Physics question (About Guns)

SixPapaCharlie

SixPapaCharlie

May the force be with you
Joined
Aug 8, 2013
Messages
16,415
Display Name
Display name:
Sixer
Einstein taught us that for every action there is an equal and opposite reaction.

So when I fire a gun, the gun hits me with the same amount of force that the bullet has. True?

It would seem that the bullet has more energy but that wouldn't be possible. As Alexander Graham Bell taught, the amount of energy in a system is fixed. It can not be created nor destroyed, it can only change forms. I believe that is the 3rd law of turbo dianetics.

Anyway is it true to say I am being hit with the same force as my victim?

Actually I would be getting harder because my victim will have been hit by a bullet that has lost some energy due to the law of diminishing returns illustrated on the Kinsey scale.

Now the gun hitting me is applying the same amount of force but distributed over a larger surface area so I end up intact, Just a little bruised.

If the back of a gun were filed into a point the size of a bullet, when I fired it, the gun then should have the same (or even worse) effect on me than the recipient of my bullet.

I feel like some energy is missing somewhere. The bullet seems to be doing orders of magnitude more damage than a gun and the surface area difference doesn't seem large enough that the difference in damage on both sides of the equation would be so different.


And by the way, I was just kidding. I know it wasn't Einstein. It was Thomas Edison.
 
It's both a difference in mass and surface area. In general a lighter gun of the same caliber and load will feel like a worse kick than a heavier one. Unless you're a delicate snowflake.

And lay off the booze. Especially during the day.
 
Alexander Graham Bell was the guy that discovered you could fire a shotgut at a bad guy, one handed, and the other guy would get blown backwards 20' through a window.
 
the energy is used and/or dissipated by the ejection mechanism. sometimes a gas ejection system is used like on my remington 11-87. sometimes, like in O/U or side by sides, you just get black and blue.
 
Hadn't considered mass. I have to think about that.
A cannon ball has roughly the same (slightly less) surface area than the back of the cannon tube but certainly the cannon has significantly more mass.

The ball does a ton of damage but the cannon doesn't destroy anything.
Equal and opposite is true but in the cannon the energy is not enough to move the cannon.

I guess since you are "a part" of the gun, the energy is the same as the bullet but it is not enough energy to move you.

I need to think on this.
 
Matthew said:
Alexander Graham Bell was the guy that discovered you could fire a shotgut at a bad guy, one handed, and the other guy would get blown backwards 20' through a window.
Click to expand...

He discovered "Action Movie Physics"? Wow. Right up there with "Western Movie Physics" that allow the hero to shoot uphill with his short barreled 6 shooter and pick the bad guy (who hasn't been able to hit him firing downhill with a rifle) off. Maybe that was Thomas Edison?
 
jsstevens said:
He discovered "Action Movie Physics"? Wow. Right up there with "Western Movie Physics" that allow the hero to shoot uphill with his short barreled 6 shooter and pick the bad guy (who hasn't been able to hit him firing downhill with a rifle) off. Maybe that was Thomas Edison?
Click to expand...

You are thinking of Jung.
 
As stevens noted neglecting friction m1a1 = m2a2 can be restated as
a1 = (m2/m1)a2 so the bigger you are compared to the bullet the less acceleration you will experience.
 
To dumb the answer down, for folks like me, the answer is that if the cartridge was fired from a gun not much larger or heavier than the cartridge itself, then there would be one helluva kick. But fired from a much larger and heavier gun the gun will absorb much of the recoil. To see this in action fire a .22 from a very small handgun and notice the large amount of recoil. Then fire a .22 from a Marlin rifle and notice that there is almost no recoil.
 
yup....
How-Rifle-Recoil-Works.png
 
SixPapaCharlie said:
Hadn't considered mass. I have to think about that.
A cannon ball has roughly the same (slightly less) surface area than the back of the cannon tube but certainly the cannon has significantly more mass.
The ball does a ton of damage but the cannon doesn't destroy anything.
Equal and opposite is true but in the cannon the energy is not enough to move the cannon.
I guess since you are "a part" of the gun, the energy is the same as the bullet but it is not enough energy to move you.
I need to think on this.
Click to expand...
Au contraire on the italicized statements above!! If you've ever seen a cannon fire, there is a LOT of movement! The if you ever visit Old Ironsides, you will see that the cannons had a pulley system to get them back in place after being fired. If you watch war movies, you will see the cannons recoil.
If you shoot a gun, you will definitely feel the recoil! You are (typically) prepared for it, so it doesn't knock you over, but you do move! The first time I shot skeet with a shotgun, I had a bruised and sore shoulder for a week!!
 
