The diagram shows a bouncing rubber ball filmed with a high-speed camera. The ball is released above a tiled floor and rebounds to a lower height. At the in...
Question 1 Report
The diagram shows a bouncing rubber ball filmed with a high-speed camera. The ball is released above a tiled floor and rebounds to a lower height. At the instant it touches the floor, the ball is compressed. The floor is hard and does not visibly bend. A microphone records a short sound when the collision occurs. The ball and floor become slightly warmer after many bounces.
(a) Describe the main energy transfer as the ball falls before it reaches the floor. [2]
(b) Name one energy store that increases while the ball is compressed. [1]
(c) Explain why the rebound height is lower than the release height. [2]
The table shows results from a technician comparing two phone chargers. Each charger is connected to the same battery pack for 20 minutes. The useful output is the increase in the chemical energy store of the battery. The remainder of the electrical energy warms the charger, cable and surrounding air. Charger B has a thicker cable.
| charger | electrical energy supplied / kJ | energy stored in battery / kJ |
|---|---|---|
| A | 72 | 54 |
| B | 72 | 63 |
(a) Calculate the efficiency of charger B. [2]
(b) Give the energy dissipated by charger A. [1]
(c) Explain how a thicker cable can help make charger B more efficient. [2]
Answer Details
Bouncing ball
- (a) As the ball falls, its gravitational potential energy store decreases and its kinetic energy store increases. [2]
- (b) While compressed, its elastic potential energy store increases. [1]
- (c) During impact, some energy is transferred to thermal energy stores and as sound. Less energy is therefore available to return to gravitational potential energy, so the rebound height is lower. [2]
Phone chargers
- (a) \[\text{efficiency}=\frac{63\text{ kJ}}{72\text{ kJ}}=0.875=87.5\%\] [2]
- (b) \[72\text{ kJ}-54\text{ kJ}=18\text{ kJ}\] [1]
- (c) A thicker cable has lower resistance. Less energy is then transferred thermally to the cable, increasing the fraction reaching the battery. [2]
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