Newtons Third Law

[itchy trigger]

I was at a barbeque last night and was talking to an 80 year old, he knew I was a shooter, as is he, during the conversation he asked if I could answer a question that has been puzzling him for some years, he has tried to find an answer but has failed to find one that satisfies him.

He was an officer on merchant ships, some number of years ago he was in The Northern Territories, Australia, shooting with a native guide, it was then acceptable to shoot saltwater croc's, he used a 12g Remmi pump firing 1 ounce slugs, thought was there nothing outstanding in this, this is what the guide provided, so one day goes out for buffalo, same guide, gun and ammo, so shoots a buffalo, head shot at about 25 - 30 yards, knocks it back onto its rear haunches, in his words "sat it down like a dog" guide says give it another to be sure, so does.

Now this guy is a clever fella, so his question is, quoting Newtons Third Law of Motion, how can he, at a slim 5 foot 7 inches possibly be able to fire comfortably, a 1 ounce slug out of a 12 g that would knock a beast the size of a buffalo back onto its rear haunches, because the momentum (Newtons Law) of the recoiling gun is equal to the momentum of the slug, so the gun should have knocked him down as well, conversely if you could have held the butt of the gun onto the head of the buffalo and fired it, the buffalo would just have shrugged it off, it would not have been knocked back,

I said it would be down to the weight of the gun, quoting game guns at around 7lb and clay guns at around 81/4lb, saying the weight is enough to dampen the effect of the recoil, along with how I now use a 32" B725 black instead of my game EELL's to reduce recoil, but he was not happy with that.

So I hope someone can come up with an answer that will satisfy him.

Sorry I was a bit long winded

Thank You All for reading and for any answers

Edited June 6, 2016 by itchy trigger

[Andy RV]

http://www.saami.org/PubResources/GunRecoilFormulae.pdf

 

Have fun!

[timps]

Acceleration is different.

 

force = mass x acceleration

 

To get up to speed the distance and time travelled comes into play, it takes time and distance for the slug to get to up to its velocity down the barrel. Once the slug hits something the negative acceleration (deceleration) is astronomical as it tries to stop instantly, so the mass is times a greater number.

 

Think of it like this, to get up to 60 mph in a car the force on the body is negligible, if you were to drive the car at 60 mph into a stationary object such as a wall the deceleration would be astronomical and the force great.

 

So while the mass and velocity could be considered the same the acceleration is totally different.

[scotslad]

Also the recoil is spread across the gun butt which is relatively large compred to the same force being put into the shot/slug.

 

Also the fact it is a slug will be irrelevant, no different to shooting 1oz of 6's really, still firing 1oz mass.

Often knacker men will use bird shot (often in a 410) in a shotgun as behaves like a solid slug at close ranges without th danger from riccochets

[Katash]

The movement of the buffalo had more to do with its muscle reaction to being shot than the physical force of the slug moving it.

 

Mythbusters did a segment on this, are people being blown through windows in Holywood westerns when they get shot real?

 

They hung a human sized/weight dummy upright from a hook on its head with only a very light "sear" point keeping it there, and shot it with various guns - when hit the dummy simply crumpled to the floor.

[Katash]

[Milo]

The movement of the buffalo had more to do with its muscle reaction to being shot than the physical force of the slug moving it.

 

 

 

This is the answer. I have seen it with cape buffalo and even mature bull elephants weighing more than 10,000lbs. When brain shot the back legs sometimes collapse in such a way as for the entire animal to appear pushed backwards. The proof of the issue is that you never see the same effect when shot broad side to the extent that I have seen cape buffalo show very little reaction to the shot when hit in the heart/lung area with .458 Lott expanding ammunition generating about 5,000 ft lbs.

[scotslad]

On a sort of similar note to do with body mechanics, some stock (think horses) has a habit of falling forwards after being shot, most experienced knacker men stand to side for that reason.

 

I watched that mythbuster clip, bullet choice would also have a lot to do with it, if using FMJ/solid bullets just put a small hole right throu very little energy dumped, with a hunting/expanding bullet or even ballistic tip u might get more movement, and probably get most movement with a shotgun using smallish shot as no penetration and all energy would be dumped.

