FORCES AND SHAPE
3 marks · NormalA cyclist is moving at a steady speed along a straight road.
Name the two horizontal forces acting on the cyclist.
Explain, using ideas about balanced forces, why the cyclist moves at a constant 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
(a) The forward (driving) force from the cyclist, and the resistive force (friction/air resistance).
(b) The cyclist moves at constant speed because the forward force is balanced exactly by the resistive force—the resultant force is zero, so there is no acceleration.
Also accept: Driving force and resistive forces
Common mistakes: Saying gravity or normal force instead of only horizontal forces.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States the two horizontal forces: forward/driving force and friction/resistive force.
B2 (b) · 1 mark · Explains that the forces are balanced so there is no resultant force.
B3 (b) · 1 mark · States that this results in constant speed (no acceleration).
Worked solution
a) The two horizontal forces are: the forward force from pedalling (driving force) and the resistive force (friction and/or air resistance). b) The cyclist moves at a constant speed because the forward and resistive forces are equal in size but act in opposite directions. This means the resultant force is zero, so there is no change in motion (no acceleration).