Physics Question Concerning Lead v. Rubber Bullets

[M.D. Holloway]

A piece of firewood is standing on end. Which is more likely to knock the piece of wood over — a lead bullet fired from a gun or a rubber bullet of the same mass traveling at the same speed (and hitting the same spot)? Explain why.

Answer provided in 6 hours...

[widebody911]

I'm not a physicist, and I don't play one on TV, but a rubber bullet of the same mass would be pretty damn big, and therefore have more surface area. The lead one would probably just tunnel through, like all those high-speed photos I've seen.

[id10t]

I think the lead one would... the rubber may compress a tad, absorbing some of the energy before it transfers to the target. Contact area may also come into play - the rubber bullet should have a larger surface area, since it is less dense... not sure how that would affect the transfer of energy.

[David]

I thought I had answer until I started writing. To solve the problem, we need to know how energy is transfered to the wood and how much is used to deform the bullet. Initially I figured the rubber bullet would use more energy when it deforms, but the lead bullet will deform too. Still thinking...

[IROC]

I'm going to go with Occam's answer and say that they would be both as likely to knock it over. The main thing is the kinetic energy of the projectile (mass times velocity squared). The piece of wood isn't going to know the difference if the kinetic energy is the same. Sure, there may be some effects right at the point of impact, but it seems that conservation of energy is going to come into play and only the duration of the impact energy would be affected, not the total amount of energy imparted.

Is it balsa or oak? :>)

As long as the lead bullet does not exit the other side, they're essentially going to be the same. Picture a piece of plywood in lieu of the piece of firewood. The lead bullet would probably exit the other side and not impart all of it's energy into the wood, where the rubber bullet (being larger) will most likely not penetrate and impart more energy into the plywood.

I'm probably wrong, but that's my 15 seconds-worth of thought...

Mike

[M.D. Holloway]

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...

[JeremyD]

lead being more dense would go through the wood, rubber would bounce off - causing wood to fall. Same reason the military uses "depleated uranium" for their tank busting bullets. More dense than lead, more likely to penetrate.

[jluetjen]

I'd say the rubber bullet since I would expect the lead bullet more likely to travel through the wood. I think (I'm a salesman, not a physicist) the the critical parameter is the density of the bullet, since this will determine the surface pressue at any given time during the impact.

1) Both bullets will have the same energy when leaving the barrel (in a simplified world which ignores barrel friction and bullet deformation within the barrel)

2) Since both bullets have the same mass, the lead bullet would be smaller since lead is denser then rubber, as a result the friction generated as the bullet travels through the air (assuming the same shape) will largely be a function of the frontal area and surface area, both of which would be larger with the rubber bullet. So by the time the projectiles reach the wood, the rubber bullet would have expended some of it's kinetic energy being heated by the air (as well as heating the air).

3) If the energy level is high enough, both bullets could just punch through the wood without knocking it over, just like the old Marx Brothers' trick where they pull the table cloth out from underneath a fully set table. (Heck! A car could even fly 200 feet and into a 2nd story appartment if the energy level was high enough -- but I digress..) But just thinking intuitively, I would expect that in most cases the rubber bullet would be more likely to knock the wood over.

4) Not to mention the surface drag as the bullet travels through the wood. Rubber is generally quite grippy when compared to a smooth metal, so once again, this would tend to increase the friction (compared to the lead bullet) as the rubber bullet travels through the wood, increasing the likelyhood that the rubber bullet would pull the wood over.

[lendaddy]

Originally posted by LubeMaster77
A piece of firewood is standing on end. Which is more likely to knock the piece of wood over — a lead bullet fired from a gun or a rubber bullet of the same mass traveling at the same speed (and hitting the same spot)? Explain why.

Answer provided in 6 hours...

The lead bullet will lodge into the wood, meaning that all of it's energy will imparted upon the wood making it as likely as possible to knock it over.

The rubber bullet will deflect, taking with it some energy, the % is not important. Therefore it is less likely to knock the wood over.

[gaijindabe]

More likely? Using usual velocities of normal firearms..