 

In theory a person/deer could still be knocked over/spun round but will depend on loads of things like how it was standing at that exact time of strike etc, have seen deer knocked clean off feet or spun round but also seen them hardly flinch or just fall down.

Edited June 8, 2016 by scotslad

[rjimmer]

The movement of the buffalo had more to do with its muscle reaction to being shot than the physical force of the slug moving it.

Just about sums it up.

[The Mighty Prawn]

I think it might have been on Mythbusters but somewhere I've seen them talking about how in war films and especially westerns made in the 50's people getting shot reacted very realistically as many men working on those films had been in the war and knew exactly what happens ie people just drop on the spot

 

Fast forward to the 80's and the ridiculous cannons of the action genre started using asian cinema wire work to catapult shot people backwards in a highly unreal way

[timps]

Unfortunately, where myth busters got it slightly wrong is by not totally understanding Newton’s laws of motion and relying on a school boys version in the monologue. They do apply but as I alluded to in my earlier post trying to decelerate so quickly by hitting a stationary object alters the equations for Newton’s laws (impulse momentum theorem). So saying for the body to travel 10ft the shooter must also travel 10 ft is not accurate.

 

It’s Momentum transfer that knocks something down and this is different when shooting the slug than when being hit by it.

This change in momentum is a force which is imparted into the body it hits. So, we need to take the time part of Newton's 3rd law then F=mA, Impulse=delta P, (the impulse-momentum theorem). The Impulse is the integral of the force over time, Force=-delta T. Meaning, Impulse = Change in Momentum

 

It’s quite easy to work out and prove how much this collision imparts by using the formulas from the impulse-momentum theorem.

The time taken is the important bit, if you catch an egg by holding your hand out stationary it smashes as the egg tries to change momentum instantaneously. Catch it whilst moving your hand backwards it takes longer for the change in moment it doesn’t smash.

 

Is it enough to move a body backwards? This is a whole other subject depending on all sorts of factors. If the slug hits something that has some elastic give such as flesh or Kevlar over flesh the time taken to change momentum will be gradual. With this gradual change the impulse will be low and doubtful if it would move anything backwards. If it hits dense or thick bone such as a skull, then this has less give so the time to change momentum will be a lot shorter hence the impulse will be greater and this might have the possibility to move the animal back.

 

So a large slug hitting solid bone with little flesh cover will give different results than hitting flesh or Kevlar over flesh. This is easily backed up and proved with the formulas.

 

Before anyone jumps down my throat about myth busters take time to read and actually understand impulse-momentum theorem. It’s all there for you to read on the net and it explains how it integrates with Newton’s laws of motion. If this wasn’t the case a hammer just wouldn’t work and be of no mechanical advantage at all.

 

Yes it’s a quiet day and I was bored.

Edited June 8, 2016 by timps

[Milo]

Unfortunately, where myth busters got it slightly wrong is by not totally understanding Newton’s laws of motion and relying on a school boys version in the monologue. They do apply but as I alluded to in my earlier post trying to decelerate so quickly by hitting a stationary object alters the equations for Newton’s laws (impulse momentum theorem). So saying for the body to travel 10ft the shooter must also travel 10 ft is not accurate.

 

It’s Momentum transfer that knocks something down and this is different when shooting the slug than when being hit by it.

This change in momentum is a force which is imparted into the body it hits. So, we need to take the time part of Newton's 3rd law then F=mA, Impulse=delta P, (the impulse-momentum theorem). The Impulse is the integral of the force over time, Force=-delta T. Meaning, Impulse = Change in Momentum

 

It’s quite easy to work out and prove how much this collision imparts by using the formulas from the impulse-momentum theorem.

The time taken is the important bit, if you catch an egg by holding your hand out stationary it smashes as the egg tries to change momentum instantaneously. Catch it whilst moving your hand backwards it takes longer for the change in moment it doesn’t smash.

 

Is it enough to move a body backwards? This is a whole other subject depending on all sorts of factors. If the slug hits something that has some elastic give such as flesh or Kevlar over flesh the time taken to change momentum will be gradual. With this gradual change the impulse will be low and doubtful if it would move anything backwards. If it hits dense or thick bone such as a skull, then this has less give so the time to change momentum will be a lot shorter hence the impulse will be greater and this might have the possibility to move the animal back.

