Let's correct your statement in-line: liokault wrote:If you are
an astronaut, you move backwards with as much force as you apply
forwards. This is true, by Newton's third law. However there
should be clarification here. Force = mass * acceleration. The
gun (which in our case is taken as "part of" the human body)
exerts a force on the bullet via the controlled explosion of
gunpowder. The force exerted on the bullet creates a change in
momentum from 0 to +10 kg*m/s (forward momentum). That same
change in momentum (remember, Force = a change in momentum), is
then exerted on the human body via the gun grip. As stated
previously, assuming a body+gun mass of 100kg, this means, the
human body will experience the same change in momentum backward
(-10 kg*m/s). If you shot that gun on a frictionless surface (eg
you are sitting on a sled floating on an enormous air hockey
table), you would then be moving backwards at 0.1m/s. The only
reason why you do not move backwards is because of friction
between you and the earth,