Momentum question

[Sammy1024]

I read a passage where there was a block sitting on a rubber floor and then the person shoots a rubber bullet and an aluminum bullet at it. The question asked which bullet will most likely deform the block?

In the passage it said that the aluminum bullet deforms when it hits the block.

I was just logistically wondering why the aluminum block would be more likely to cause deformation?

The answer said: Choice B is the best answer. Because of the elastic nature of rubber, the rubber bullet bounces off the block, carrying away most of its original speed, and therefore carrying away most of its kinetic energy. By contrast, the aluminum bullet penetrates the block, surrendering all of its kinetic energy to the block. This kinetic energy becomes heat energy and the energy causing deformation of the bullet and the block (work is necessary to change the shape of both objects). The best answer is choice B.

I wasn't sure which one so I was thinking of the scenario in my head. If a rubber bullet hits a wooden block it is likely to cause deformation because the rubber won't deform. And then I thought that since the aluminum itself deforms, then when it hits the block, it is less likely to cause deformation.

Can someone explain logistically why it is the opposite?

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[syoung]

I would say this is a common sense-type of question, where if you think about it...

Let's say you have a rubber sword and a metal sword. Use it to try to cut a tree. What happens?

 

[Sammy1024]

Well thinking of that way it makes sense, but the question had an aluminum bullet that deforms, while the rubber one doesn't. Would you suggest making the objects a little bit more extreme and trying to go about it that way?

 

[syoung]

So I might have mis-read that a little.

The aluminum bullet has two things going on. It's much harder than the rubber bullet, but it also deforms. So by that nature it penetrates the block, and the heat generated from the penetration also deforms the bullet.

The rubber bullet on the other hand, being so elastic, it's nearly impossible for it to penetrate the wood. Because of that, it won't lose energy to the wood and become deformed.

That's my best understanding.

 

[Sammy1024]

Ahhh I see. I guess I was inferring in information because I was imagining the aluminum being weak like aluminum foil. Thanks so much! :]

 

[syoung]

Yeah, it is a bullet afterall... There's some amount of structure there.

 

[NextStepTutor_1]

This question likely tests your knowledge of the three types of collisions. The rubber bullet bouncing off the block is an elastic (somewhat) collision where most of the KE is conserved. The aluminum bullet deforming shows that the collision is likely inelastic, meaning that both objects lose KE and thus, both objects are deformed.