Brainlag PlusMomentum is always conserved, kinetic energy only sometimes. Compare kinetic energy before and after to tell an elastic collision from an inelastic one, then analyse trolley and light gate data.
Elastic or inelastic?
Momentum is conserved in every collision.
Kinetic energy is not always:
- Elastic: the total kinetic energy after equals the total before.
- Inelastic: some kinetic energy is transferred to other stores (the thermal stores of the objects and surroundings, and away as sound). Objects that stick together always lose kinetic energy.
kinetic energy = 0.5 × mass × (speed)²
Ek = ½ m v²
AQA, Edexcel and OCR: recall this one.
- A 2.0 kg trolley at 0.6 m/s hits a stationary 1.0 kg trolley and sticks
- v = (2.0 × 0.6) ÷ 3.0 = 0.4 m/s
- KE before = ½ × 2.0 × 0.6² = 0.36 J
- KE after = ½ × 3.0 × 0.4² = 0.24 J
- 0.12 J transferred to other stores, so the collision is inelastic
Add up the kinetic energy of every object, before and after. Velocities are squared, so a minus sign makes no difference to kinetic energy.
The rest of this lesson and its 7 practice questions are in Plus.
Screen 1 of 4, free to read.
- 7 exam-style questions, marked as you go, each with a worked solution
- The Momentum and collisions simulation beside every question
- Anything you get wrong is saved to your flashcards