WEBVTT

1
00:00:00.000 --> 00:00:10.786
Here is the problem: A 2 kg ball at 3 m/s hits a 1 kg ball at 0 m/s in a perfectly elastic head-on collision. Find both final velocities.

2
00:00:11.236 --> 00:00:16.460
One-dimensional collision; positive velocities point one way, negative the other.

3
00:00:17.260 --> 00:00:18.642
Total momentum before.

4
00:00:19.092 --> 00:00:22.854
Elastic collision: conserve momentum and kinetic energy.

5
00:00:23.304 --> 00:00:25.487
Kinetic energy before and after.

6
00:00:26.287 --> 00:00:34.878
So the answer is v₁ equals 1 m/s, v₂ equals 4 m/s (elastic; kinetic energy conserved).

7
00:00:35.328 --> 00:00:52.011
Momentum is conserved in every collision when no external force acts. Kinetic energy is conserved only in elastic collisions; in a perfectly inelastic collision the objects stick together and the maximum possible kinetic energy is converted to heat, sound and deformation.

8
00:00:52.811 --> 00:00:58.303
Momentum before 6 equals after 6 kg times m/s, which checks out.

9
00:00:58.753 --> 00:01:03.130
Kinetic energy before 9 equals after 9 J, which checks out.