Newtons law of motion and how it applies to recoil / impact

[Black Tail Hunter]

A coworker and I were recently having a conversation regarding a target I built that automatically stands itself up after you shoot it. He asked if the impact of the bullet would be sufficient to knock it down, which I replied I was pretty certain that my bullets retained energy should be enough to do it.

He replied that due to newtons 3rd law of motion that the bullet could have no more impact on the target than the butt of my rifle had in recoil against my shoulder.

I am admittedly not a physicist but due to various factors like bullet weight vs rifle weight and velocity this does not seem right, but I would be curious to know if anyone on the forum has better insight on this topic.

 

[Ingwe]

Great question...always wondered that myself

 

[ATH]

He's right.

"Every action has an equal and opposite reaction". The amount of energy the bullet has going forward will have equal energy working the other direction on the rifle.

However, this will be felt differently. The rifle is much heavier than the bullet, moves more slowly, and therefore the energy is imparted on your shoulder over a much longer timeframe than the bullet impacting the target. So while the recoil impulse won't knock you off the bench, the sharper energy discharge on target may cause the target to snap down more forcefully.

 

[Laelkhunter]

Isn't that where the term "foot-pounds of energy" comes from? If it generates 1000 foot-pounds of energy, that is enough energy to move 1000 pounds 1 foot? I seem to remember something like that. So if it generates more energy in foot-pounds that the target weighs, it should knock the target over? It makes my head hurt thinking about it. Someone else PLEASE explain it.

 

[Deleted member 103481]

Force = Mass x acceleration

However, you muddy the waters when you consider shape of recoil pad, material etc. i.e. Not all of that force is being applied to you, the shooter.

This does not include recoil from propellants out the end of your barrel. Which is why muzzle brakes work.

 

[Ingwe]

Either way it doesn't seem like the bullet impacts with THAT much energy...certainly not enough to knock a big animal off its feet....theoretically anyway

 

[Hand Skills]

This is an interesting question and the domain of time must be considered to accurately quantify the action/reaction here.

F=MA

Force = mass x acceleration

The 'magic' at play here is actually time. A bullet dumps its energy on a steel plate almost instantly, the rate of acceleration in this case is very high relative to the rate at which the bullet accelerates as it traveles down the rifle barrel.

Take ballistic gelatin (or a box of newsprint) for example - upon impact the bullet decelerates at a much lesser rate - compared to hitting the plate.

F=MA

So, for a given a mass (the bullet), force is directly correlated to the rate at which velocity changes.

Acceleration is an interesting concept. How much 'force' do you feel driving down the highway at 60mph? In this example we feel force only when the car speeds up or slows down. If a car accelerates from 0-60mph over 5 seconds, as part of the car, the driver will feel a lot of force during this acceleration. However, the driver will experience a lot more force if his car decelerates to 0mph in 0.5sec...

May the force be with you

 

[Black Tail Hunter]

Excellent input on this guys, I figured there would be some knowledge on this topic here. Any other thoughts?

 

[dmoon]

How do you calculate muzzle energy?
E = (M x V²) ÷ K where K = 450,435 and is derived from (2 x 32.1739 x 7000), M is the weight of the projectile, in grains, V is the velocity in feet per second and E is the energy in foot pounds.

 

[dmoon]

I believe that is the right formula, and then Newton's law would come into play because of the different masses (bullet vs rifle) and speeds of the bullet versus the rifle recoil in ft/second.

 

[J E Custom]

How about in layman's terms.

The reason the rifle does not produce the same amount of energy as the bullet energy, is the rifle to bullet weight ratio. the rifle weighs many times the bullets weight and even though the energy produced in both directions is the same, the recoil Velocity of the rifle is many times slower that the bullets velocity because of this difference. The bullet delivers its energy at the velocity produced. knock down ti based on mominum

With the rifle weighing many times more than the bullet, the bullet energy is measured in ft/lbs (Energy delivered in One foot distance). the rifle however,recoils far less distance to use up the bullets energy based on it's weight. The shooters weight and position will also affect the distance traveled by the rifle. In simple terms, If the rifle weighs 100 times the bullet weight, the recoil energy will be 100 times less in theory . In fact most rifles weigh hundreads of times what the bullet weighs.

The rifle is a low velocity projectile and the bullet is a high velocity projectile. They produce the same amount of recoil, but over a different time and distance.

If it were not for the weight of the rifle and its effect, we would not be able to shoot firearms with over 1 or 200 ft/lbs of energy. Muzzle brakes cannot change this energy, They work on the extra recoil produced by the gass by reducing the added recoil to the rifle.

J E CUSTOM

 

[Rich Coyle]

Let's take a sledgehammer let's say a 3 lb sledge hammer. We put the head of the Hammer against your shoulder with the handle sticking straight out from your shoulder push it at 20 feet per second. Then we take that same hammer turn it on its end so that the small part hits you on the shoulder and run it about a hundred feet per second and see if there's any difference.

 

[Edd]

A rifle that weighed thousands of times what a bullet weighs would be a very heavy rifle.

500 140gr bullets will weigh 10 pounds.

 

[Ingwe]

So what would a 308 impact in the sternum feel like....a hammer blow?