Kekekalan Energi 2

One person drops a ball from the top of a building

while another person at the bottom observes its

motion. Will these two people agree (a) on the value

of the gravitational potential energy of the ball–

Earth system? (b) On the change in potential energy?

(c) On the kinetic energy of the ball at some point in

its motion?

Does everything have energy? Give the reasoning for

your answer.

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 as shown in Figure CQ8.5. The demonstrator remains stationary. (a) 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?

Can a force of static friction do work? If not, why not? If so, give an example.

By what transfer mechanisms does energy enter and leave (a) your

television set? (b) Your gasoline-powered lawn mower? (c) Your hand-cranked

pencil sharpener?

Consider a block sliding over a horizontal surface with friction.

Ignore any sound the sliding might make. (i) If the system is the block, this system is (a) isolated (b) nonisolated (c) impossible to determine. (ii) If the system

is the surface, describe the system from the same set of choices. (iii) If the system

is the block and the surface, describe the system from the same set of choices.

A ball is thrown straight up into the air. At what position

is its kinetic energy a maximum? At what position is the

gravitational potential energy a maximum?

When a person runs in a track event at constant velocity,

is any work done? (Note: Although the runner moves with

constant velocity, the legs and arms accelerate.) How does

air resistance enter into the picture? Does the center of

mass of the runner move horizontally?

Consider a ball fixed to one end of a rigid rod whose

other end pivots on a horizontal axis so that the rod can

rotate in a vertical plane. What are the positions of stable

and unstable equilibrium?