Work and Power
4 marks · EasyA cyclist with mass is riding at a speed of . The cyclist and bicycle come to rest after applying the brakes, stopping over a distance of .
Calculate the initial kinetic energy of the cyclist and bicycle.
State what happens to this kinetic energy as the cyclist comes to rest.
Calculate the average braking force exerted on the cyclist and bicycle to bring them to rest.
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) 3500 J
(b) It is transferred to heat (and sound) due to friction/brakes.
(c) 175 N
Common mistakes: Forgets to square the velocity; Wrong direction for force
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses KE = 1/2 mv^2.
A1 (a) · 1 mark · KE = 0.5 × 70 × (10)^2 = 3500 J.
B1 (b) · 1 mark · States energy is transferred to heat (allow: heat and sound, friction).
A2 (c) · 1 mark · Ave force = KE / distance = 3500 J ÷ 20 m = 175 N. (follow-through)
Worked solution
a) KE = 1/2 × m × v^2 = 0.5 × 70 × (10)^2 = 0.5 × 70 × 100 = 3500 J.
b) The kinetic energy is converted to heat by the brakes (and some sound).
c) Work done by brakes = loss in KE = force × distance; so force = KE / distance = 3500 J ÷ 20 m = 175 N.