Newton's Third Law

[paidinfull]

For every action there is an equal and opposite reaction. Why, then, does anything move?

[Calvesy]

The total amount of momentum in the universe is, and always has been zero. Forces acting between bodies over time cause them to exchange momentum, but none is created nor destroyed.

[badams]

At a localised level, other forces come into play, the most important being friction.

[Hippy]

One thing pushes against another thing and they move apart. Take a boat. The propellor imparts enrgy into the water that then moves away from the boat. At the same time the boat moves away from the acted upon water. As the boat has less inertia (resistance to movement) than the water, the boat moves more with respect to the water.

Athletics: the runner crouches with foot on starting block which is firmly attached to the Earth. Bang! The runner pushes hard against the block, trying to push planet Earth backwards (with respect to the athlete) but because the Earth has more inertia (harder to move or deviate from its course) than the athlete, the runner is propelled forward. The force acing on the athlete causing motion is exactly the same as that imparted to the Earth, but being much more massive, the Earth is able to absorb the energy with little change to shape/trajectory/speed etc.

[johnny5]

so its poo when in a film u see someone blown away when whoever fires the shot is stood still ?

[nigidivitch]

Not if the person firing the gun is really really fat with a big inertia...

[Hippy]

Johnny 5, if you mean using a hand gun or even rifle, then yes. The recoil of the bullet leaving the gun is absorbed by the shooter, who will tense their muscles against the force. When the bullet hits the target person its rapid deceleration in their body causes tissue damage, and may knock them off balance if say leaning forward to get something out of reach. In this case they would stumble and perhaps fall over.

If the bullet came from a cannon, then the shell is more massive and carries much more energy, so would "blow away" the target.

The ludicrous bit in films is where the baddies are crouched on the rooftop shooting from behind a barrel/chimney or whatever, and the Lone Ranger (Hii Hoo Silvia!) tosses off a shot and the baddies leap upwards into the sky and then hurtle to their presumed death on the ground shouting "Arrrgh!"

[Hamish]

it comes down to intertia and conservation of momentum. Momentum is defined as the velocity (which has a sign, either + or -) multiplied by the mass. If a 5g bullet leaves the gun at 300kms-1 then the 100kg person will have a momentum of 15e2 kgms-1. Man weigs 100kg, velocity - 15ms-1 in the opposite direction. This assumes elastic behavour - not usually the case.