H1 Physics 8867 · objective lesson
Momentum conservation and elastic interactions
Momentum is conserved for a closed system; kinetic energy and relative-speed conditions distinguish a perfectly elastic interaction.
Repair the common break
Common misconception: Kinetic energy is conserved in every closed-system collision.
Correct model: Momentum is conserved for a closed system; kinetic energy and relative-speed conditions distinguish a perfectly elastic interaction.
Worked transfer
For a one-dimensional elastic collision, conserve total momentum and use relative speed of separation = relative speed of approach. Conservation of momentum alone cannot establish elasticity.
Guided practice
Explain why “Kinetic energy is conserved in every closed-system collision.” fails. Then construct one example that correctly applies momentum conservation and elastic interactions.
Independent practice
A 2.0 kg trolley at 5.0 m s⁻¹ hits a stationary 3.0 kg trolley and rebounds at 1.0 m s⁻¹. Find the other velocity and test whether the collision is elastic.
Return to the recorded repair plan when the explanation and application are secure enough to re-test.