Momentum Questions - Solution
Q1. The diagram below represents part of an experiment that is being used to estimate the speed of an air gun pellet. The pellet, which is moving parallel to the track, strikes the block, embedding itself. The trolley and the block then move along the track, rising a vertical height, h .
(a) Using energy considerations explain how the speed of the trolley and block immediately after it has been struck by the pellet, may be determined from measurements of h . Assume frictional forces are negligible.
[3 marks]
The kinetic energy of the pellet changes to potential energy of the block, trolley and pellet 
You can find the potential energy gained by measuring h and using E p = mg D h
You then equate initial kinetic energy to potential energy to find speed
[OR use Newton ’s equations of motion – the acceleration is uniform ( g sin θ is a)
s = h and v=0 (1 mark) so you can use v 2 = u 2 + 2 as to find u]
[or use h as sand time how long it takes to travel s
and calculate the average velocity
v = 2 x average velocity]
(b) The following data is collected from the experiment:
mass of trolley and block = 0.50 kg
mass of pellet = 0.0020 kg
speed of trolley and block immediately after impact = 0.40ms-1
Calculate
(i) the momentum of the trolley and block immediately after impact,
p = mv = 0.50 × 0.40 = 0.20 (1 mark) N s (or kg m s − 1 )
(ii) the speed of the pellet just before impact.
m p v p = m t v t
0.0020 v = 0.20
v = 100 ms-1
[4 marks]
(c)
(i) State what is meant by an inelastic collision.
In an inelastic collision kinetic energy is not conserved
(ii) Use the data from part (b) to show that the collision between the pellet and block is inelastic.
initial kinetic energy = ½ × 0.002 × 100 2 = 10 J
final kinetic energy = ½ × 0.5 × 0.4 2 = 0.040 J
hence there is a change in kinetic energy
[4 marks]
(Total 11 marks)
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