The clever thing about momentum is how it is conserved
after a collision
(eg. a fly hitting a car, an explosion etc.). This translates to Momentum before
= Momentum after
. So, if you have two air cars on an air track
of the same weight, with one at rest and one moving, in a totally elastic collision
the momentum of the moving car will be transferred to the stationary car completely, leaving the originally moving car at rest and the originally stationary car moving at the velocity
the first car was moving.
This is very much like the effect from Newton's Cradle. Things get interesting when one car is heavier than the other. If the moving car is twice the mass of the stationary car, then you can say that the mass of the moving car is "2M" (2*M) and the stationary car is "M". If the moving car is moving at "V" ms-1, then the momentum is 2*1 = 2 kg ms-1.
When the moving car hits the stationary car in an elastic collision all momentum will be transferred, so car number 2 also has momentum of "2M". Because its mass is M, its velocity will be 2V, ie. twice as fast as car number 1.
This effect can also be seen in golf when the ball leaves the surface of the club at a much faster velocity than the club was moving, because the club has a much higher mass than the ball. An explosion can be said to have momentum. An unexploded bomb has no momentum because it is not moving. A second after it explodes shrapnel of varying sizes will be travelling in all directions. Because velocity, and momentum, are vectors, something travelling in one direction will cancel out the thing travelling in the opposite direction. This is because the thing travelling in one direction has a mass of M and a velocity of V, so a momentum of MV, while its opposite counterpart will also have a mass of M but a velocity of -V, so a momentum of -MV. If you add MV and -MV you get 0, so the momentum before the explosion, and the momentum 1 second after is equal. Shrapnel from an explosion will always be balanced in all directions due to this law of physics (or should i say that this law of physics is so because shrapnel is always balanced?).
This is not to say an explosion is perfectly symmetrical, just that all the momentums will add up to 0 in every direction (there may be a few slow big things going in one direction, and the same amount of small things going in the opposite direction but much faster).