IB Physics SLGOHS

5.3 – 5.4 Worksheet: Potential Energy and Conservation of Energy

Conceptual Questions

8. 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 Q5.8. If the demonstrator remains stationary, explain why the ball does not strike her on its return swing. Would this demonstrator be safe if the ball were given a push from its starting position at her nose?

12. As a sled moves across a flat, snow-covered field at constant velocity, is any work done? How does air resistanceenter into the picture?

14. The driver of a car slams on her brakes to avoid colliding with a deer crossing the highway. What happens to thecar’s kinetic energy as it comes to rest?

Problems

MORE ON BACK!!!

19. Find the height from which you would have to drop a ball so that it would have a speed of 9.0 m/s just before it hitsthe ground.

21. An athlete on a trampoline leaps straight up into the airwith an initial speed of 9.0 m/s. Find (a) the maximumheight reached by the athlete relative to the trampolineand (b) the speed of the athlete when she is halfway up toher maximum height.

ANSWERS

Concept Questions

8. The total energy of the bowling ball is conserved. Because the ball initially has gravitational potential energy mgh and no kinetic energy, it will again have zero kinetic energy when it returns to its original position. Air resistance and friction at the support will cause the ball to come back to a point slightly below its initial position. On the other hand, if anyone gives a forward push to the ball anywhere along its path, the demonstrator will have to duck.

12.Both the force of kinetic friction exerted on the sled by the snow and the resistance force exerted on the moving sled by the air will do negative work on the sled. Since the sled is maintaining constant velocity, some towing agent must do an equal amount of positive work, so the net work done on the sled is zero.

14.The kinetic energy is converted to internal energy within the brake pads of the car, the roadway, and the tires.

Problems

5.19With only a conservative force acting on the falling ball,
or
Applying this to the motion of the ball gives
or

5.21Once the athlete leaves the trampoline, only the conservative gravitational force acts on her. Thus, with if we choose the level of the trampoline to be the reference level for gravitational potential energy.

(a)At, and conservation of mechanical energy yields

(b)When during the upward trip, conservation of mechanical energy gives
,or