When to use each of these equations

[tubworld]

Just some thoughts on solving questions relating to forces, etc. How do I know when to use momentum to solve the problem or when to use conservation of energy in solving a problem?

For example: A bullet travelling at 1000m/s strikes and pass through a 2kg block initially at rest from the bottom of the block. The bullet emerges from the top of the block with a speed of 400m/s. To what maximum height will the block rise above its initial position?

 

[LeonhardEuler]

A hint to use conservation of energy is when they say a collision is elastic. In the case of a bullet hitting a block, this is not likely to be the case since a lot of energy probably becomes heat. Conservation of momentum still holds, though.

 

[Doc Al]

Some problems require using both conservation of momentum and conservation of energy. This is one of them.

During the collision of bullet with block, energy is not conserved, but momentum is (as LeonhardEuler explained). But after the collision, the energy of the block is conserved. You'll need to use conservation of momentum to find the speed of the block and then use conservation of energy to find the maximum height.