WEBVTT

1
00:00:00.000 --> 00:00:10.495
Here is the problem: A 1 kg ball at 2 m/s hits a 1 kg ball at 0 m/s in a perfectly elastic head-on collision. Find both final velocities.

2
00:00:10.945 --> 00:00:15.972
One-dimensional collision; positive velocities point one way, negative the other.

3
00:00:16.772 --> 00:00:18.142
Total momentum before.

4
00:00:18.592 --> 00:00:22.226
Elastic collision: conserve momentum and kinetic energy.

5
00:00:22.676 --> 00:00:24.882
Kinetic energy before and after.

6
00:00:25.682 --> 00:00:34.680
So the answer is v₁ equals 0 m/s, v₂ equals 2 m/s (elastic; kinetic energy conserved).

7
00:00:35.130 --> 00:00:51.697
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
00:00:52.497 --> 00:00:57.919
Momentum before 2 equals after 2 kg times m/s, which checks out.

9
00:00:58.369 --> 00:01:02.850
Kinetic energy before 2 equals after 2 J, which checks out.