Small one man space fighters

[vitalis6969]

I've seen video of soldiers in Iraq knocked off their feet by rifle hits stopped by their armor, so I think I will disagree on this point.

12.7mm machine gun bullets, weighing in at over fifty grams, or shotgun slugs massing in at well over that can and have if they strike body armor.

It isn't the inertia of the bullet knocking them over, it is the shock of being struck. It is simple physics, a bullet will not knock someone over. The shock can drop them in their tracks.

As for the .50 cal, it won't be stopped by body armor so that isn't really applicable to knocking someone over. It will pass clean through the armor and trauma plates, and of course, all the squishy bits inside.

Depends on how many megajoules we are talking. For starship weapons? We could be talking hundreds. At four-and-change megajoules being one kilo of TNT equivalent, yes, I bet they could. And it depends on what we mean when we say "move". Moved as in the whole ship shifting? Or moved as in a nasty shockwave reverberating through and flexing the ship hull and interior components?

Is the 1 kilo of TNT equivalent in energy or explosive power? Not being a jerk, actually wondering.

-V

 

[vitalis6969]

Just like a beam of coherent light, vaporizing armor plate, will never move a ship massing in at 10+ tons.

Laser and maser are considered as means to accelerate solar sail craft
with a mass of considerably more than 10 tons, so I would not bet that
a megawatt laser would be unable to move a starship, especially under
zero-g conditions.

Using the laser to solar sail theory, wouldn't the firing ship be moving just as much but in the opposite direction?

-V

 

[vitalis6969]

Sorry for the triple post, but again back to the bullet.

Mythbusters also proved the bullet will not knock over the person thing, in addition, if a bullet had the force to knock over a person, it would knock over the person firing the gun as well.

Oh, and there is this: f= 1/2 (mv) squared

And also this: http://www.youtube.com/watch?v=aaS_2l8nGdg&feature=player_embedded

-V

Still digging up the info on the laser.

 

[DFW]

Depends on how many megajoules we are talking. For starship weapons? We could be talking hundreds. At four-and-change megajoules being one kilo of TNT equivalent, yes, I bet they could. And it depends on what we mean when we say "move". Moved as in the whole ship shifting? Or moved as in a nasty shockwave reverberating through and flexing the ship hull and interior components?

Actually, it ALL depends on the amount of kinetic energy imparted.

KErelativistic = m γ c2 - mc2

where γ = 1/(√(1-v2/ c2), 'c' is the velocity of light, 'm' is the mass of the object, v is the velocity of object according to a reference frame and 'c' is velocity of light. To calculate KE, just substitute values in this formula.

Since photons have no mass. Nada will happen.

Space sails work by http://en.wikipedia.org/wiki/Radiation_pressure

 

[rust]

Using the laser to solar sail theory, wouldn't the firing ship be moving just as much but in the opposite direction?

I do not think so, as the photons of the laser beam do not have mass.

Still, the radiation pressure of a megawatt laser should be sufficient to
move the target ship, although of course not much.

A second mechanism that could cause even more movement than the
radiation pressure could be the vapourizing of the armour, because
this vapour is streaming away from the target ship, and it does have
mass.

 

[rinku]

The explosion caused by the laser vaporising hull material will have an effect on the ship's momentum, but it's not likely to be noticable compared to whole G thrusts. You'd have to have some numbers for the mass of the ship and the energies involved in vaporising hull material to work it out.

*edit*
Whoops, rust pretty much just said that

 

[rust]

... if a bullet had the force to knock over a person, it would knock over the person firing the gun as well.

I have seen people knocked over by firing a rifle.

I think for the bullet impact problem the jury is still out. There are too ma-
ny variables that make things difficult, from the material and shape of the
bullet (determines how much of the impact energy is transferred directly
to the target's body or used for penetration) to the stance/balance of the
target's body.

So I would consider a strong movement of the target's body highly unlike-
ly for any kind of normal handgun ammunition, but I am not at all sure
when it comes to rifle ammunition, for example something like the heavy
minie balls with its 500 grains used in muskets.

 

[DFW]

I think for the bullet impact problem the jury is still out. There are too many variables that make things difficult, from the material and shape of the bullet (determines how much of the impact energy is transferred directly to the target's body or used for penetration) to the stance/balance of the target's body. So I would consider a strong movement of the target's body highly unlikely for any kind of normal handgun ammunition, but I am not at all sure when it comes to rifle ammunition, for example something like the heavy minie balls with its 500 grains used in muskets.

