Physics Question Concerning Lead v. Rubber Bullets

[svandamme]

Originally posted by Eric Coffey
What about the expansion (mushroom effect) that takes place with a lead bullet upon impact? Isn't that a large factor in the "knockdown" power of a lead projectile? If not, then why are hollowpoint bullets so effective?

1 Hollow point have less kinetic energy then Full lead cores

2 the air pocket is designed, so it expands after penetration , causing massive interior damage as opposed to staying intact and exiting the target.

Rubber bullets on the other hand are designed not to penetrate at all, but on a block of wood, i think HP's and Rubber would behave more similar to eachother then to full Lead..

[Eric Coffey]

Originally posted by lendaddy
They simply prevent the bullet from passing through and not imparting it's full potential on the target.

This doesn't make sense to me. Assuming same mass and impact velocity: If a bullet passes completely through, then it obviously still has kinetic energy. If a bullet mushrooms out, and doesn't pass through, then all of it's energy is spent on the target, no?

[lendaddy]

Originally posted by Eric Coffey
This doesn't make sense to me. Assuming same mass and impact velocity: If a bullet passes completely through, then it obviously still has kinetic energy. If a bullet mushrooms out, and doesn't pass through, then all of it's energy is spent on the target, no?

We agree, I was just clarifying.

[svandamme]

Originally posted by Eric Coffey
This doesn't make sense to me. Assuming same mass and impact velocity: If a bullet passes completely through, then it obviously still has kinetic energy. If a bullet mushrooms out, and doesn't pass through, then all of it's energy is spent on the target, no?

but it discards it's energy sideways in shards
not in the direction the bullet was initially traveling...

[lendaddy]

Originally posted by svandamme
but it discards it's energy sideways in shards
not in the direction the bullet was initially traveling...

Also true, it uses a portion of it's energy ripping up your inerds as opposed to knocking you down. All I was saying is that by staying in the body, all of it's energy is spent on that body.

[jluetjen]

Originally posted by LubeMaster77
Bounce a rubber ball on the pavement. The force you deliver to that ball is reciprocated back in the bounce back - but not 100%. Try dropping a rubber ball instead of throwing it down. It will bounce back up but not to the height you dropped it at - it can't (unless a chemical reaction ensued on the way down that changed its mass on the way back - different discussion though) There is lost energy? It had to go someplace. With a bouncing ball the energy that seems lost is so small it is un-noticed but that is not the same for a projectile traveling at 800 to 1500 ft/sec. The bullet absorbs so much but the wood (or any surface has too as well). The total energy has to be accounted for. It this case it is transfered to the surface where action has to occur (and some may even be given off in the form of heat).

This is correct, and also why it's important to keep shock absorbers cooled with fresh air on bumpy circuits.

Anyhow...

Yes, the log has potential energy while it's standing on end, but only in relation to it's laying down position. But irrelevent in the case of the question at hand. The log is at rest, and wants to stay at rest -- and has no energy in relation to the bullet.

If the rubber bullet bounces, it is basically the bullet's kenetic energy being reflected by the log. Some portion will be transfered to the long -- most likely resulting in an imperceptable movement. But most of it will (less any heat generated by the internal friction of the rubber) will used to accelerate the rubber bullet in roughly the opposite direction.

The only way that the log will impart any energy on the bullet would be if the velocity of the bullet in relation to the log is enough to annihelate the log's matter, (AKA nuclear fusion) which will then most likely liberate enough energy to destroy a small neighborhood -- and create a PhD for someone.

[Joeaksa]

Had a friend who was with the Royal Army and stationed in Belfast Northern Ireland. He was a really nice guy but after several of the locals tried to kill his friends the gloves came off.

They all were banned from shooting directly at the locals with their rifles, which had rubber bullets in them. He told me that with a bit of practice you could bounce or riccochet them very accurately. Evidently they could still break a bone or injure internal organs if they hit on the right spot. Once the locals figured this out they became a bit more peaceful he said...

[Overpaid Slacker]

Lube I think your post proved mine...

Or it proved I'm more confused than I thought.

I guess what I'm saying is that if the rubber ball ricochets or rebounds that energy had to come from somewhere, and it's not the log.

