WEBVTT

1
00:00:00.000 --> 00:00:10.902
Here is the problem: A 2 kg object moving at 3 m/s collides and sticks to a 1 kg object moving at 0 m/s. What is their common velocity?

2
00:00:11.352 --> 00:00:16.344
One-dimensional collision; positive velocities point one way, negative the other.

3
00:00:17.144 --> 00:00:18.572
Total momentum before.

4
00:00:19.022 --> 00:00:22.644
Perfectly inelastic: the objects move together.

5
00:00:23.094 --> 00:00:25.335
Kinetic energy before and after.

6
00:00:26.135 --> 00:00:36.305
So the answer is v equals 2 m/s (perfectly inelastic; 3 J of kinetic energy lost, 33.33 percent).

7
00:00:36.755 --> 00:00:53.322
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
00:00:54.122 --> 00:00:59.753
Momentum before 6 equals after 6 kg times m/s, which checks out.