Author Topic: math question (Read 710 times)

tree rat · « **on:** April 10, 2003, 06:38:48 PM »

what is the formula to figure out energy, if you know weight and velocity?

Castaway · « **Reply #1 on:** April 11, 2003, 01:48:12 AM »

formula for ft/lbs is 1/2 mvv/ 32.16

(Mass is grams of bullet divided by 7000)

BigEd22 · « **Reply #2 on:** April 11, 2003, 04:58:07 AM »

First, calculate the mass (lets call it 'm') as follows:

e = 1/2 * m * v * v

In a simplified version, it is :

e= ( Bullet(grains) * Velocity(fps) * Velocity(fps) ) / 448000

That is, multiply the weight of the bullet by the velocity, and again by the velocity, then divide the WHOLE THING by 448000. It will give you the foot pounds of energy.

Tony D · « **Reply #3 on:** April 11, 2003, 09:21:42 AM »

BigEd, your constant is a little low. I've ususally seen it as 450240. I come up with a little different number. Acceleration gravity in english units is 32.174, 7000 grains a in pound times 2 comes to: 450436.

BigEd22 · « **Reply #4 on:** April 11, 2003, 11:19:17 AM »

quite possible.... For 'simplicity', I round of the gravity to a nice and even 32 :grin:

For example, taking a 180 grain bullet at 3000 fps (Weatherby, for ex). With the 32.174 rounded off to 32, you will get 3616 ft.lbs instead of the 3598 ft.lbs with the 450240 (and 3596.5 ft.lbs with K=450436).

They are basically all the same.... 3600 ft.lbs.... Of course, one could argue that the gravity might be a a little different in my neck of the woods ! :lol:

Castaway · « **Reply #5 on:** April 12, 2003, 09:56:29 AM »

Never-the-less, both are the same formula. The second formula justs combines some variables. Depending on where you are (altitude) 32.16 or even 32.17 is an accepted value. The figure 450240 is what you are looking for as you divisor in the second formula given above. The number is the product of multiplying 7000*32.16*2. Same thing, different way of getting there.