A ball of clay travelling horizontally at 4 m/s collides with an identical stationary ball of clay. The two balls stick together and move off as one combined mass. Explain, using the principle of conservation of momentum, why the combined mass moves off at a lower speed than 4 m/s.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a physics investigation, two identical balls of clay, each of mass 0.5 kg, are used. One ball is moving at 4 m/s and the other is stationary. They collide and stick together.
Model answer (4 marks)
The total momentum before the collision equals the total momentum after the collision (conservation of momentum).
Before the collision only one ball has momentum: 0.5 kg × 4 m s⁻¹ = 2 kg m s⁻¹. The stationary ball has zero momentum, so the total initial momentum is 2 kg m s⁻¹.
After the collision the two balls stick together, giving a combined mass of 1.0 kg. The total momentum is still 2 kg m s⁻¹.
Because momentum = mass × velocity, the velocity after the collision is 2 kg m s⁻¹ ÷ 1.0 kg = 2 m s⁻¹, which is lower than the original 4 m s⁻¹.
Thus the combined mass moves off at a lower speed because the same momentum is now carried by a larger mass, requiring a smaller velocity.
Before the collision only one ball has momentum: 0.5 kg × 4 m s⁻¹ = 2 kg m s⁻¹. The stationary ball has zero momentum, so the total initial momentum is 2 kg m s⁻¹.
After the collision the two balls stick together, giving a combined mass of 1.0 kg. The total momentum is still 2 kg m s⁻¹.
Because momentum = mass × velocity, the velocity after the collision is 2 kg m s⁻¹ ÷ 1.0 kg = 2 m s⁻¹, which is lower than the original 4 m s⁻¹.
Thus the combined mass moves off at a lower speed because the same momentum is now carried by a larger mass, requiring a smaller velocity.
Examiner tips
- State conservation of momentum first; calculate initial momentum; note the stationary ball’s momentum is zero; calculate final velocity using p = mv; explain why velocity decreases when mass increases.
Common mistakes
- Using the wrong mass (e.g., 0.5 kg instead of 1.0 kg) for the final calculation; forgetting that the stationary ball contributes zero momentum; not showing the division step to find the final velocity.
Mark scheme (4 marks)
- The total momentum before the collision equals the total momentum after the collision (conservation of momentum).
- Before the collision, only one ball has momentum; the stationary ball has zero momentum, so total momentum = mass × 4 m/s.
- After the collision, the combined (larger) mass carries the same total momentum.
- Because momentum = mass × velocity, if mass increases and momentum stays the same, the velocity must decrease / the combined mass moves more slowly.
Key terms in this question
momentum · conservation of momentum · mass
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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