Power
5 marks · HardA 0.25 kg ball is dropped from a balcony 25 m above the ground. Ignore air resistance.
Calculate the gravitational potential energy lost by the ball as it falls.
Determine the speed of the ball just before it hits the ground.
Explain why, in real life, the ball’s speed might be less than your calculation.
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) 62.5 J
(b) 22.4 m/s
(c) Air resistance does work on the ball, converting some energy to heat, so not all GPE is converted to KE.
Common mistakes: Not using correct significant figures; Leaving answer in wrong units; Forgetting to take square root when finding speed
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses GPE = mgh with correct values.
A1 (a) · 1 mark · Calculates GPE lost: 62.5 J.
M2 (b) · 1 mark · Sets KE = GPE lost, uses . (follow-through)
A2 (b) · 1 mark · Correct final speed: 22.4 m/s. (follow-through)
B1 (c) · 1 mark · States that air resistance means not all GPE is transferred to KE (energy dissipated).
Worked solution
(a) GPE lost = mgh
(b) Assuming all GPE is converted to KE just before hitting the ground:
(c) In real life, the ball will experience air resistance which transfers some energy to the surroundings as heat, so not all the lost GPE is converted into kinetic energy.