[Stevie Bain (OP)]

Madhu says:

In a recent film, the hero is falling off a bridge and while falling he fires a bullet which hits and knocks down a helicopter.  I was wondering what the physics behind this is? Can we actually fire a bullet while free-falling to the ground? Can it hit a target? What effect does this have on the falling shooter?

Can you help?

[Janus]

A falling person will reach terminal velocity at somewhere between 54-90 meters/sec (depending on whether he maximizing or minimizing air drag).  A bullet typically has a muzzle velocity of over 1000 meters/sec. So firing a bullet upward will slightly reduce its upward velocity, but not by much.   The recoil will accelerate the persons fall a bit, causing him to reach terminal velocity sooner if he hasn't reached it yet, or pushing him temporarily past terminal velocity if he has already reached it. 
Depending on how he is aiming the guns relative to his body, the recoil could also induced a slight rotation to his body. ( for instance, if he was falling with his back parallel to the ground and with arms straight out in front of him.)  The degree of the recoil effect depends on what type of weapon they are using.

[chris]

This would become an issue with a gun mounted on a powered aeroplane moving very quickly though, wouldn't it?

[alancalverd]

Indeed, and firing all the guns - particularly cannon - on a low-powered plane has a noticeable effect on its speed. The laws of physics seem to be immutable, and the conservation of momentum is one such.

[Bill S]

Can it hit a target?

That would depend on a few factors, including the shooter's aim.  I suspect there is a greater chance of success on film than in real life.

[jeffreyH]

If the shooter has oriented his body such that the gun is pointing directly away from his centre of gravity then no rotation will occur. I doubt that this is taken into account by Hollywood.

[helter]

Can it hit a target?

That would depend on a few factors, including the shooter's aim.  I suspect there is a greater chance of success on film than in real life.

Most of the things in the films and movies are exaggerated. Hitting the target is still possible but less chance and impact compared to a gun fired by a man on the ground.

[chris]

Indeed, and firing all the guns - particularly cannon - on a low-powered plane has a noticeable effect on its speed. The laws of physics seem to be immutable, and the conservation of momentum is one such.

So if a big plane doing about 500kph has a rear-pointing gun with a muzzle-velocity of e.g. 1000kph:

1) If the gunner fires at a stationary target behind them, presumably the shells effectively hit that target at  500kph and with half the energy imparted as if the gun were static on the ground?

2) If the gunner fires at a pursuing aircraft, also travelling at 500kph, how big is the impact on the pursuing aircraft if it is hit?

[Colin2B]

2) If the gunner fires at a pursuing aircraft, also travelling at 500kph, how big is the impact on the pursuing aircraft if it is hit?

The closing speed of the pursuing plane and the bullets is 1000kph and this determines the momentum of the bullet impact.
This why KE is relative.

"Relativity rules, OK!"