Chapter 6: Work and Energy

Answer MisConceptual Question 10 assuming a small amount of friction. refer to book for picture

(c)

13. You push a heavy crate down a ramp at a constant velocity. Only four forces act on the crate. Which force does the greatest magnitude of work on the crate? (a) The force of friction. (b) The force of gravity. (c) The normal force. (d) The force of you pushing. (e) The net force.

(a) The force of friction.

A ball is thrown straight up. Neglecting air resistance, which statement is not true regarding the energy of the ball? (a) The potential energy decreases while the ball is going up. (b) The kinetic energy decreases while the ball is going up. (c) The sum of the kinetic energy and potential energy is constant.

(a) The potential energy decreases while the ball is going up.

A car accelerates from rest to 30 kmh. Later, on a high- way it accelerates from 30 km/h to 60 km/h. Which takes more energy, going from 0 to 30, or from 30 to 60? (a) 0 to 30 km/h. (b) 30 to 60 km/h (c) Both are the same.

(b) 30 to 60 km/h

Two balls are thrown off a building with the same speed, one straight up and one at a 45° angle. Which statement is true if air resistance can be ignored? (a) Both hit the ground at the same time. (b) Both hit the ground with the same speed. (c) The one thrown at an angle hits the ground with a lower speed. (d) The one thrown at an angle hits the ground with a higher speed. (e) Both (a) and (b).

(b) Both hit the ground with the same speed

You push very hard on a heavy desk, trying to move it. You do work on the desk: (a) whether or not it moves, as long as you are exerting a force. (b) only if it starts moving. (c) only if it doesn't move. (d) never—it does work on you. (e) None of the above.

(b) only if it starts moving A common misconception is that all forces do work. However, work requires that the object on which the force is acting has a component of motion in the direction of the force.

Whenthespeedofyourcarisdoubled,bywhatfactordoes its kinetic energy increase? (a) square root of 2 (b) 2 (c) 4 (d) 8

(c) 4

A bowling ball is dropped from a height h onto the center of a trampoline, which launches the ball back up into the air. How high will the ball rise? (a) Significantly less than h. (b) More than h. The exact amount depends on the mass of the ball and the springiness of the trampoline. (c) No more than h. probably a little less. (d) Cannot tell without knowing the characteristics of the trampoline.

(c) No more than h—probably a little less.

A satellite in circular orbit around the Earth moves at constant speed. This orbit is maintained by the force of gravity between the Earth and the satellite, yet no work is done on the satellite. How is this possible? (a) No work is done if there is no contact between objects. (b) No work is done because there is no gravity in space. (c) No work is done if the direction of motion is perpendicular to the force. (d) No work is done if objects move in a circle.

(c) No work is done if the direction of motion is perpendicular to the force

A man pushes a block up an incline at a constant speed. As the block moves up the incline, (a) its kinetic energy and potential energy both increase. (b) its kinetic energy increases and its potential energy remains the same. (c) its potential energy increases and its kinetic energy remains the same. (d) its potential energy increases and its kinetic energy decreases by the same amount.

(c) its potential energy increases and its kinetic energy remains the same.

A car traveling at a velocity v can stop in a minimum distance d. What would be the car's minimum stopping distance if it were traveling at a velocity of 2v? (a) d. (b) square root of d (c) 2d (d) 4d (e) 8d.

(d) 4d

Engines, including car engines, are rated in horsepower. What is horsepower? (a) The force needed to start the engine. (b) The force needed to keep the engine running at a steady rate. (c) The energy the engine needs to obtain from gasoline or some other source. (d) The rate at which the engine can do work. (e) The amount of work the engine can perform.

(d) The rate at which the engine can do work.

A ball is thrown straight up. At what point does the ball have the most energy? Ignore air resistance. (a) At the highest point of its path. (b) When it is first thrown. (c) Just before it hits the ground. (d) When the ball is halfway to the highest point of its path. (e) Everywhere; the energy of the ball is the same at all of these points.

(e) Everywhere; the energy of the ball is the same at all of these points.

A skier starts from rest at the top of each of the hills shown in Fig. 6-34. On which hill will the skier have the highest speed at the bottom if we ignore friction: (a), (b), (c), (d), or (e) c and d equally? refer to book for picture

(e) c and d equally