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IGCSE · 1 · structured

Motion, forces and energy · Question 8

Motion, forces and energy · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
discriminating
Marks
9
Topics
2
Answer
Complete
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
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.
structured9 marks

Trolley A has a mass of 0.80 kg and moves along a straight, level track at 2.5 m/s. It collides with trolley B, of mass 1.2 kg, which is stationary. After the collision the two trolleys move off joined together. Friction is negligible.

  1. (a)

    State State the principle of conservation of momentum.

    2 marks
  2. (b)

    Calculate Calculate the speed of the joined trolleys immediately after the collision.

    3 marks
  3. (c)

    Show (that) Show that kinetic energy is not conserved in this collision, and determine how much kinetic energy is transferred to other stores.

    3 marks
  4. (d)

    Suggest Suggest what happens to the kinetic energy that is not accounted for after the collision.

    1 mark
Ready to self-mark?Reveal the detailed answer guide
Answer overviewKey answer: the total momentum of a system (of interacting bodies) before an event equals the total momentum after it provided no external resultant force acts momentum before = 0.80 × 2.5 = 2.0 kg m/s
01

(a)

2 marks

State State the principle of conservation of momentum.

How to approach it

Answer the command word directly and use precise physical vocabulary. Include only the distinct features or facts that earn marks, without burying them in unrelated background information.

  1. 1

    the total momentum of a system (of interacting bodies) before an event equals the total momentum after it

  2. 2

    provided no external resultant force acts

02

(b)

3 marks

Calculate Calculate the speed of the joined trolleys immediately after the collision.

How to approach it

List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.

  1. 1

    momentum before = 0.80 × 2.5 = 2.0 kg m/s

  2. 2

    total mass after = 0.80 + 1.2 = 2.0 kg

  3. 3

    v = 2.0 / 2.0 = 1.0 m/s

03

(c)

3 marks

Show (that) Show that kinetic energy is not conserved in this collision, and determine how much kinetic energy is transferred to other stores.

How to approach it

List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.

  1. 1

    E_k before = ½ × 0.80 × 2.5² = 2.5 J

  2. 2

    E_k after = ½ × 2.0 × 1.0² = 1.0 J

  3. 3

    1.5 J transferred away, so kinetic energy is not conserved

04

(d)

1 mark

Suggest Suggest what happens to the kinetic energy that is not accounted for after the collision.

How to approach it

Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.

  1. 1

    transferred to the internal (thermal) energy store of the trolleys and surroundings, and/or emitted as sound

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