 

So a large slug hitting solid bone with little flesh cover will give different results than hitting flesh or Kevlar over flesh. This is easily backed up and proved with the formulas.

 

Before anyone jumps down my throat about myth busters take time to read and actually understand impulse-momentum theorem. It’s all there for you to read on the net and it explains how it integrates with Newton’s laws of motion. If this wasn’t the case a hammer just wouldn’t work and be of no mechanical advantage at all.

 

Yes it’s a quiet day and I was bored.

And now so are we.

 

That's a very cheap joke; sorry. Great to learn the true cause behind an effect. Thanks.

[Dougy]

Would it make much difference if it was rifled or not.

 

 

 

Just wondering...

[fortune]

YEAH. But back when I was a kid the guns were a lot more powerful than now. Every time John Wayne shot the bad guy The Guy was either blown backwards through a window or door and JW was an excellent shot as well. He Was always shooting the gun out of the hands of the bady making him spin around and when the bady Was a dot on the horizon he could take him with a single shot.

[Good shot?]

YEAH. But back when I was a kid the guns were a lot more powerful than now. Every time John Wayne shot the bad guy The Guy was either blown backwards through a window or door and JW was an excellent shot as well. He Was always shooting the gun out of the hands of the bady making him spin around and when the bady Was a dot on the horizon he could take him with a single shot.

 

On horseback at a gallop.

[figgy]

Do rabbits get knocked into the next field when shot do pheasants suddenly go backwards when hit with shot so why would anyone think a human weighing a lot more would get flung far and wide.

Only way to move an object is to hit it with more weight in energy than it weighs. If someone wears body armor and gets hit with a big bullet it will move them.

 

On firing a gun the projectile is pushed forward on a cushion of expanding gas so it's accelerating and you are somewhat removed from the full affect by that gas.

Edited June 10, 2016 by figgy

[James19306]

What if the person shot with a 1 oz slug was wearing a bullet proof vest?? Would the fact that the slug stops suddenly transmit more or less force to the shot person or the person who fired the gun?

[The Mighty Prawn]

What if the person shot with a 1 oz slug was wearing a bullet proof vest?? Would the fact that the slug stops suddenly transmit more or less force to the shot person or the person who fired the gun?

Clearly the most force is with the stopping, the person starting gets essentially a fast shove as the gas expands to push the slug forwards, but the person on the end would have all the energy dump virtually instantly

[timps]

What if the person shot with a 1 oz slug was wearing a bullet proof vest?? Would the fact that the slug stops suddenly transmit more or less force to the shot person or the person who fired the gun?

Just to give a rough idea on impulse felt by the person/animal shot compared to that of the shooter.

 

Impulse is the change in the momentum of a body caused over a very short time. So m is the mass and v and u the final and initial velocities of a body.

 

Impulse = Ft = mv – mu

 

Force(N) time(s) impulse (N*s) momentum change mass kg velocity change (M/s)

4000n 0.010 -40 -40 kg*m/s 10kg -4

400n 0.100 -40 -40 kg*m/s 10kg -4

 

The figures used above are low so it is easier to understand but you can see that an increase in the time taken to change momentum directly changes the force required.

 

Therefore extending or decreasing the time of the collision will extend or decrease the impact force by the same factor.

 

If you input the figures for a slug and say it took 1 second to reach maximum velocity in the gun barrel, then that slug took 0.010 seconds to stop due to impact the difference in force is a factor of 100.

 

In the above circumstances a 10N force applied to the shooter would now be 1000N applied to what it hit.

 

Now the above is just used for a simple example on the equation of the force required, altering the time must alter the force by the same factor, in very simple terms the force felt by the target must be more if it stops the slug dead in a shorter period of time than it took to accelerate it up to velocity.

 

In terms of knocking someone back 10 feet a lot of the bullets energy would be dissipated in deformation of the bullet and target, as well as heat and a whole host of other factors. The only thing that is guaranteed to happen is the energy put in must be the same as the overall energy put out, how that energy is broken down (heat, deformation, force etc) will determine how much goes into knocking someone back if at all.

Edited June 15, 2016 by timps