Well, if you want to say it technically is the log, then the log got that energy to give from the rubber bullet itself so not all of the rubber bullet's energy stays with the log. So, maybe (maybe....) all of the rubber bullet's energy is given to the log, but the log gives at least a portion of it back.

So.... is what you're saying that, given equal and opposite reactions, the log is "pushed" twice? Once when the rubber bullet hits it, and then again when it pushes the rubber bullet away? And that "extra" energy comes from the elasticity of the bullet?

The conservation principle you mentioned was demonstrated very vividly. As a kid, I sat in on a Physics course at SU -- The Physics of Toys. GREAT course. Anyway, the auditorium was enormous, and one day we arrived to find the prof had suspended a bowling ball from the ceiling by a cord, such that it hung just off the floor.

Letting the tension build during class (wtf is up with the bowling ball?), near the end he took the ball, stood with his back to the side wall, at which point the suspended bowling ball was right at his face level, touching his nose, and let the ball go. It crossed the room, coming close to the opposite wall and made a massive "whooooooommmmm" sound as it arced back toward the prof, who was just standing exactly still, casually blah blah blahing with the class. There were a whole lot of gasps as the ball almost hit his head and swung back across the room.

He solicited volunteers, and a girl assumed the position, back against the wall, with the ball at head level, touching her nose. The prof said "go ahead". And the girl pushed the ball away...

Well, being a good guy, he pushed her out of the way, but being a showman, he waited until the last possible second. He said of the dent the ball left in the wall "oh, they've got to fix that wall every year after this demonstration."

JP

[nostatic]

what about the monkey?

[David]

Glad this wasn't on the PE exam, I would have wasted too much time. Now where's that physics book?

[Eric Coffey]

Originally posted by nostatic
what about the monkey?

Who do you think fired the gun?

[Overpaid Slacker]

That is one of the most inspired replies, and true to the spirit of OT.
This Sapphire Gibson is dedicated to you, Eric Coffey!!

Slainte!

JP

[artplumber]

Originally posted by IROC
I've had to do impact calculations before. It really all comes down to how quick the "mass" is decelerated. Maybe that's the key here. The lead bullet penetrates and decelerates at a slower rate than the rubber bullet, which decelerates more rapidly.

I think I just proved myself wrong. Oh well. BTDT.

Mike

Actually, this issue is all about dp/dt Mike and where the energy goes (thermal deformation of the projectile etc). You have it right IMO.

[Don Plumley]

How much wood would a woodchuck chuck if a woodchuck could chuck wood?

Having bad physics flashbacks. Monkeys throw poo at wood.

[Eric Coffey]

Originally posted by Overpaid Slacker
This Sapphire Gibson is dedicated to you, Eric Coffey!!

Slainte!

JP

Ha! Right back at ya:


Slainte agat, agus bas in Eirinn!

[john70t]

Break it down the rubber example into steps(in order I hope):

1. On the moment of impact, the mass of the rubber bullet transfers partially to the wood(velocity=0).
-A small part of the knietic energy is used to deform the round rubber to a quazi-disk shape laterally with minute frictional surface forces.
-A small part of the energy is used to deform and expand the impact sites wood fibers latterally(assume it could be rubber as well)
-A small part of the energy is used to permanently deformand/or compress the wood surface(transfer)
-Most of the bullets kinetic energy is transfered to the wood's total unmoved mass through fiber compression along the line of travel.

2. Resistance/rebound forces begin to act
-The two "sides" of rubber disk expand outwards while returning to a globe shape, equally along the line of travel, so only half is imparted to the surface of the now-accelerating wood, which may or may not be in surface contact with it. If it is, additional minute frictional forces between the rubber and wood can take place.
-The mass of the wood begins to accelerate(away from the original impact location)
-The fibers of the (minutely) deformed wood begin to contract back to its original shape(on itself) independantly.

Vs: The lead:
1. More line-of-travel(transfer)compression(transfer) from lack of rebound, but more permanent compression and lateral deformation of the wood fibers
2. More energy used in the deformation/shattering of the kinetic carrier(bullet).

Is the wood Lignum Vitae or Balsa? I give up, 10am headache.
Rubber sounds right.