Can someone explain to me--in straight forward terms--the difference between energy (ft-lbs) and momentum (ft-lbs/sec)?

It's been a long time since my physics classes!

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p.s. Can we do this in less than 30 posts w/o it degenerating into a conversation about SD?

quote:

Can we do this in less than 30 posts w/o it degenerating into a conversation about SD?

Can't be done as you have 28 posts left!!

Momentum = Mass*Velocity

is a vector quantity and is conserved.

Kinetic energy = (1/2)*Mass*Velocity^2

is defined as the ability to do work

(Remember that the unit pound is not a unit of mass. It is a unit of force. A "slug" is the unit of mass such that when it is accelerated at 1 ft/sec^2 yields 1 lb-ft of force. F=ma)

The simple equations are

Momentum is mass*velocity

Energy is 1/2Mass*velocity*velocity

so if you increase the velocity from 1 to 2 the momentum doubles while the energy goes up by 4.

From here you will now get your 30+ posts discussing the merits of momentum and energy.

The energy of a bullet is its kinetic energy due to its mass and velocity, (mV^2)/2

The momentum is its mass times velocity, mV.

Both energy and momentum are always conserved, blah, blah. Just depends on what you want to know about them, if anything.

In the case of a 400 gr. bullet going 2400 fps, you can calculate energy in ft-lb(f) [the lb(f) is the English version of force as opposed to Newtons in metric, blah, blah] as

((1/2)(400)(2400)^2)/(7000(32.17)) = 5115 ft-lb

Doesn't it depend on the rate of twist?

Momentum -a property of a moving body that the body has by virtue of its mass and motion and that is equal to the product of the body's mass and velocity. (broadly) : a property of a moving body that determines the length of time required to bring it to rest when under the action of a constant force or moment.

Kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its current velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. Negative work of the same magnitude would be required to return the body to a state of rest from that velocity.

Momentum = [(bullet weight)(velocity)
-------------------------------
450,400

To me, momentum means the ability to penetrate.
The higher the momentum number, the further it will penetrate. Which to me is an indicator for tough skined dangerous game.

Kinetic energy seems to me to be used more with expanding bullets and used as a judgment with thin skinned game.

Of course this is all contingent with the rate of twist!

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Doesn't it depend on the rate of twist?

If your bullets were made in the southern hemisphere on a Tuesday - yes.

quote:

Originally posted by mstarling:
. A "slug" is the unit of mass such that when it is accelerated at 1 ft/sec^2 yields 1 lb-ft of force. F=ma)

I thought a slug was something in the air as in "slugs per cubic foot" and didn't need to be accelerated anywhere.

No, slugs are used in boxing. Usually to the head in "blows per second"....

quote:

Originally posted by dogcat:
No, slugs are used in boxing. Usually to the head in "blows per second"....

I'm afraid that you are both incorrect. Slugs are a shelless snail found in moist areas in the Southern USA.

[QUOTE]Originally posted by Pegleg:
To me, momentum means the ability to penetrate.
The higher the momentum number, the further it will penetrate. Which to me is an indicator for tough skined dangerous game.

Kinetic energy seems to me to be used more with expanding bullets and used as a judgment with thin skinned game.QUOTE]

I guess this was basically what I was wondering. At Norma's website, they've got a series of heavy for caliber bullets, beginning at .375/350 gr.

For a non-expanding bullet, is momentum (as opposed to KE) a better indicator of penetration, all things being equal?

(Fjold, perhaps your icon is a good example of MOmentuMMM? ).

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