Bullet Energy Equation Questions

April 21, 2018, 04:26 PM

duke762

While playing around with my TI 86 and the energy equation I programed in, I noticed that the equation is mass*vel sguared divided by 450240.
My question is what is 450240 derived from. I've been poking at it, assuming it has to do with converting MPH to FPS and Grains to LBS but so far I haven't cracked it. Any ideas anyone?

April 21, 2018, 04:39 PM

Pipe Smoker

Dunno where 450240 comes from, but I usually see bullet muzzle energy calculated as mass (in grains) * V^2, where V is muzzle velocity in thousands of FPS. It’s not a consistent metric or English system of units. But it’s a handy system for bullets, and provides a good relative measurement.

E.g., a 115 grain bullet at 1200 FPS would have an energy of 115 * 1.2^2 = 165.6.

April 21, 2018, 04:45 PM

KenS

Since the basic equation is KE= m×v^2 / 2×g it is undoubtedly a result of converting units. However, since you didn't provide the resulting units of your calculation, there is no way to determine what factors constitute the overall conversion. Typically, KE is given as ft-lbf and g is roughly 32.2 ft/s^2, so the basic equation is m×v^2 / 64.4. v is in ft/s and m is in slugs.

Ken

April 21, 2018, 05:19 PM

sigfreund

I don’t know the specifics, but it’s to allow the use of velocity in feet per second and weight in grains to produce an answer in foot-pounds rather than the usual physics textbook inputs. And the constant varies among sources; I’ve also seen it as 450400, 450500, and IIRC 450800.

April 21, 2018, 05:56 PM

maladat

quote:
Originally posted by KenS:
Since the basic equation is KE= m×v^2 / 2×g it is undoubtedly a result of converting units. However, since you didn't provide the resulting units of your calculation, there is no way to determine what factors constitute the overall conversion. Typically, KE is given as ft-lbf and g is roughly 32.2 ft/s^2, so the basic equation is m×v^2 / 64.4. v is in ft/s and m is in slugs.

Ken

Kinetic energy is KE = 1/2 * m * v^2. There is no g involved.

You would divide by g if you were using weight instead of mass because weight = mass * g, or alternatively, mass = weight / g. If you wanted to use the equation with pounds, you would use 1/2 * pounds / g * v^2 because pounds are a unit of weight. pounds = slugs * g.

In English units, you would use slugs for mass and fps for velocity. Grain is also a unit of mass, not weight.

1 slug is 225,218.3 grains. Dividing the mass in grains by 225,218.3 gives the mass in slugs.

1/2 * mass in grains * (1 slug/225,218.3 grains) * (velocity in fps)^2 = 1/(2*225,218.3) * mass in grains * (velocity in fps)^2 = 1/(450,437)* mass in grains * (velocity in fps)^2.

April 21, 2018, 07:42 PM

reginaldcrumpepper

quote:
Originally posted by duke762:
While playing around with my TI 86 and the energy equation I programed in, I noticed that the equation is mass*vel sguared divided by 450240.
My question is what is 450240 derived from. I've been poking at it, assuming it has to do with converting MPH to FPS and Grains to LBS but so far I haven't cracked it. Any ideas anyone?

Grain is not a unit of mass, nor is pounds. There are 7000 grain per pound. The acceleration of gravity is 32.2 feet per second per sec. Presumably this is why you want to divide by 450240. However, it seems to me you should divide by 450800. The number 450800 is because there are 7000 grains in a pound, and the acceleration of gravity is 32.2 fps/second.

Miles per hour doesn't enter into the calculation.

Well i guess i cannot answer your question, because i don't know where the 450240 came from. Me, i would divide your grains times speed in feet per second squared, calculation by 450800.

April 21, 2018, 07:50 PM

mr kablammo

When I took physics in college the SI system was used. As posted above, I assume that is a conversion factor to foot-pounds. I mostly detest the metric system but for this sort of stuff I am an SI guy.

April 21, 2018, 07:57 PM

KenS

quote:
Originally posted by maladat:
quote:
Originally posted by KenS:
Since the basic equation is KE= m×v^2 / 2×g it is undoubtedly a result of converting units. However, since you didn't provide the resulting units of your calculation, there is no way to determine what factors constitute the overall conversion. Typically, KE is given as ft-lbf and g is roughly 32.2 ft/s^2, so the basic equation is m×v^2 / 64.4. v is in ft/s and m is in slugs.

Ken

Kinetic energy is KE = 1/2 * m * v^2. There is no g involved.

You would divide by g if you were using weight instead of mass because weight = mass * g, or alternatively, mass = weight / g. If you wanted to use the equation with pounds, you would use 1/2 * pounds / g * v^2 because pounds are a unit of weight. pounds = slugs * g.

In English units, you would use slugs for mass and fps for velocity. Grain is also a unit of mass, not weight.

1 slug is 225,218.3 grains. Dividing the mass in grains by 225,218.3 gives the mass in slugs.

1/2 * mass in grains * (1 slug/225,218.3 grains) * (velocity in fps)^2 = 1/(2*225,218.3) * mass in grains * (velocity in fps)^2 = 1/(450,437)* mass in grains * (velocity in fps)^2.

Doh! Thanks for catching my mistake. I hate flipping between SI and IP systems.

Ken