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Unread postAuthor: jackssmirkingrevenge » Thu Oct 01, 2009 1:23 am

Insomniac wrote:Ah, thanks for clearing that up. I'm starting to understand it now. So would that mean that two objects with the same momentum won't neccissarily have the same amount of kinetic energy?


Picture two objects:

10 Kg object travelling at 10 metres per second

5 Kg object travelling at 20 metres per second.

Same momentum (10x10 = 5x20), but if you work out the kinetic energy, the smaller object has double the kinetic energy of the first.
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Unread postAuthor: Ragnarok » Thu Oct 01, 2009 9:06 am

Insomniac wrote:So would that mean that two objects with the same momentum won't necessarily have the same amount of kinetic energy?

Two objects with equal momentum will ONLY have equal kinetic energy if they are of equal mass.

And as Jack has already begun to show, the lesser the mass for the same momentum, the higher the kinetic energy - and of course, vice versa. Which is why guns, at usually about two or three orders of magnitude more massive than their projectiles, don't simply tear the shooters arm off.

You need to find the most violently recoiling weapons for them to have a recoil energy that exceeds the muzzle energy even of a .22 Long Rifle.
More than 100 ft-lbf of recoil energy is very unusual (although .700 Nitro rifles can be as much as 200 ft-lbf, best described with the word "ouch".) and most people, were they presented with such a rifle, would probably drop it on firing. I'm sure you'll've seen the videos of people firing .577 T-Rex on Youtube.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: Insomniac » Fri Oct 02, 2009 8:02 am

Ah, that makes sense. I had always thought that 'equal and opposite reaction' meant equal kinetic energy, and had thought that momentum and kinetic energy were different ways of measuring the same thing. Turns out I was wrong. :oops:
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I wonder how much deeper the ocean would be without sponges.
Right now I'm having amnesia and deja vu at the same time. I think I've forgotten this before.

Add me on msn!!! insomniac-55@hotmail.com
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 02, 2009 8:07 am

Ragnarok wrote:I'm sure you'll've seen the videos of people firing .577 T-Rex on Youtube.


Classic stuff, I think "ouch" is a slight understatement :D
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Unread postAuthor: nakarti » Tue Oct 13, 2009 4:49 pm

On the energy and momentum conservation: you're not just pushing the bullet forward and ignoring the gases pushing forward!

The push on the bullet is split equally between the barrel->gun->shooter, and 1/3 sounds fair for how much goes to the bullet, so 1/3 would also go to the gun.

Some of the other third goes to heating things up, but some goes to pushing the gases forward, so the recoil energy is actually greater than the bullet energy.

That much is logically obvious. I would need more knowledge of physical mechanics to attest to the precision of everything else, but it looks good (the kgm/s divided by kg directly into m/s is fuzzy, but so is the softening of that rifle trying to fly at 1.5m/s by holding it against your shoulder, so it's ok.)

And MOMENTUM is solely subject to the objects' mass! Two 1kg balls, one at 2/3 the speed of light, one at 100m/s, will have the same momentum. They will have hugely different kinetic energies, and the kinetic energy is what knocks big holes in things, so the near-light-speed ball definitely wins.

So again: momentum, or resistance to change in motion, is directly proportional to mass. Kinetic energy divided by mass will tell you velocity. Kinetic energy added, divide by momentum, will tell you change in velocity.

It's like that class on acceleration, with different units. In fact, it defines that class, just without time.
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