Muzzle Energy Math

[Action_Can_Do]

Hello everyone. I believe there is an equation to figure out muzzle energy through a bullets velocity and weight. Does anybody here know it?

 

[max popenker]

E = M* V**2 / 2
that is, Kinetic Energy is Mass multiplied by Velocity squared and divided by two.

And I've thought they teach that in elementary school.

 

[Davo]

they teach it in college physics, trust....

 

[max popenker]

they teach it in college physics, trust

d@mn, i knew that our educational systems are different, but i thought it was not THAT much - we were taught on kinetic and potential energy, job, force and such in 7th or 8th grade...
On the other hand, and back to topic, i (and all my classmates, girls included!) was taught on basic infantry squad tactics and basic AK handling on 9th grade, and first shoot 7,62mm AK as a part of the school lesson on 10th grade
Old good Soviet days.
Now, no teenager can tell CS from VX or Zarine by its effect , and does not know his or her size of gas mask. Nor they can dissassembly AK in 35 seconds and reassembly in 40

PS i graduated from school in 1989, 2 years before fall of USSR

 

[BIGDADDYLONGSTROKE]

Man, I shoulda gone to school in Russia, but then again were I went to school if we forgot to take a rifle off of our gun rack in our trucks, usually theyd just tell us to take it home. Now though Im sure there would riminal charges and jail time.

 

[Nightcrawler]

Now, no teenager can tell CS from VX or Zarine by its effect , and does not know his or her size of gas mask. Nor they can dissassembly AK in 35 seconds and reassembly in 40

Kids these days!

I learned how to field strip an M16 when I was fifteen years old, a Freshman in high school. I was in Junior ROTC and we had a Nat'l Guard Armory right next to the school, so we got to use their stuff sometimes.

FWIW, I was taught all that in middle school as well. I just don't remember any of it.

 

[Davo]

I know it, but it escapes me at the moment...and believe me it is anything but common knowledge in southern california these days.

 

[Davo]

perhaps mv2xweight in grains/450400=ft lbs? crap Jill would know

 

[razorburn]

E = M* V**2 / 2
that is, Kinetic Energy is Mass multiplied by Velocity squared and divided by two.

And I've thought they teach that in elementary school.

So what unit of kinetic energy is that? Not much help if that's not provided. The one davo provided is the formula for converting to ft-lbs. Close enough, anyways. I've sometimes seen it as 450000, 450250.

 

[max popenker]

razorburn, the formula is universal
It will give you ft*lbs if using velocity in FPS and weight in Lbs, or it will give you Joules if using meters per second and kilograms

The 450000 thing is just to use grains instead of pounds.

 

[LaEscopeta]

So what unit of kinetic energy is that? Not much help if that's not provided.

Very true. But the above equation works for ALL units. You decide the units for mass and velocity, and that tells you the units energy will be. If you decide to use an inconsistent system of units (like feet per second and grains) you have to use a conversation factor to get to decent energy units, and since you picked the input units you should be able to figure out what that factor is.

Some places have used the consistent SI units (the meter/kilogram/second etc) universally for so long everyone automatically knows what the units are, without having to have them stated. Sort of like no one here in the US needs to say “inches” when they say .22 caliber; everyone knows the units.

It will give you ft*lbs if using velocity in FPS and weight in Lbs, or it will give you Joules if using meters per second and kilograms

Well, sort of. In the inconsistent English system of units we have lbs for force and slugs for mass. If you use lbs for the weight of the projectile you have to divide by 32.2 to get the mass unit of slugs before you get ft-lbs for energy. Anyone who never had to deal with science in English units should feel lucky.

 

[bhk]

(weight of bullet in grains)(velocity in fps squared)
450240


Above formula takes care of all conversions and gives energy in foot-pounds, the unit we shooters are used to.

 

[Threeband]

This is Correct, and WORKS

Here's the correct way to figure kinetic energy for firearms.


(Velocity in feet-per-second SQUARED) times (Bullet weight in grains)
-------------------- divided by ---------------------------
450240

But if you try it on a regular eight-place calculator, it won't work. It'll overload the calculator as soon as you multiply in the bullet weight.

So here's the work-around:

Divide the bullet weight by 10, then multiply the "final" result by 10


So here's how to do it. (And I used to do this SO freaking much, like 20 years ago, that I have "450240" burned in my memory.)

What's the energy of a 230 grain bullet at 880 fps?

(880 x 880) = 774,400

(230 grains) divided by 10 is 23. ---So:


(774,400) times (23) equals 17,811,200.

17,811,200 divided by 450240 is 39.559...

39.55... times 10 equals 396 foot-pounds.

 

[ATAShooter]

Muzzleloader x Too much smoke = "Fuzzy Math"

 

[max popenker]

Well, sort of. In the inconsistent English system of units we have lbs for force and slugs for mass. If you use lbs for the weight of the projectile you have to divide by 32.2 to get the mass unit of slugs before you get ft-lbs for energy. Anyone who never had to deal with science in English units should feel lucky.

