MOMENTUM
4 marks · NormalA 0.5 kg football is kicked so that it moves with a velocity of 8 m/s towards a goal.
Calculate the momentum of the football.
If the football comes to rest in 0.4 s when caught by the goalkeeper, calculate the average force applied by the goalkeeper to stop it.
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) 4 kg m/s
(b) -10 N
Common mistakes: Not using correct formulae; Using time in wrong place
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Using p = m × v and substituting correct values.
A1 (a) · 1 mark · Final answer = 0.5 × 8 = 4 kg m/s with unit.
M2 (b) · 1 mark · Finds change in momentum (final - initial), uses F = change in p / t formula. (follow-through)
A2 (b) · 1 mark · Correct answer, including negative sign (-10 N), with correct unit. (follow-through)
Worked solution
a) p = m × v
b) Change in momentum = final - initial = 0 - 4 = -4 kg m/s
F = (change in p) / t = (-4) / 0.4 = -10 N
Negative sign shows force acts in opposite direction to motion.