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Old December 19, 2009, 10:56 PM   #62
JohnKSa
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Join Date: February 12, 2001
Location: DFW Area
Posts: 24,929
Quote:
Not correct.
The bullet is carrying the energy it received from the expanding powder gases, a far larger number than any recoil energy.

If you stopped a 2000 ft-lb energy projectile in 1 foot it would exert 2000 pounds of force.
I was very careful to speak NOT in terms of the DISTANCE required to stop the bullet but rather in terms of the TIME required to stop the bullet.

IF the bullet stops in exactly the amount of time that the rifle's recoil is arrested then you can use conservation of momentum to see that the rifle applied more force to the shooter than the bullet applied to the shootee.

Your examples are (correctly) pointing out that it is highly unlikely that the bullet will stop in the same amount of time as the rifle was stopped by the shooter's shoulder but I didn't claim that it was realistic to expect the bullet to stop in the same amount of time that the rifle was stopped by the shooter's shoulder. It was just one of several thought experiments to help people get a feel for how momentum and force are related.

If you work through what I said the way I said it you will see that it's correct.

What you said is also correct but your post is looking at it from the perspective of energy and distance instead of from the perspective of momentum and time as my post was.
Quote:
Force = mass x feet per second/second

Using high school algebra, multiply both sides of the equation by the unit second to simplify:

Force x second = mass x feet per second.

Now what does mass times feet per second equal?
That's right, the momentum.
I'm not sure where you're going with this. You are agreeing with brickeye's correct definition of momentum.

Force x time = momentum = mass x velocity = mass x distance / time
Force = momentum / time = mass x velocity / time = mass x distance / ( time x time) = mass x acceleration
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