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IGCSE Physics · guided topic map

Forces and momentum for Cambridge IGCSE Physics

Forces and momentum for IGCSE Physics, organized into 1 syllabus topic and 1 mapped concept guide.

Syllabus topics
1
Mapped concept guides
1
Educational level
Cambridge IGCSE Core and Extended

Syllabus to lesson

Choose the exact concept

Work in order or jump to the concept named in your specification, course outline, or assignment.

1.6

Momentum

Motion, forces and energy

1 guide
  1. 01MomentumMapped lesson

Diagrams

Forces and momentum as IGCSE Physics draws it

The figures from the IGCSE Physics practice papers that sit on these syllabus points — the apparatus, circuits and graphs an exam question actually puts in front of you.

01Fig. 8.1Momentum · EnergyIGCSE
Two trolleys on a level track, before the collision and after itbefore the collisionAB2.5 m/sat rest0.80 kg1.2 kgafter the collisionABvjoined together

Figure comment

Fig. 8.1Two panels, one above the other. In the upper panel, labelled before the collision, trolley A of mass 0.80 kg stands on a level track with an arrow showing it moving to the right at 2.5 m/s towards trolley B of mass 1.2 kg, which is at rest. In the lower panel, labelled after the collision, the two trolleys are drawn joined together and moving to the right with a speed marked v.

Read the comment once, then trace every arrow, label, axis or component in the drawing before opening the questions.

1.6 Momentum1.7.1 EnergyOpen IGCSE question · Motion, forces and energy Q8
Guided questions 5 parts

Reading cue. In the upper panel the 2.5 m/s belongs to A alone, B at rest carrying none; in the lower panel v is the speed of the joined 2.0 kg, not of either trolley on its own.

  1. aState State the momentum of trolley B before the collision, and give the reason from the upper panel of Fig. 8.1.

    recall2 marks

    Check answer 2 marks
    1. zero (0 kg m/s) (1)
    2. B is at rest, so its velocity is zero, and momentum is mass x velocity (1)

  2. bDetermine After the collision the joined trolleys move to the right at 1.0 m/s. Determine the change in momentum of trolley A, and give its direction.

    routine3 marks

    Check answer 3 marks
    1. momentum of A before = 0.80 × 2.5 = 2.0 kg m/s to the right (1)
    2. momentum of A after = 0.80 × 1.0 = 0.80 kg m/s to the right (1)
    3. change = 1.2 kg m/s, directed to the left, that is opposite to A's motion (1)

  3. cCalculate The two trolleys are in contact for 0.15 s. Calculate the average force that trolley B exerts on trolley A during the collision.

    demanding3 marks

    Check answer 3 marks
    1. force = change in momentum / time taken (1)
    2. = 1.2 / 0.15 (1)
    3. = 8.0 N, acting to the left on A (1)

  4. dExplain Using both panels of Fig. 8.1, explain why the momentum gained by trolley B is exactly equal to the momentum lost by trolley A, even though B is the heavier trolley and its velocity changes by less.

    top of the paper4 marks

    Check answer 4 marks
    1. the force B exerts on A is equal in size and opposite in direction to the force A exerts on B (1)
    2. the two trolleys are in contact for the same length of time, so force x time is the same for both and the changes in momentum are equal and opposite (1)
    3. A's velocity changes by 1.5 m/s, giving 0.80 × 1.5 = 1.2 kg m/s; B's changes by 1.0 m/s, giving 1.2 × 1.0 = 1.2 kg m/s (1)
    4. B's larger mass is offset exactly by its smaller change in velocity, so the total momentum of the two trolleys is unchanged (1)

Transfer challenge

A skater of mass 50 kg stands at rest on ice holding a ball of mass 2.0 kg. She throws the ball horizontally away from her at 6.0 m/s. Determine the speed at which she moves backwards, and state the total momentum of the skater and ball after the throw.

Check answer 4 marks
  1. momentum of ball after the throw = 2.0 × 6.0 = 12 kg m/s (1)
  2. the skater must carry 12 kg m/s in the opposite direction, since the total was zero before (1)
  3. speed = 12/50 = 0.24 m/s (1)
  4. total momentum after the throw = zero, the same as before (1)