- Define and explain Elastic and Inelastic Collisions in your own words
- Use key terms such as momentum accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Elastic and Inelastic Collisions: applying conservation of momentum to elastic and inelastic collisions.
Applying conservation of momentum to elastic and inelastic collisions.
Key ideas
Momentum is conserved in every collision
Total momentum before an interaction equals total momentum after, provided no external impulse acts. In elastic collisions kinetic energy is conserved too; in inelastic collisions some becomes heat and sound. Crumple zones lengthen impact time, cutting the peak force via F = Δp/Δt — the physics that saves lives.
Circular motion needs a constant centre-seeking force
An object circling at steady speed is accelerating because its velocity direction keeps changing. The centripetal acceleration is v²/r towards the centre, so the required force is F = mv²/r. Remove the force — cut the string — and the object flies off along a tangent, which is why mud flings off a spinning wheel.
Key term — momentum: Mass × velocity (p = mv), a vector quantity measured in kg m/s.
Why do airbags reduce injury in a crash?
They increase the time over which momentum changes, so the average force on the passenger is smaller.
Answer: They increase the time over which momentum changes, so the average force on the passenger is smaller.
- Treating momentum as a scalar Momentum has direction; opposite momenta subtract, which is why head-on collisions need a sign convention.
- Inventing an outward 'centrifugal force' In an inertial frame there is only the inward centripetal force — the outward feeling is just inertia resisting the turn.
Practice
(1200 × 400) ÷ 50 = 480,000 ÷ 50 = 9600 N.
√(0.2 ÷ 80) = √0.0025 = 0.05, so T = 2π × 0.05 ≈ 0.31 s.
Δp = 0.5 × 6 = 3 kg m/s; F = 3 ÷ 0.2 = 15 N.
Before: 2 × 3 = 6 kg m/s. After: 3 kg × v = 6, so v = 2 m/s.
Quick check
Which of these best defines "momentum"?
In SHM, where are speed and acceleration greatest?
- Elastic and Inelastic Collisions: applying conservation of momentum to elastic and inelastic collisions.
- Momentum is conserved in every collision: Total momentum before an interaction equals total momentum after, provided no external impulse acts.
- impulse: Force × time (FΔt), equal to the change in momentum.
- Watch out for: treating momentum as a scalar