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GCSE Questions: Energy Changes

Q5. The graphic shows a cyclist riding along a straight, level road at a constant speed.

Complete these sentences:

As the cyclist rides along the road, the chemical energy store in the cyclist's body decreases. The speed of the cyclist is constant when the work done by the cyclist is equal to the work done against air resistance.

[2 marks]

The graph below shows how the speed changes as the power output of the cyclist changes.

(a)

(i) Write down the equation that links power, time and work done.

Power = Work done/ time taken

P = W/t

[1 mark]

(ii) Calculate the work done by the cyclist when his power output is 200 W for 1800 seconds.

W = Pt

W = 200 x 1800 = 360,000 J

(or 360 kJ)

[3 marks]

(b) Calculate the percentage increase in speed of the cyclist when the power output changes from 200 W to 300 W.

See the graph - it is always a good idea to mark your working on the graph.

The change in velocity is from 9.5 m/s to 11 m/s - an increase of 1.5 m/s

Percentage increase = increase/original value x 100%

= 1.5/9.5 x 100%

= 16 %

[2 marks]

(c) The maximum speed this cyclist can travel on a level road is 14 m/s. How does cycling uphill affect the maximum speed of this cyclist? Explain your answer.

When cycling uphill the maximum speed will be lower because the maximum power output of the cyclist is constant but when going uphill additional work is done (against gravity) (or you could say the gravitational potential energy (of the cyclist) is increased).

[3 marks]

[Total 11 marks]