Well, then I'll stick to my 7,62mm and leave all those .300's, .307's and .308's to you Americans
To be honest, when i have to calculate ME in Ft*Lbs i use metric measures and then simply multiply result by 1,36. Works for me every time.
But then, i was born metric

 

[Deavis]

Wikipedia is your friend:

http://en.wikipedia.org/wiki/Muzzle_energy

 

[benEzra]

To convert grains to kilograms:

(weight in grains / 7000) *.45359237 = bullet mass in kg

ft/sec times 0.3048 = m/sec

Kinetic energy (in joules) = 0.5 * M * V^2

Convert joules to ft/lb: J * 0.7375621493

You could combine some of those terms and get a simpler formula if you want only the energy in U.S. customary units. I personally prefer joules...

 

[Threeband]

[/ If you decide to use an inconsistent system of units (like feet per second and grains) QUOTE]


There is nothing "inconsistent" about the English system.

A Rod is ALWAYS 5 and-a-half yards.
An acre is ALWAYS 43,560 square feet.
And of course, 100 fathoms is ALWAYS an ordinary cable's length.
(though in the U.S. Navy it's ALWAYS 120 fathoms, and in the
British Navy, it's ALWAYS 101 and-a-third fathoms.)

It should go without saying that 4 pecks are ALWAYS a bushel.
What could possibly be more consistent?

 

[Action_Can_Do]

That is a lot of info. Thanks everybody.

 

[Cesiumsponge]

I think he means its inconsistent because SI units are based off magnitudes while the English system is based off seemingly arbitrary conversions and jumps likely established through historical reasons.

1m = 100cm = 10^3mm =10^6nm, = 10^-3km

1L = 1000cm^3 = 10^3mL = 1^-1m^3, and so on.

It allows you to simply deal with linear unit conversions using various powers of 10 and different units from the base use a universal prefix system.

Then you have this crap to memorize:

1 mile = 5280 feet = 1760 yards = 63360 inches = 0.3333 leagues = 8 furlongs = 320 perches = 15840 hands = 8000 links = 760300 lines.

Bushels, leagues, furlongs, rods, perches, links, lines, stones, pounds, ounces, drams, grains, short tons, long tons, townships, acres, all with arbitray conversion constants to memorize.

The English system even attempts to emulate SI by creating things like the Rankine scale and kips. I grew up with the English system so pounds and feet make more sense than kilograms and centimeters, but when doing physical calculations using formulas, its much easier to use SI and convert the answer to English rather than trying to convert each variable as you go through.

 

[Donut]

Wow, It looks like I always did it the hard way,

[(bullet weight/437.5)/16]/2.205 = M (In kg)
Velocity/3.28 = V (in meters per second)

[(.5M)(V^2)] x 0.74 = Muzzle energy in Ft-Lbs

 

[terry264]

actioncando, the easiest formula to remember is:
muzzle vel. squared, divided by 450,436, multiplied by bullet wt. in grains.

 

[Vern Humphrey]

E = M* V**2 / 2
that is, Kinetic Energy is Mass multiplied by Velocity squared and divided by two.

And I've thought they teach that in elementary school.

So what unit of kinetic energy is that? Not much help if that's not provided. The one davo provided is the formula for converting to ft-lbs. Close enough, anyways. I've sometimes seen it as 450000, 450250.

Muzzle energy (in the US) is usually given in foot-pounds, which is defined as the amount of energy needed to lift X pounds one foot. (Or to lift one pound X number of feet.) (But note also that bullets move so fast that anything you try to "lift" will be penetrated before any lifting occurs.)

Note that the "M" in the equation is Mass, not weight. To convert weight to mass, you must divide by the gravitational rate of acceleration, 32.2 Ft/sec/sec.

Let's take a 150 grain bullet at 3,000 fps. First we convert the bullet weight to pounds -- 150/7000 (there are 7,000 grains in a pound.) Now we convert weight to mass -- 150/(7000 * 32.2)

Now we apply the formula -- 1/2 * 150/(7000*32.2) * 3000^2 and get 2,994.6 Ft/lbs.

 

[Donut]

According to this site: Weight vs Mass, What's the difference?

by converting the weight of the bullet into Kg, the conversion to mass from weight is already incorporated...

So wouldn't a simple: (Bullet weight/7000)/2.205 = a proper mass unit for conversion into Joules?

Then a quick (fps)(.3048) for the velocity.

Then Joules x 0.74 = ft-lbs

Right? I'm confused by the mass/weight thing of pounds and Kg. This is why I always stuck to chemistry. Physics makes my head hurt.

 

[waynedm]

Splitting hairs, but I've seen it both using the 450400 number and 450240. Which one is correct?