EASA/GCAA Part-66 · Module 2 — Physics · Mechanics

Momentum & Collisions

Objective: in every collision the total momentum (mass × velocity) is conserved. Kinetic energy is conserved only if the collision is elastic — in an inelastic collision some becomes heat and deformation. Set the masses and speeds and watch both totals before and after.

Two carts on a frictionless track

Mass 1 (blue) 2.0 kg

Mass 2 (amber) 2.0 kg

Velocity 1 +3.0 m/s

Velocity 2 -2.0 m/s

Momentum before

0kg·m/s

Momentum after

0kg·m/s

KE before

KE after

Momentum: p = m·v, always conserved · KE = ½mv², conserved only if elastic

Momentum is conserved in every collision, because the forces the carts exert on each other are equal and opposite (Newton's third law), so the total momentum can't change. Kinetic energy is different: in an elastic collision it is conserved (the carts bounce cleanly), but in an inelastic collision the carts deform or stick and some kinetic energy turns into heat, sound and permanent deformation — so KE after is less than before, while momentum still balances exactly.

Try this: Equal masses, elastic, one moving — they swap velocities (Newton's cradle).
Try this: Switch to inelastic — the carts stick and move together; watch KE after drop while momentum is unchanged.
Try this: Make cart 1 very heavy and cart 2 light — the heavy one barely slows.
Try this: Send them at each other with equal and opposite momentum — after an inelastic hit they stop dead.