Topic 6: Further Mechanics — Edexcel A-Level Physics
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Topic 6: Further Mechanics explained
A collision is elastic when total kinetic energy is conserved as well as total momentum; it is inelastic when kinetic energy is lost to other stores such as internal energy, sound or deformation.
Read the full explanation
To decide, calculate the total kinetic energy before and after using ½mv² for each body, then compare. Momentum is conserved in both types, so momentum alone cannot classify a collision. Example: a 2.0 kg trolley at 3.0 m/s hits a stationary 1.0 kg trolley and they move together at 2.0 m/s. Before: ½ × 2.0 × 3.0² = 9.0 J. After: ½ × 3.0 × 2.0² = 6.0 J. Kinetic energy falls from 9.0 J to 6.0 J, so the collision is inelastic. If the totals were equal, it would be elastic.
Your focus
- Distinguish elastic from inelastic collisions using kinetic energy conservation.
- Calculate total kinetic energy before and after a collision.
- Explain why momentum conservation alone cannot classify a collision.
Topic 6: Further Mechanics exam tips
Marking Points
- State that elastic collisions conserve total kinetic energy as well as momentum.
- State that inelastic collisions conserve momentum but total kinetic energy decreases.
- Calculate kinetic energy of each body before and after using ½mv² and compare totals.
- Recognise that momentum conservation holds in both elastic and inelastic collisions, so it cannot distinguish them.
- Identify that lost kinetic energy is transferred to internal energy, sound or deformation.
- Apply the test correctly to a numerical example, showing before and after totals.
Examiner Tips
- 💡Always show before and after kinetic energy totals with units in joules.
- 💡State explicitly that momentum is conserved in both cases to justify why you used energy.
- 💡If bodies stick together, treat them as a single combined mass with the common velocity.
Common Mistakes
- Assuming momentum conservation proves a collision is elastic; correction: momentum is conserved in both types, so compare kinetic energy totals instead.
- Comparing speeds rather than kinetic energies; correction: kinetic energy depends on v², so compute ½mv² for each body.
- Forgetting to include every body after the collision, such as a joined pair treated as one mass; correction: sum kinetic energy over all moving masses.
- Treating a small kinetic energy loss as zero; correction: any decrease means inelastic, only exact equality means elastic.