When two objects collide, energy is only conserved if they bounce off each other [an elastic collision.] In this case, the objects stuck together, which means a lot of the energy was wasted and turned into heat.

Because of this, we can’t just say that the energy gained by the block = the energy lost by the bullet, which is what I did in the problem. Here is the correct way to do it:

1: Use energy to find out the block’s velocity after collision
2: Find the momentum of the block after collision
3: Set that equal to the momentum of the bullet before collision.

In short, when two objects collide, momentum is ALWAYS conserved. However, KE may or may not be conserved [elastic vs. inelastic collisions]

Mr. Martin could you explain what you said was wrong with this video in class.

Hey, sorry I missed this comment earlier.

When two objects collide, energy is only conserved if they bounce off each other [an elastic collision.] In this case, the objects stuck together, which means a lot of the energy was wasted and turned into heat.

Because of this, we can’t just say that the energy gained by the block = the energy lost by the bullet, which is what I did in the problem. Here is the correct way to do it:

1: Use energy to find out the block’s velocity after collision

2: Find the momentum of the block after collision

3: Set that equal to the momentum of the bullet before collision.

In short, when two objects collide, momentum is ALWAYS conserved. However, KE may or may not be conserved [elastic vs. inelastic collisions]

Please let me know if any of that was unclear