Muzzle Energy Formula in Ft. Lbs.
- Thread starter Mr. Mosin
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Docendo Discimus
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Here is the link to the ft lb formula that Saami uses:
https://saami.org/glossary/kinetic-energy-formula/
It looks like yours is pretty close. The GC is a little different.
https://saami.org/glossary/kinetic-energy-formula/
It looks like yours is pretty close. The GC is a little different.
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F-111 John
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I thought I was somewhat physics and mathematics literate, but what does gravitational constant have to do with calculating kinetic energy?
mcb
Member
Yes your formula is right. The 450240 might be off a touch. It would appear that someone used 32.16 ft/sec^2 for the acceleration due to gravity when most use 32.174 as the standard value. Which results in 450436 the same as the SAAMI equation posted above.
KE = 0.5 * m * V^2
m has to be in slugs to correctly get ft-lbs. So if your bullet weight is in grains you need to divide by 7000 gr/lb to convert it to lb and then divide the lb by the acceleration of gravity (32.174 ft/sec^2) to covert it from lb (weight/force, base units being slug-ft/sec^2) to mass (slugs). If you absorb the 0.5 into to those two conversions you get the 450436 number.
Obviously velocity need to be in ft/sec to be compatible with the mass.
I personally just remember the number 4.44e-6 and multiply that onto the bullet weight in grains and this converts it to mass in slugs. Then plug that into the 0.5mV^2
KE = 0.5 * m * V^2
m has to be in slugs to correctly get ft-lbs. So if your bullet weight is in grains you need to divide by 7000 gr/lb to convert it to lb and then divide the lb by the acceleration of gravity (32.174 ft/sec^2) to covert it from lb (weight/force, base units being slug-ft/sec^2) to mass (slugs). If you absorb the 0.5 into to those two conversions you get the 450436 number.
Obviously velocity need to be in ft/sec to be compatible with the mass.
I personally just remember the number 4.44e-6 and multiply that onto the bullet weight in grains and this converts it to mass in slugs. Then plug that into the 0.5mV^2
mcb
Member
Remember in US customary units we most often report things in weight rather than mass. F=ma Your scale is measuring the force that gravity is pull down on that mass. Grains and pounds are units of weight or force not mass. The kinetic energy equation needs the mass of the object in motion. To convert pounds to mass you divide by the acceleration due to gravity and the resulting units are slugs in US customary units for mass.
With SI (metric) units your scale reports mass (grams) with your scale doing the division by the acceleration of gravity internally (9.80665 m/sec). SI units for force are Newtons (N) and SI units for Mass is grams(g) F=ma again.
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mcb
Member
For most things that is plenty close. My 4.44e-6 is a round off to but is easy to remember and gets me close enough for most things.
I've always used the one in the OP - thought it was right.
(V^2)(M)÷450240= ft. lbs. energy
All these calculations were using average velocity from 5-6 shots through my chrono and formula in OP.
Kahr PM9: Corbon 115 +P JHP @ 1,264 fps / 408# KE
Glock 19: Federal HST 124 gr. +P @ 1,210 fps / 403# KE
Glock 19: Federal HST 147 gr +P @ 1,044 fps / 356# KE
Glock 35: Federal HST 180 @ 1,019 fps / 415# KE
Those are all bullets I carry.
They are all above 350# muzzle energy, coincidence? Nice, but muzzle energy alone is not it.
They all penetrate at least 12'' and consistently expand, which is my primary selection criteria.
This bullet has more muzzle energy than the previous loads but I don't carry FMJ in any caliber.
Glock 35 with 357 Sig Barrel: 125 PPU FPJ @ 1,461 fps / 593# KE
(V^2)(M)÷450240= ft. lbs. energy
All these calculations were using average velocity from 5-6 shots through my chrono and formula in OP.
Kahr PM9: Corbon 115 +P JHP @ 1,264 fps / 408# KE
Glock 19: Federal HST 124 gr. +P @ 1,210 fps / 403# KE
Glock 19: Federal HST 147 gr +P @ 1,044 fps / 356# KE
Glock 35: Federal HST 180 @ 1,019 fps / 415# KE
Those are all bullets I carry.
They are all above 350# muzzle energy, coincidence? Nice, but muzzle energy alone is not it.
They all penetrate at least 12'' and consistently expand, which is my primary selection criteria.
This bullet has more muzzle energy than the previous loads but I don't carry FMJ in any caliber.
Glock 35 with 357 Sig Barrel: 125 PPU FPJ @ 1,461 fps / 593# KE
mcb
Member
It is right, or right enough, it just used a slightly different constant for gravity. The difference is less than .05%. The modern common standard is 32.174 ft/sec^2 but using 32.16 32.174 ft/sec^2 is close enough and no double correct at some points on the surface of the planet. We think of the acceleration of gravity as a constant but in reality it varies across the surface of the planet due primarily to variations in the density in the planets crust and altitude plays a secondary role too. Acceleration due to gravity at the surface can vary from as little as ~32.033 ft/sec^2 to as high a ~32.263 ft/sec^2. Most of the time using the the standard/average is close enough. Sometimes is does matter and there are maps you can use to get a fairly accurate estimated and even specialized equipment to measure the local gravity exactly for a specific location. Most old pendulum clocks had a moveable weight in their pendulum to tweak the effective length of the pendulum to tune the rate at which the clock ran for the specific local gravity.
-rambling...
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F-111 John
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Mauser fan
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This is a great topic for the hand loading forum.
Vern Humphrey
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The formula calls for "Mass" which is not the same thing as "weight". Mass is calculated by dividing the weight by the Gravitational Constant (32.3 F/S/S)
Demi-human
maybe likes firearms a little bit…
This calculating is wonderful and all, but I can think of very few times I’ve shot something touching it with the muzzle…
Of course, those few times were quite memorable!
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