Power
3 marks · NormalA cyclist rides up a hill at a constant speed. Explain, using ideas about energy stores and transfers, why the cyclist must keep pedalling to maintain this speed.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
The cyclist must keep pedalling to provide energy to overcome resistive forces (like friction and air resistance) and to increase gravitational potential energy as they rise. If no energy is provided, friction will slow the bike and GPE will stop increasing, causing the cyclist to lose speed.
Also accept: Energy from pedalling is transferred to increase GPE and overcome resistive forces, so pedalling is needed to keep the speed constant
Common mistakes: Ignoring friction; Ignoring GPE increase
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Mentions energy transfer to GPE in going up the hill
B2 · 1 mark · Mentions work done against resistive forces (friction/air resistance)
B3 · 1 mark · Explains the need to keep supplying energy to maintain constant speed
Worked solution
As the cyclist goes up the hill, their kinetic energy stays the same (constant speed), but their gravitational potential energy increases.
To increase GPE and overcome resistive forces (like friction and air resistance), the cyclist must keep providing energy by pedalling.
Without pedalling, these forces would slow the cyclist and stop them climbing the hill.