Momentum 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.
What you will learn
- Calculate the kinetic energy transferred in a collision
- Decide whether a collision is elastic
- Analyse light gate data to test conservation of momentum
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
- as a percentage of the kinetic energy before: 0.12 ÷ 0.36 × 100 = 33%
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 locked.
In this lesson
- Elastic or inelastic?
- Explosions make kinetic energy
- Trolleys and light gates
- Try it: Elastic or sticky in the simulation
7 practice questions, marked as you go; about 25 minutes in all.
In this unit: Momentum
Momentum as mass times velocity, its conservation in collisions and explosions, force as the rate of change of momentum, safety features and kinetic energy in collisions (Higher tier).
- Momentum, p = mv
- Conservation of momentum in collisions
- Explosions and recoil
- Force and the rate of change of momentum
- Collisions and kinetic energy