# Conservation of momentum

The conservation of momentum is an important concept in physics. Make sure you understand this lesson before studying collision.

The word conservation may already gives you an idea of what we mean. Some synomyms for conversations are preservation, protection, or keeping.

Therefore, conservation of momentum refers to the momentum that does not change.

In what situation will the momentum not change?

Recall that from Newton's second law of motion that an object will not move unless a force is applied to it.

The analogy between Newton's second law and the momentum is that to change the momentum, an impulse must be exerted on the object.

The important thing to keep in mind here is that the impulse must be external

Internal forces that are found inside an object will not change the momentum of an object.

Consider this situation!

If superman sits inside an airplane, and he pushes the back of the chair in front of him, the airplane will not move. Since he is so strong, he may even break or dammage the chair in front of him. However, to give some momentum to the airplane he needs to apply some force for a certain period of time, which is what impulse is, on the outside of the airplane. Period!

When superman pushes the back of the chair in front of him, the back of the chair pushes superman's hand with the same force. This is the law of action and reaction. These forces just cancel each other.

### Law of conservation of momentum

When the momentum remains the same before and after some internal interation, we say that the momentum is conserved. You can also say that the momentum is constant.

Mathematically speaking, you could write momentum = constant

Here is what the law states as shown below.

If no external force acts on a system, the momentum of the system will not change.

When no external forces act on the system, we say that the system is isolated. Earlier, we talked about an airplane with superman inside of trying to push the back of the chair in front of him.

The airplane is an isolated system since no external forces are pushing the airplane.

The system must also be a closed system. In other words, the system will not allow particles to enter or leave the system.

When two cars collides as shown above, the cars go through some changes since they can be severy dammaged. However, notice that the cars experienced no external forces.

In this case, you can also say that the net momentum before and after is the same.

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