Multiple choiceQuestion 1 · Recall
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By the end you can recall what makes a collision elastic, explain why momentum is always conserved while kinetic energy may change, and decide whether a collision is elastic by calculating kinetic energies.
The main symbols used in this lesson:
| Symbol | Meaning |
|---|---|
| velocity |
Drop a super-bouncy ball and a lump of clay. The ball bounces back almost to where it started; the clay goes splat and stays put.
Momentum is conserved in both cases (the Earth takes up the difference). What's different is the kinetic energy: the ball keeps nearly all of it; the clay's goes into warming and squashing the clay, and a satisfying thud.
| Interaction | Total momentum | Total kinetic energy | Examples |
|---|---|---|---|
| Elastic collision | conserved | conserved | gas molecules; very nearly, snooker balls |
| Inelastic collision | conserved | decreases | most real collisions: cars, footballs |
| Objects stick together | conserved | decreases (by the most that momentum conservation allows) | clay, coupling trucks, a dart in a board |
| Explosion | conserved | increases | fireworks, recoil, a released spring |
The momentum column is boring on purpose: it never changes. All the drama is in the kinetic energy column.
Elastic collision
A collision in which the total kinetic energy is conserved. (Total momentum is conserved too, as it is in every collision.)
Inelastic collision
A collision in which the total kinetic energy is not conserved: some is transferred to other forms, such as thermal energy (internal energy) and sound. Total momentum and total energy are still conserved.
Worked example: Checking a collision
A trolley at hits a stationary trolley and they stick together. Find their common velocity, and show that the collision is inelastic.