Physics / Unit 6: Momentum and Collisions
6.5 Elastic and Inelastic Collisions
Notes
Momentum is conserved in every collision. Kinetic energy is not always conserved.
| Type | Momentum | Kinetic energy | Example |
|---|---|---|---|
| elastic | conserved | conserved | pool balls (nearly), atoms |
| inelastic | conserved | some lost | car crash, bouncing ball |
| perfectly inelastic | conserved | most lost; objects stick | clay hitting clay |
- Lost kinetic energy becomes heat, sound and deformation.
- In a head-on elastic collision between equal masses, the moving one stops and the other moves off with its velocity.
- Example: a 2 kg cart at $3 \text{ m/s}$ sticks to a 1 kg cart at rest. $KE$ before $= 9 \text{ J}$; after $\displaystyle = \frac{1}{2}(3)(2^2) = 6 \text{ J}$. So $3 \text{ J}$ was lost.
Practice
In a perfectly inelastic collision, which is true?
Momentum is always conserved; "perfectly inelastic" means they stick.
A moving pool ball hits an identical ball at rest head-on in an elastic collision. What happens?
Equal masses in an elastic head-on collision exchange velocities.
A 4 kg ball at $5 \text{ m/s}$ collides with a 1 kg ball at rest. Afterward the 4 kg ball moves at $3 \text{ m/s}$ in the same direction. What is the 1 kg ball's velocity?
$4(5) = 4(3) + 1v \Rightarrow 20 = 12 + v \Rightarrow v = 8 \text{ m/s}$.
Ready for more? Take the Unit 6 test.