# Momentum and law of conversation of momentum.

1. Sep 5, 2006

### winkle

can anybody plz explain these two definations?
i know momentum is the mass in motion..but how do we use the formula...explain plz

thx

2. Sep 5, 2006

### dmoravec

basic non-relativistic formulas:
momentum = mass * velocity (note: velocity is not speed, velocity is a vector quantity)
and the conservation of momentum means:
momentum (before) = momentum (after)

For example:
a 5 kg ball is traveling at 1m/s : its momentum is 5*1 = 5 kg*m/s
if it hits a different ball (which is 10 kg) directly on and bounces directly back at 0.5 m/s. (directly on so that the other ball will head in the same direction as the inital ball was.
then the total momentum after the collision will still have to be 5 kg*m/s
we know that the first ball has momentum of -0.5 m/s * 5 kg = -2.5kg*m/s. Therefore the momentum of the second ball must be 7.5 kg*m/s so that the total momentum in the system is still 5 kg*m/s. From this the velocity of the second ball can be determined.

3. Sep 5, 2006

### HallsofIvy

No, momentum is not "the mass in motion". You have to be a lot more precise than that: momentum is "mass times velocity". It is, as dmoravec said, a vector quantity; scalar mass times vector velocity.
To use that formula, calculate the total (vector) momentum of each particle before a collision, then after the collision and set them equal. The momentum of individual particles might have changed but the total is "conserved"- remains the same.