WEBVTT

1
00:00:00.000 --> 00:00:13.584
Here is the problem: A 1200 kg object moving at 15 m/s collides and sticks to a 800 kg object moving at negative 10 m/s. What is their common velocity?

2
00:00:14.034 --> 00:00:19.142
One-dimensional collision; positive velocities point one way, negative the other.

3
00:00:19.942 --> 00:00:21.347
Total momentum before.

4
00:00:21.797 --> 00:00:25.361
Perfectly inelastic: the objects move together.

5
00:00:25.811 --> 00:00:28.156
Kinetic energy before and after.

6
00:00:28.956 --> 00:00:40.078
So the answer is v equals 5 m/s (perfectly inelastic; 150000 J of kinetic energy lost, 85.71 percent).

7
00:00:40.528 --> 00:00:57.502
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
00:00:58.302 --> 00:01:04.676
Momentum before 10000 equals after 10000 kg times m/s, which checks out.