Mine has hit the green stage of bruising. It is ugly.
 
jsstevens said:
He discovered "Action Movie Physics"? Wow. Right up there with "Western Movie Physics" that allow the hero to shoot uphill with his short barreled 6 shooter and pick the bad guy (who hasn't been able to hit him firing downhill with a rifle) off. Maybe that was Thomas Edison?
Click to expand...
Now just wait a minute, Pilgrim.
 
SixPapaCharlie said:
Mine has hit the green stage of bruising. It is ugly.
Click to expand...

A few years ago I was on heavy duty anti-inflamitories for some carpal tunnel issues. (Worked like a charm with the wrist brace.) I went and played paintball with folks from work. Normally I don't bruise from paintball (and, really, I don't get hit much). This time I got a bad "friendly fire" hit on the back of my shoulder from ~2 feet and a <6 feet shot on my right buttock. I had huge, I mean YUUGE! weeping bruises for a couple of weeks. I never made the mistake again.

John
 
SixPapaCharlie said:
Hadn't considered mass. I have to think about that.
A cannon ball has roughly the same (slightly less) surface area than the back of the cannon tube but certainly the cannon has significantly more mass.

The ball does a ton of damage but the cannon doesn't destroy anything.
Equal and opposite is true but in the cannon the energy is not enough to move the cannon.
Click to expand...
As Dennis pointed out, this is WAY not true. Are you familiar with the term "loose cannon"? Old nautical term referring to what would happen if the breeching ropes failed when the gun fired. Ordinarily, the cannon (usually) didn't destroy anything because the recoil was expected and planned for.

Historical reports indicate that the recoil would sometimes make the guns jump clear of the deck...not bad, for a ton and a half of iron. And yes, sailors were mangled and killed from not getting out of the way.

The recoil was useful, as it threw the gun back inboard and made it easier to swab out and reload. But running that mass of iron out after reloading took work, you might have eight or a dozen sailors on the training tackle.

Some guns had fixed carriages (carronades) but the gun itself would be set atop a sliding element, typically greased with the fatty residue produced when the salt beef was boiled (known as "slush"). However, the carrronades were less efficient; shorter barrels, much bigger calibers. The intent was to fire a big, heavy cannonball slowly, on the believe it would create more damage.

Ron Wanttaja
 
Recoil begins when the bullet begins to move, too. I once had a guy offer to let me try his .454 Casull pistol. I naively held it like I was used to holding a .45 wad gun with soft target hand loads. The gun moved enough before the bullet left the barrel that the bullet impacted the paper at about 11 o'clock in or beyond the outside ring. Lesson learned.
 
ChemGuy said:
next time hold the gun farther from your shoulder so it will decelerate more. It may not even touch your shoulder, unless your a pansy. best way to try it is a 3"magnum 12ga shell.
Click to expand...

^^Note to self for Bryan's next outing....

...I got some slugs around here somewhere...
 
If you fire a gun such as a revolver, the energy is dissipated into your hand and would be approximately equal to the force in the bullet. The difference between the bullet and the gun handle is the area over which the energy is dissipated. The force from the gun to your hand is spread over a few square inches, enough so it shouldn't damage you. The bullet on the other hand dissipates its energy over a very small area, generally to very messy results.
 
Then there's this classic.

.577 Tyrannosaur IIRC

 
Just to clarify that "equal and opposite" refers to momentum (aka inertia), not energy.
So depending on the relative masses of the gun and bullet, you could well have vastly more energy in the bullet than in the recoiling gun.
For example, say the gun's mass is 1 and bullet is 1/100 (arbitrary mass units), and let's say the gun recoil velocity is 100 (again arbitrary velocity units). To conserve momentum (mass * velocity), the bullet velocity will be 1*100*100 = 10000, i.e. 100 times the gun recoil (this is assuming an unattached gun). On the other hand, since kinetic energy is a function of the square of the velocity, the bullet will have 100000000/100 = 1000000 energy units, while the gun recoil will have 10000 units, i.e. the bullet will have 100 times more energy than the gun's recoil.
So OP's intuition is actually correct: the bullet, travelling much faster, will in fact have a lot more energy ("ability to do work", or damage), since the conservation rule applies to the recoil's inertia matching the bullet's inertia, not its kinetic energy.
(Feel free to check/correct my math.)
 
Last edited:
SixPapaCharlie said:
Hadn't considered mass. I have to think about that.
A cannon ball has roughly the same (slightly less) surface area than the back of the cannon tube but certainly the cannon has significantly more mass.

The ball does a ton of damage but the cannon doesn't destroy anything.
Equal and opposite is true but in the cannon the energy is not enough to move the cannon.

I guess since you are "a part" of the gun, the energy is the same as the bullet but it is not enough energy to move you.

I need to think on this.
Click to expand...

