The table below shows measurements from a student investigating the useful power of a stair-climbing exercise. Each student climbs the same vertical distanc...
Question 1 Report
The table below shows measurements from a student investigating the useful power of a stair-climbing exercise. Each student climbs the same vertical distance of 3.0 m. The students have different mass and take different time. Take g as 10 N/kg. The teacher reminds the class that the useful energy output is the increase in gravitational potential energy, although some chemical energy from the body is also transferred by heating and sound.
| Student | Mass / kg | Time / s |
|---|---|---|
| Jai | 50 | 6.0 |
| Mina | 60 | 9.0 |
| Omar | 75 | 10.0 |
(a) What is Jai's increase in gravitational potential energy? Show your working. [2]
(b) Which student has the greatest useful power? [1]
(c) When a student breathes heavily after climbing, what energy transfer to the surroundings has increased? [1]
The table below shows data from a harbour authority testing a floating wave-energy generator. The waves push a buoy up and down. This motion drives a generator, which transfers electrical energy to a battery on the shore. The authority records the energy in the moving water and the electrical energy stored in the battery during equal 10-minute periods. The device also makes sound and its moving joints become warm.
| Test | Energy from waves / kJ | Electrical energy stored / kJ |
|---|---|---|
| 1 | 120 | 42 |
| 2 | 180 | 72 |
| 3 | 240 | 84 |
(a) What is the efficiency in test 2? Show your working. [2]
(b) Which test has the greatest electrical energy output? [1]
(c) When the joints become warm, what has happened to some of the wave energy? [1]
(d) Sketch an energy pathway from the moving buoy to the battery. [1]
Answer Details
Stair climbing
(a) Use \(\Delta E_p=mgh\):
\[50\times10\times3.0=1500\text{ J}\] [2]
(b) Jai has the greatest useful power:
\[P_{Jai}=\frac{1500}{6.0}=250\text{ W}\]
Mina's power is \(\frac{60\times10\times3.0}{9.0}=200\text{ W}\), and Omar's is \(\frac{75\times10\times3.0}{10.0}=225\text{ W}\). [1]
(c) Energy transfer by heating to the surroundings has increased. [1]
Wave-energy generator
(a) \[\text{efficiency}=\frac{\text{useful output}}{\text{input}}=\frac{72}{180}=0.40=40\%\] [2]
(b) Test 3 has the greatest electrical output, \(84\text{ kJ}\). [1]
(c) Some wave energy has been dissipated by heating due to friction in the warm joints. [1]
(d)
[1]
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