Elastic vs Inelastic
Crash carts, compare energy loss. Builds on Conservation of Momentum.
Two identical clay balls moving toward each other at the same speed collide head-on and stick together.
What happens after they collide?
They stop dead! Equal masses, equal speeds, opposite directions: total momentum = mv + (−mv) = 0. After sticking together, they must have zero momentum → zero velocity. The kinetic energy is gone — converted to heat, sound, and deformation. This is the MOST inelastic collision possible.
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Crash carts into each other. Compare elastic (bouncy) vs inelastic (sticky) collisions. Watch the energy bars — momentum is always conserved, but energy may not be!
KE_lost = ½μv_rel²
In a perfectly inelastic collision between equal masses, exactly HALF the kinetic energy is lost. The other half remains as kinetic energy of the combined mass.
A tackle is a perfectly inelastic collision — the players stick together. The heavier player's momentum dominates, which is why bigger players are harder to stop.
The desk toy where balls click back and forth is a nearly elastic collision. Energy bounces between the first and last ball with minimal loss.
The Moon likely formed from a giant inelastic collision between proto-Earth and a Mars-sized body (Theia). The energy released was enough to melt the entire Earth.
“Momentum is ALWAYS conserved in collisions. Energy is conserved only in elastic ones. Nature is elastic at atomic scales (gas molecules) but inelastic at human scales (car crashes). The difference is whether kinetic energy converts to heat.”
Crash Lab
Set up elastic and inelastic collisions between carts. Adjust masses, velocities, and 'bounciness.' Compare momentum and energy before and after.
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Set restitution to 1.0 (perfectly elastic) and collide equal masses. Then set it to 0 (perfectly inelastic). Compare how much kinetic energy survives in each case.
The coefficient of restitution (0 to 1) tells you how 'bouncy' a collision is. Real-world values: tennis ball ≈ 0.75, baseball ≈ 0.55, clay ≈ 0.