Conservation of Momentum

In physics and engineering we use a law called the conservation of momentum to predict the motion of objects which are in a collision. We can also use it to design rocket propulsion systems.

Momentum of an object is influenced by its mass and its velocity. It has to have both to have momentum.

Mathematically we express momentum as the mass of the object times its velocity. The momentum of a stationary object is zero as a result of its velocity being zero. From the definition you can see that the momentum of an object increases as its mass or velocity increases.

The law of Conservation of Momentum

A powerful concept is the law of conservation of momentum. This says that the total momentum before a collision equals the total momentum after the collision. This allows us to do some amazing calculations to predict the motion of objects after a collision.

For example, let us look at the situation where we have two train freight wagons in a collision.

We can use the conservation of momentum to calculate the velocity of the two wagons after the collision. In this case we have only one of the wagons moving before the collision. We will also assume the wagons become locked together after the collision.

Calculating velocity after a collision

The way you would use the conservation of momentum to calculate the velocity after the collision is:

1. Identify the objects to be considered. In this case it is the two freight wagons. We refer to this as the system under consideration.
2. Calculate the momentum of each freight wagon before the collision. The total of this is the total momentum before the collision.
3. Calculate the total momentum of the system after the collision. The freight wagons are joined together and moving as one. The momentum is therefore the combined mass of the two wagons times their velocity. Note that since they are joined together, both wagons have the same velocity.
4. Use the conservation of momentum to equate the momentum after the collision to the momentum before the collision. This is the key step as it enables us to calculate the velocity of the two wagons after the collision. Full details of the calculation are shown in the video at the top of this article.

External forces acting on the system

There is an important point to remember when using the conservation of momentum. It only works when there are no external forces acting on the system under consideration. If there are external forces acting on the system then the momentum of the system will change and you cannot use the conservation of momentum to do the calculation.

An example of external forces acting on the freight wagons would be where they collided with a fixed barrier on the track. You can see this in the video above.

Before you go

Why don’t you have a look at my video on Energy and Work. In it you will learn how to calculate the velocity of a dropped object just before it hits the ground.