## Momentum

Momentum is the mass times the velocity of the object.

It is an interesting quantity because it is always conserved, like energy!

Conservation of momentum may be very helpful when solving problems, regarding a collision, for instance.

**Examples**

1) What is the momentum of a bullet that weighs 20 g and moves at 500 m/s ?

P = 0.020 * 500 = 10 m kg/s

2) What is the momentum of an elephant that weighs 1 ton and moves at 1 m/s ?

P = 1000*1 = 1000 m kg/s

Notice that although the momentum of the elephant is much higher than the momentum of the bullet, the kinetic energy of the bullet is higher than the KE of the elephant.

3) A snooker ball weighing 300 g and moving at 1 m/s collide with another one that weighs 200g and is initially at rest, and hits it right in the middle. After the collision, the 300g ball stops. What is the speed acquired by the other ball?

We can use conservation of momentum to solve this one.

The initial moment is:

p = 0.3*1 = 0.3

the final moment must be the same, but now it comes from the 200g ball:

0.3 = 0,2*v

v = 1.5 m/s

4) The same as the previous, but this time the 2 balls move together after the collision.What is the velocity of the 2 balls after the collision?

The initial moment is:

p = 0.3*1 = 0.3

The final momentum(with the 2 balls together) is: p = (0.3+0.2)*v

equating the 2 moments:

0.3 = 0.5 v

v = 0.6 m / s

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