Physics Question Concerning Lead v. Rubber Bullets

[lendaddy]

Originally posted by LubeMaster77
I'm dying to give the answer but I know there are a few more guys who have not had their coffee and smoke yet and would like to take a shot at it. maybe in 3 hours I will provide the answer. It is rather interesting and maybe obvious but the explanation is rather cool as to why...

Edit: sorry., bit of a spoiler. Wasn't thinking.

[Nathans_Dad]

Wouldn't the rubber bullet transfer as much if not more energy to the wood? Both bullets will come to a stop, thus imparting their kinetic energy to the wood. The rubber bullet, however, will store some of the energy inside itself as it deforms and then will impart some energy back to the wood as it rebounds.

Of course there is that energy cannot be created nor destroyed thingy to worry about...I knew I hated physics for a reason.

[lendaddy]

Originally posted by Nathans_Dad
The rubber bullet, however, will store some of the energy inside itself as it deforms and then will impart some energy back to the wood as it rebounds.

Yes, but would not that energy used to deform the bullet be energy that would otherwise be used to move the log against the resistance?

[David]

Realistically I think both will knock the wood over.

Since I must choose, I'm going with the lead bullet due to the yield strength of each item. I think the deformation of the rubber bullet will absorb most of the energy, leaving little to move the wood. The lead and wood have similar yield strengths so the energy will be more even distributed leaving more energy to knock over the wood.

[hardflex]

I'll say the rubber one, since it would be less likely to penetrate the wood and dissipate the energy. Isn't that why they use them in riots?

I'm assuming that the area of impact is roughly the same, since they would be fired out of the same gun barrel. I may be wrong there.

[Nathans_Dad]

Originally posted by lendaddy
Yes, but would not that energy used to deform the bullet be energy that would otherwise be used to move the log against the resistance?

Well, I would think so, but doesn't the wood also have to give some energy to the bullet when it rebounds? I mean, some of the energy from the bullet will go into deforming the bullet, but then when that bullet bounces off, doesn't the wood have to push back against it?

[lendaddy]

Originally posted by Nathans_Dad
Well, I would think so, but doesn't the wood also have to give some energy to the bullet when it rebounds? I mean, some of the energy from the bullet will go into deforming the bullet, but then when that bullet bounces off, doesn't the wood have to push back against it?

I agree. When the bullet enters the log it pushes laterally(while spreading the wood) as well, this is wasted energy. But I cannot get my head around the idea that the rebound is additional energy. I'm sure I'm wrong, but to me that is leached energy not additional.

[Jared at Pelican Parts]

If they are the same mass and travelling at the same speed, wont they both knock the wood over at the same time?

[Nathans_Dad]

Originally posted by lendaddy
I agree. When the bullet enters the log it pushes laterally(while spreading the wood) as well, this is wasted energy. But I cannot get my head around the idea that the rebound is additional energy. I'm sure I'm wrong, but to me that is leached energy not additional.

I agree, the only way I can figure is that there is energy stored in the wood from its position, i.e. someone put it on end and therefore stored energy in it which will be spent when it hits the ground. Therefore the wood has some energy stored which it can give back to the bullet when the bullet rebounds.

I really do hate physics...

[masraum]

This is fun

[dhoward]

You guys have never been to a bowling pin match....

[hardflex]

"agree, the only way I can figure is that there is energy stored in the wood from its position, i.e. someone put it on end and therefore stored energy in it which will be spent when it hits the ground. Therefore the wood has some energy stored which it can give back to the bullet when the bullet rebounds."

I think all the energy of compression would be in the rubber bullet, not in the wood. I would think wood is not very "elastic"

Not to say it wouldn't bounce off the wood, just where the energy goes as it does.

I think he must have read this in the Mensa quizzes on the airplane ride. ;->

[nostatic]

was the monkey in the tree when the bullet was fired?

[widebody911]

There had to be more than one shooter.

[bryanthompson]

edit: crap, totally wrong.

[M.D. Holloway]

OK - here is the answer:

The rubber bullet will be more likely to knock the wood over. The lead bullet will embed itself in the wood, so the wood will have to absorb enough energy to stop the bullet's momentum.

The rubber bullet will ricochet off of the wood, so the wood will have to absorb enough energy to stop the bullet plus send the bullet back in the opposite direction.

The rubber bullet will supply more total force to the wood and will therefore be more likely to knock it over.

[bryanthompson]

I call BS. someone get a log and some bullets... we gots an experiment to prove.

[M.D. Holloway]

Problem is, none of these clowns would ever shoot a rubber bullet! Drop'm with lead or cut'm wit steel...

[lendaddy]

Originally posted by LubeMaster77
so the wood will have to absorb enough energy to stop the bullet plus send the bullet back in the opposite direction.

That's the part I don't get. The energy to send the bullet back is simply a % of the energy it originally carried. Instead of being used to knock over the log it was used to deform/energize the bullet.

[Nathans_Dad]

I think it's because the wood has to push back against the bullet in order for it to rebound. That energy comes from the friction between the wood and the ground as well as gravity which tends to want to keep the wood upright (until it gets to a 45 degree angle anyhow). If the wood did not push back against the bullet, it would simply fall over and there would be no rebound.

I still really hate physics.