Cannons do roll back with a lot of force. New systems have shocks to dissipate the energies, but the old style cannons had to be on wheels or they would brake the stand. Try to stand behind a firing cannon and it won't be all that different than standing in front of it
 
RotorDude said:
Just to clarify that "equal and opposite" refers to momentum (aka inertia), not energy.
So depending on the relative masses of the gun and bullet, you could well have vastly more energy in the bullet than in the recoiling gun.
For example, say the gun's mass is 1 and bullet is 1/100 (arbitrary mass units), and let's say the gun recoil velocity is 100 (again arbitrary velocity units). To conserve momentum (mass * velocity), the bullet velocity will be 1*100*100 = 10000, i.e. 100 times the gun recoil (this is assuming an unattached gun). On the other hand, since kinetic energy is a function of the square of the velocity, the bullet will have 100000000/100 = 1000000 energy units, while the gun recoil will have 10000 units, i.e. the bullet will have 100 times more energy than the gun's recoil.
So OP's intuition is actually correct: the bullet, travelling much faster, will in fact have a lot more energy ("ability to do work", or damage), since the conservation rule applies to the recoil's inertia matching the bullet's inertia, not its kinetic energy.
(Feel free to check/correct my math.)
Click to expand...

It's been awhile, but I'm pretty sure "equal and opposite" refers to energy, in other words if 500 joules is applied to a bullet, there is also 500 joules applied to the gun, pretty much the same principle as a rocket engine in space, propellant being ejected out of the engine causes an equal and opposite force applied to rocket which accelerates it through space.
 
PaulS said:
It's been awhile, but I'm pretty sure "equal and opposite" refers to energy, in other words if 500 joules is applied to a bullet, there is also 500 joules applied to the gun, pretty much the same principle as a rocket engine in space, propellant being ejected out of the engine causes an equal and opposite force applied to rocket which accelerates it through space.
Click to expand...

You are right about one point, it is the same principle that applies to rocket propulsion (or any propulsion, for that matter). But it's not energy, it's momentum (or inertia) which is conserved in this case (in a useful way, by being equally divided between the two moving bodies since it's a vector quantity). Energy is conserved too, of course, but that only means that the total original gunpowder chemical energy got converted into heat, sound and kinetic energy, of both the recoil and bullet (as well as escape gases), not necessarily in equal amounts.
 
Last edited:
One other thing to consider is time. A round of ammo, moving at 2,000 FPS decelerates to 0 FPS as it impacts the target. Let's say it stops after a 1' penetration and averages 1,000 FPS from impact until full stop. All of that energy is concentrated on one bullet shaped "point" over 1/1000th of a second. Which is what a projectile is supposed to do - concentrate force at a time and place, doing damage in the process.

On the other hand, the gun recoils at a fraction of the speed of the projectile - say 20 fps. Your body absorbs that recoil in a quarter of a second, and the force is spread across the entire footprint of the pistol grip or the butt stock where the firearm contacts your body. You've spread the force across a lot more time and area compared to the impact of the bullet...
 
kyleb said:
One other thing to consider is time. A round of ammo, moving at 2,000 FPS decelerates to 0 FPS as it impacts the target. Let's say it stops after a 1' penetration and averages 1,000 FPS from impact until full stop. All of that energy is concentrated on one bullet shaped "point" over 1/1000th of a second. Which is what a projectile is supposed to do - concentrate force at a time and place, doing damage in the process.

On the other hand, the gun recoils at a fraction of the speed of the projectile - say 20 fps. Your body absorbs that recoil in a quarter of a second, and the force is spread across the entire footprint of the pistol grip or the butt stock where the firearm contacts your body. You've spread the force across a lot more time and area compared to the impact of the bullet...
Click to expand...

That's true, but bear in mind that the bullet has orders of magnitude more energy compared to the recoiling gun, per my above post.
 
RotorDude is correct and gave the most correct explanation. Another way to say it: we're more familiar with kinetic energy in the form (1/2)mv^2, but it can also be written as (p^2)/2m where p is momentum. Bullet and gun have the same (and opposite) momentum, but the bullet, having much less mass, has much more kinetic energy and therefore way more potential to do damage.
 
Last edited:
RotorDude said:
Just to clarify that "equal and opposite" refers to momentum (aka inertia), not energy.
Click to expand...

To be pedantic, Newton's third law refers to force, not momentum. In a collision of two objects, there will be an equal and opposite force applied to each object. If they have different masses, then the resulting acceleration of each object will be different. Remember that force involves mass and acceleration whereas momentum involve mass and velocity. Conservation of momentum can be derived from Newton's third law, but "equal and opposite" is not talking about momentum.
 
You must log in or register to reply here.
Back
Top