The rubber bullet.

It does not have the same density and would not pass through. Without passing through - it would unbalance the wood.

[lendaddy]

We are talking about firewood guys, a lead bullet is not going to pass through a log.

[id10t]

lendaddy - if it don't, you aren't using enough gun...

[masraum]

I think a lot depends upon the wood and the bullets. If the bullets are going really fast or relatively slow (in bullet terms) would make a pretty big difference. If the wood is really soft or really hard that may make a difference (lead passes through or embeds in... In some circumstances both bullets will impart the same energy and either neither, or both would knock the thing over (depending upon the mass and stability of the firewood.

Rubber would be more likely to knock the thing over. The bullet would either pass through or get stuck in the wood. If the initial
The (big, as Thom suggested) rubber bullet would hit, deform, and then reform which would impart more energy. Assuming that this is some sort of magic rubber bullet that doesn't disintegrate when it's fired that fast.

I wonder how the MythBusters test with firing into water relates. When they fired a relatively low powered bullet into water it went much, much farther than a really high powered bullet. The really high powered bullets, especially the 50 cal disintegrated in the first 12 or 18 inches.

I'm terribly curious to hear the answer that Lube has for us.

[Nathans_Dad]

It depends on the speed of the swallow and that of course depends on whether it is a common swallow or an African swallow.

[Rot 911]

Just off the top of my head, I'm no engineer or anything (can't even remember the last time I read cr4.globalspec.com ) but, commonly accepted practice in ballistics is to ignore potential engergy, thermal energy and concentrate on kinetic energy. KE=1/2 m v^2, which both bullets have the same amount when leaving the muzzle of the gun. The effiency of the transfer of this energy is the key to the solution. If we assume that the lead bullet penetrates and does not exit the wood, then it has transferred all of its KE to the wood. If the rubber bullet does not penetrate the wood, then it has only transferred a portion of its KE to the wood. The diameter of the bullets only dictates how large of an area their respective amounts of energy are transferred. On the other hand, if the bullets are fired at sufficient velocity that both bullets penetrate the wood, it is likely that the lead bullet will pass through entirely and the rubber bullet will remain lodged in the wood. In that case the rubber bullet transferred the most energy to the wood block, since it was stopped by the wood, it expended all of its KE in the wood, while the lead retained some of it's KE on exiting the wood. If the bullets were fired at a velocity that resulted in exiting the wood in both cases, then internal ballistics would be a factor and bullet diameter would have to be considered.

[JohnC]

I’m going to vote rubber bullet.

Thinking out loud….it’s about energy transfer…the rubber bullet is more likely to transfer its energy to the target …as opposed to passing through and imparting very little energy.

Thinking out loud again…if the rubber bullet rebounds nearly 100% of the energy is imparted to the target? Excluding the energy dissipated, in the form of heat, due to the deformation of the rubber. Lead is less likely to deform generally speaking, compared to rubber.

….things are slow around here today

[Hugh R]

energy=1/2 mass x velocity squared. Same mass, same velocity, question to me is how is the terminal energy dissipated. I think rubber will "knock down" more in that all of its energy is dissipated faster at the surface of the wood versus the lead bullet which loses energy over a greater period of time as it bores into the wood.

[ChrisBennet]

A suspect a rubber bullet of the same mass would knock a piece of wood over about the same as a lead bullet if it stayed intact. F=ma
Note: As someone who has shot thousands of bowling pins I know that pistol bullets won't easily pass through them.

-Chris

[Tervuren]

Assuming the same velocities upon impact, I beleive rubber would be capabl of transferrering more energy into the impact.

[JohnC]

Incidently Depleted uranium is not used because its heavier although I'm sure that helps in penetration. It's because depleted uranium when under tremendous pressure........burns......probably turning into plasma (fourth state of matter). Plasma can't be contained by anything in contact with it....sooooo it goes right through....until it cools. The effect is the same as a shaped charge but with more "ass" behind it (it has mass, kinetic energy). When depleted uranium rounds strike a targewt notice the "sparks" fly off?...Thats molten steel....