Question

Two bullets of the same size, mass and horizontal velocity are fired at identical blocks, only one is made of steel and the other is made of rubber. The steel bullet has a perfectly inelastic collision with the block, while the rubber bullet has an elastic collision. Which bullet is more likely to knock over the block, or are both equally likely to do so? Justify your choice based on physics principles. Make an initial posting before replying to other threads.

Answer 1

Since we're talking about bullets, the relevant thing is how the energy is converted as the bullet is decelerated to a stop when it hits something. KE is proportional to mass times speed squared, so indeed, the 2 cases you give have almost identical KE. The amount of energy is definitive, so the two cases will do roughly the same damage provided of course we compare cases when all the KE is used up. One difference that will be easily apparent is that the gun in the heavy-bullet case will have more recoil.

One can think of energy absorption as force times distance, and momentum absorption as force times time. Hence, the heavier but slower bullet with the same energy will travel the same distance in the absorbing material, but because of larger momentum, will take a longer time doing it. It will therefore also impart a greater "kick" to the absorber object.