Watching derivative New Year television, I scoffed loudly when a single shotgun blast sent its recepient hurtling back a few feet through the air. But my Physics Major Friend, to my continuing surprise, DISAGREED.
He pishawed my blundered and incomplete citing of equal and opposite momentums (I being only a modest high-schooler). However, while I lack the expression, I intuit he is wrong - Dope me comprehensively, please.
First, it’d help if you’d cut the lingo when posing the question. What the heck is derivative TV? And, is it as :rolleyes: “legit” :rolleyes: as those reality shows…or what? Maybe that’s what your friend intuit knows but says not.
I’m sure your physics friend understands about recoil and conservation of momentum. But, could it be s/he scoffs at the TV source you quote?
There was a thread along these lines a few months ago. The upshot was that if the shotgun blast is going to send the victim flying backwards, then the shooter (more accurately the system composed of the shooter and the shotgun), being of approximately similar mass (within considerably less than an order of magnitude, anyway) ought to experience a similar force. But he doesn’t, and neither does the person being shot.
QED, I don’t quite follow…
Is what you’re saying mean that the shooter won’t feel any recoil, if the victim also doesn’t go flying?
No, I’m saying both the shooter and the victim will feel roughly the same force acting on them. Since the shooter doesn’t go flying back several feet, neither will the victim. Sorry if I was unclear.
Jinx - ‘Derivative television’ is xerox of a xerox, predictable and usually lame television - sorry if that’s not obvious - and the ensuing debate had nothing to do with the “legitimacy” of the programme, only the physics of the shotgun blast. Sorry if the post seems a bit glib.
QED - thanks for the outline, much appreciated
Could someone link that thread? I need details to bait him with.
Here.
And so it was shown…Thanks QED! Consider him humiliated. 
Another site that covers this topic (as well as a number of other movie physics fallacies) is Stupid Movie Physics. The relevant part is under the section ‘The Attractive Force of Glass’ in the second from the last paragraph.
I believe I found out about this site from someone on SDMB.
The actual energy delivered in a gunshots is in total, relatively small when applied to the whole of the mass of the human body, but of course that’s not what happens.
All the energy is directed across a very small area and this exceeds the structural strength at that particular point, hence penetration at that site.
In the case of high velocity rounds, the bullets does not transfer all its energy, it often passes right throught the target person, and its energy it spent on some other obstacle.
As QED points out, Newtonian physics proves the matter very conclusively.
Mention Newtons equations of motion, and ask how it is that the victim move further away than the person shootin, for the same force, recoil and impact absolutely must equal each other.
Thats just not true the shooter is expecting the impact and can brace for it. The victim on the other hand isn’t expecting it and thus gets knocked off his ass. If I walk up to you and push you from the side when you aren’t expecting it you will go flying but I will be in the same place.
Oops. Actually the whole section from the site I mentioned covers shotgun blasts - with equations even! That should please a Physics Major.
Making somebody fall backwards, or perhaps sprawling or stumbling, would be one thing. Making them ‘fly backwards’ in the bad-Hollywood fashion would need something else - LIFT! Assuming the gun is fired horizontally, no matter what the strength of the forces involved, there’s no reason for the victim to go upwards.
Equations!?
Get the following items.
A tape measure
A side of beef
A shotgun
Now find out if you can knock it back and up.
I guess it would depend on the size of the shotgun. Those that I have fired have a recoil, yes, but nothing that would’ve knocked me back several feet. However, I’m sure it’s possible to come up with a big enough shotgon that would.
At any rate, your point is well taken. And, depending on how far away for a person’s center of mass the hit is, the victim would be just as likely to be knocked over as he would be knocked back. (And, if shot from below, he might be knocked up. )
We’re talking about a shotgun here, not a rifle. The pellets are not going to pass all the way through the target. They will, indeed, transfer all of their energy.
Remember, also, that we’re dealing with appearances here. When you fire a shotgun, there are a number of recoil-absorbing things going on, in the gun and in your stance. When that load hits the target, he will probably not be braced for it. It’s highly likely that a blow of that intensity would knock him off his feet. If you stood completely unbraced and fired a shotgun that was firmly pressed against your ribcage, you’d definitely go flying.
That said, I agree with everyone who has said the impact of the pellets isn’t going to lift the recipient off his feet and propel him several feet up and back.
It’s the opposite of integral TV 
Just to be nitpicky, momentum is the more relevant quantity here, not energy. Momentum must be conserved during the impact, while KE is not (some will be converted to heat. Energy is conserved of course, but not necessarily as KE.)
This sort of pedantic hair splitting is practically REQUIRED of a physics major. The OP’s friend should hang his head in shame. He probably doesn’t even smirk when people talk about “cetrifugal force”.
Though the newtonian physics quite clearly shows that the forces to firer and target are very similar, it should be pointed out that the effect may well be quite different due to the relitive situation of either person. Consider trying to fire a shotgun whilst badly ballanced on a slope firing up the slope (or being shot in a similar situation) though the force is not huge it could easily be enough to break the ballance sufficiently to send the firer or shot person down the slope. Or in the same way that firing a gun whilst badly ballanced might knock you over, also being shot whilst badly ballanced may also knock you over quite easily.
Also someone who believes they may be about to get shot could well be throwing themselves to the ground or away from the gun as the shot is fired in an attempt to save themselves. Both of these facts lead me to believe the person shot will often appear to receive far more kinnetic energy than the person shooting.
Of course non of this would cause the oft seen image of a person being lifted into the air and thrown several feet into a glass wall that is the staple of many cheesy movie stunts.
You could wait until you are in a crowd at a bar and then start talking about the time you walked in on him…