A bullet doesn't have enough momentum. According to Newton’s 3rd law, if the bullet were to knock the target 10 feet back, the shooter would have to be knocked back 10 feet.

 

[rust]

According to Newton’s 3rd law, if the bullet were to knock the target 10 feet back, the shooter would have to be knocked back 10 feet.

Since recoil and bullet impact do not occur simultaneously, Newton's 3rd
law does not really apply here. Moreover, at least some of the recoil ener-
gy is used up by the gun's mechanism, which also spreads the event out
over time, which is not the case for the bullet impact. This is more com-
plicated than a classical action-reaction-pair, which is why I would hesi-
tate to make a general final statement concerning all possible cases.

Of course, no chance whatsoever for a knockback of 10 feet, I could
imagine no more than the target being knocked back less than a foot
at most.

 

[DFW]

Since recoil and bullet impact do not occur simultaneously, Newton's 3rd law does not really apply here. Moreover, at least some of the recoil energy is used up by the gun's mechanism, which also spreads the event out over time, which is not the case for the bullet impact.

A bullet hitting a body has LESS energy than at the muzzle, PENETRATES the body thus the body is dispersing the energy and the body's much larger inertia prevents movement. Your 1st sentence "...simultaneously..." has no validity I'm afraid and doesn't fall outside of Newton's laws of motion at all. Good try though.

I have much real world experience with this exact subject from hunting. The animals DON'T go flying back when hit. They drop straight down. This of course is in line with the physics of the problem. Also the two people I have seen shot with a .30-06 Springfield cartridge dropped and showed no horizontal movement to speak of.

 

[Ishmael]

the shooter is braced in a shooting position and ready to handle the recoil. Because of this, the greatest part of his body mass is considered when figuring out how far back he'd move ( he will move because of conservation of momentum, just not very far ).
The target is not braced in a ready position and the bullet may hit somewhere other than his chest/center of gravity, so the body mass is much less as a leg or head masses much less than the entire body. The 'jump/reaction' is the body's reflex trying to compensate for some body part being thrown out of balance. Consider the apparent contradiction in the Kennedy assassination between the head-shot bullet's entry and the movement of the target in the Zapruder film.

This transfer of momentum is also why big bore guns have greater 'stopping power' because they carry more momentum per unit muzzle energy..better ballistic coefficient too. This is the reason the Colt m1911 was made. This also follows the real-world experiences of a friend of mine in combat. This leads me to experiment with using bullet momentum to determine damage instead of muzzle energy. Muzzle energy would be for determining penetration.

 

[rust]

Your 1st sentence "...simultaneously..." has no validity I'm afraid and doesn't fall outside of Newton's laws of motion at all.

A language problem, sorry.

What I intended to say, now from another angle: Newton's Third Law de-
scribes the mutual forces of action and reaction between two bodies, but
this covers only the gun and the bullet, the target is a third body, so New-
ton's law does not really cover this situation: While action and reaction
between gun and bullet are of the same force according to Newton's Third
Law, it says nothing about the situation between the bullet and the target.

This does not change the result, but it would be wrong to cite Newton's
Third Law as the explanation why recoil and bullet impact should be of
the same force - and indeed they are of course not.

 

[BP]

Ack - I am afraid my poor wording in an attempt to find a terrestrial analogy to the OPs initial post has opened a can of bad worm...

My sincere apologies!

Firstly, the bullet question depends on a lot of factors - not just its KE (mass & velocity), but very importantly the nature of the impact.

• ANY Kinetic Energy will impart motion in a purely ELASTIC COLLISION.

KE is, at non-relativistic speeds, equal to one half of mass times velocity squared. In an elastic collision, like two steel ball on a friction free table, KE is conserved - meaning the mass and velocity of the bullet would become part of that of the target. Without other forces (gravity, friction, air resistance) - a change in velocity will occur and the target will thus be moving even if it was not. [Of course, all motion, velocity being relative].

• Almost all real world examples are not perfect elastic collisions - they are INELASTIC - so KE is not conserved.

Most of the above posts - excepting any categorically denying others or those stating physics was violated - are true! Just depends on context...

So yes most target of bullets don't 'fly back'. I.e., since I presume DFW wasn't shooting armoured animals standing on ice - the animals tissue absorbed the KE of his bullets, or allowed it to pass thru - and therefore just fell (good shots if they didn't move off...)

