Fizzics Fizzle: Beginner: Momentum

Momentum--you probably heard that word before, but what does it exactly mean? The heavier something is and the faster it goes, the more momentum it has, right? A train has more momentum than a car if they both go at the same velocity. A car can stop from 100 km/hr in a few seconds but it will take a couple of minutes for a train to stop from 100 km/hr. Momentum of an object is defined as the product of its mass and its velocity. Multiply mass and velocity and you have the momentum of the object. Thus, a train has a higher momentum than a car moving at the same velocity because it has a greater mass.

Collisions

So what happens to momentum when you start colliding with other objects? What if there is a car accident? What happens to all that momentum because obviously one of the cars slowed down, right? The law of the conservation of momentum happens.

The Law of the Conservation of Momentum:

The total momentum of an isolated system of bodies remains constant.

But what does that mean? That means when you add all the momentum from each object before and after the collision, they will equal each other as long as another force does not affect the system. That is what an isolated system means, where no outside force jumps in and ruins everything. So what would be an outside force? Well, if you started with two cars and a third car smashed into them, then the system of two cars was disturbed by the third car. However, if you include the third car in the system, then it would not be an outside force anymore. It is a little complicated but understandable.

So does anyone play pool? That's a game of collisions and momentum. When two balls collide all or part of the momentum from one ball is transfered to the second ball. If all of the momentum is transferred to the second ball, then the first ball will be at rest and the second ball will move with the same speed as what the first ball originally moved at (this is called an elastic collision).

Explosions

Momentum is conserved in an isolated system; that is the law. Since an explosion is actually an isolated system, that means momentum must be conserved. So what does that mean? Well, that means you can actually calculate where all the pieces of the thing that exploded will go.

Let's say we have a bomb that splits into two pieces when it explodes. To make it more simpler, we will say that both pieces have the same mass. When the bomb explodes, one piece flies off in one direction. The other piece will fly off in the exact opposite direction with the exact same velocity. Why? Since the original momentum of the bomb was zero (it wasn't moving), the momentum of both pieces must cancel each other out.

If the two pieces were of different masses, then the heavier piece would be moving in the opposite direction at a slower speed than the lighter peice. This is also because the total momentum must cancel out.

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Clyde Law, Chetan Taralekar, Jim Wang

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