This was a nit that may have bothered only me (and my ilk. My ilk is somewhat sensitive to such things). In this past week’s episode of NUMB3RS, there’s a scene where Liz and Nikki go to arrest some 350-pound badassMF™. One of them tries some FBI-fu on him, is thwarted, and the other (I forget which did which) grabs a fire hose and knocks him over with the jet of water and a cliché. Except that momentum is conserved, or is supposed to be. The impulse from the water leaving the hose should knock the person holding it back, and given that either of these characters has somewhere around a third of the mass of the target, should have not been able to easily wrangle such an instrument of havoc.
This is similar to the magic shotgun, that is recoil-free to the wielder, but is able to knock the target a meter or so backward when struck.
I was yelling at the screen about that, it’s starting to piss me off so much.
That whole scene made no sense “You can’t shoot me, I’m unarmed” “Oh, then I guess I had better wrestle you”
Good catch Tom.
I suspect that it is difficult to control a fire hose, but you are wrong about the underlying mechanism.
Imagine that instead of a hose, you are aiming a rigid section of pipe. In this case, the pump at one end of the pipe pushes water forward through the pipe, which hits the bad guy near the other end. The bad guy moves in one direction; the pump is attached to the earth, so the pump-and-earth move in the opposite direction to conserve momentum. Although you are aiming the pipe, none of these forces act on you.
By contrast, if you fire a shotgun, the explosion pushes against the gun which pushes against your body which pushes against the earth. If you were wielding a fire hose and holding the pump then you would take the impulse in the same manner as when firing a shotgun.
The problem which fire hoses is that they are not rigid sections of pipe. Forcing water through them at high pressure causes them to whip around unless forcibly controlled.
No, it’s the same principle. The whipping around is due to conservation of momentum — that is the action/reaction of sending the water out. It is because the wielder is not firmly affixed to the earth — so their mass is small — that they (should) recoil.
Yes, it’s conservation of momentum that causes the hose to whip around. But it’s not quite right to say that the hose-wielder is knocked back. A shotgun knocks the shooter “back” in the direction opposite the projectile. The direction a hose pushes the you depends on the curvature of the hose, and it shouldn’t push you around at all if it’s perfectly straight (hmm…. maybe that’s an unstable equilibrium, though).