceiling of an elevator. The elevator is slowing down while moving upward. What is the magnitude of the tension in the string?

The magnitude of the tension in the string is less than the magnitude of the weight of the object.

Two identical balls are thrown vertically upward. The second ball is thrown with an initial speed that is twice that of the first ball. How does the maximum height of the two balls compare?

The maximum height of the second ball is four times that of the first ball

An object moves in a circular path at a constant speed. What is the direction of the net force acting on the object?

The net force is directed toward the center of the circular path.

A person applies a 50 N force on a crate, causing it to move horizontally at a constant speed through a distance of 10 m. What is the net work done on the crate?

The net work done on the crate is zero joules

If the net work done on an object is positive, what can you conclude about the object's motion?

The object is speeding up.

If the net work done on an object is zero, what can you determine about the object's kinetic energy?

The object's kinetic energy remains the same.

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.

Two objects are moving at equal speed along a level, frictionless surface. The second object has twice the mass of the first object. They both slide up the same frictionless incline plane. Which object rises to a greater height?

The two objects rise to the same height.

Which of the following statements is/are true? Check all that apply. A neoconservative force permits a two-way conversion between kinetic and potential energies. The work done by a nonconservative force depends on the path taken. A potential energy function can be specified for a nonconservative force. The work done by a conservative force depends on the path taken. A potential energy function can be specified for a conservative force. A conservative force permits a two-way conversion between kinetic and potential energies.

The work done by a nonconservative force depends on the path taken. A potential energy function can be specified for a conservative force. A conservative force permits a two-way conversion between kinetic and potential energies.

An object weighing 20 N moves horizontally toward the right a distance of 5.0 m. What is the work done on the object by the force of gravity?

The work done on the object by the force of gravity is zero joules.

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

True

A ball is thrown straight upward. How does the sign of the work done by gravity while the ball is traveling upward compare with the sign of the work done by gravity while the ball is traveling downward?

Work done by gravity is negative while the ball is traveling upward and positive while the ball is traveling downward.

Consider a ball dropped from the top of a 50 m building. Is it possible for the ball-Earth system to have negative potential energy?

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

Suppose a highway curve is properly banked to eliminate friction for a speed of 45 mph. If your tires were bald and you wanted to avoid sliding on the road, you would have to drive

at exactly 45 mph.

A crate is sliding down an inclined ramp at a constant speed of 0.55 m/s. The vector sum of all the forces acting on this crate must point down the ramp.

false

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.

false

A box of mass m is pressed against (but is not attached to) an ideal spring of force constant k and negligible mass, compressing the spring a distance x. After it is released, the box slides up a frictionless incline as shown in the figure and eventually stops. If we repeat this experiment but instead compress the spring a distance of 2x

just as it moves free of the spring, the box will be traveling twice as fast as before.