Likewise, an armoured marine shot with a high velocity sniper rifle and/or high caliber round could go flying several feet (note, only part of his mass has to move that way - the rest is done by gravity unless the round were to strike in his exact center of gravity and moment of inertia).

As to lasers not imparting motion - this is not entirely true.

Photons do have relativistic mass. They only have zero rest mass. The term mass is actually a simplification that does not apply universally except in non-relativistic situations (i.e. newtonian physics).

Photons do have momentum which they impart on a lasing device and a target. Again, most materials will not conserve the KE of the collision, turning the potential energy of an impact not back into KE, but into heat (and subsequent physical changes).

The classical KE = mv^2/2 is actually only a simplification for normal speeds we encounter.

 

[srogerscat]

Vitalis6969 writes:

As for the .50 cal, it won't be stopped by body armor so that isn't really applicable to knocking someone over. It will pass clean through the armor and trauma plates, and of course, all the squishy bits inside.

Oh, I very much disagree. Even when penetrating armor, a bullet is transferring energy to the object being penetrated. If the bullet is punching through body armor, a body, and the other side of the body armor vest a big load of energy gets applied to the target.

Now, your point about wound shock being what drops soldiers has a lot of merit, but I have seen a video of a soldier getting struck in the shock plate over his chest and being flipped back by the impact, then springing back to his feet instantly and returning fire. No wound shock, just sheer impact from the strike.

 

[srogerscat]

DFW writes:

Since photons have no mass. Nada will happen.

But they do have *energy* which is transferred to target. Otherwise laser welding and cutting would not work. When a very powerful laser hits a solid, the matter literally explodes away from the strike point. That is what causes the concussion shock.

 

[srogerscat]

Vitalis6969 writes:

Is the 1 kilo of TNT equivalent in energy or explosive power? Not being a jerk, actually wondering.

Depends on the intensity of the beam. A 100 megajoule microsecond pulse is functionally identical to a 25 kilo spot charge of TNT. A one hundred megajoule one second beam would be more like a 25 kilos of TNT goop spread over a line on the hull a couple of centimeters wide and a couple of meters long.

 

[vitalis6969]

Oh, I very much disagree. Even when penetrating armor, a bullet is transferring energy to the object being penetrated. If the bullet is punching through body armor, a body, and the other side of the body armor vest a big load of energy gets applied to the target.

Now, your point about wound shock being what drops soldiers has a lot of merit, but I have seen a video of a soldier getting struck in the shock plate over his chest and being flipped back by the impact, then springing back to his feet instantly and returning fire. No wound shock, just sheer impact from the strike.

And the target still will not be "flipped back" or any other thing unless they were moving that way in the first place or had a physical reaction to getting shot or already had their weight leaned that way.

http://www.youtube.com/watch?v=QCzD5uhSViY

It is just not going to happen. The target in the above video is man weight with a vest AND steel plate. And still, only two inches of movement.

-V

 

[rust]

And the target still will not be "flipped back" or any other thing unless they were moving that way in the first place or had a physical reaction to getting shot or already had their weight leaned that way.

I suspect where we still disagree is only the amount of movement caused
by the impact of a bullet, that we could easily agree that "Hollywood Phy-
sics" depicting people knocked back several meters by a bullet impact is
highly nonsensical, and that the actual movement caused by a bullet im-
pact normally is in the centimeter range only ?

 

[BP]

Yep - either KE will be conserved (the target must move or portion(s) of mass must) - or it will not (the potential energy is converted into other forms).

A standing human target shot in the chest, with armour that does not deform or give -
- with shoes welded to the ground, will pivot - probably just from knees on up depending on KE imparted (assuming he tied his shoes well ).
- on ice, would be put in motion, his speed dependent on friction and the KE imparted.

Kinetic energy is just one form of energy - mass energy is conserved, but types do not have to be (epitomized by dE = dMC^2, where d is delta (change in))...

(P.S. - while Hollywood is well, Hollywood - movement depends on too many factors to define as 'centimeter range only' - as in the case above 'on ice' and in space as well...)

 

[rust]

... movement depends on too many factors to define as 'centimeter range only' - as in the case above 'on ice' and in space as well...

Yes, of course - this is why I inserted the "normally" in the post above,
there are lots of possible exceptional cases, up to a target jumping into
the air and hit at the highest point of the jump, and so on.