# Momentum is conserved in a collision

## Main Question or Discussion Point

Momentum is conserved in a collision so, the momentum before collision = the momentum after the collision. But i have a hard time understanding this, as P=m*v in a collision doesn't velocity decrease? And if velocity decreases so does the momentum of the object as the mass is constant. It doesn't make sense how momentum is is conserved, can someone help me please? i'm quite confused.

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Simon Bridge
Homework Helper
Welcome to PF;
You have described the change in momentum of one object in a collision. For a collision to happen, though, there must be more than one object.

When one object in a collision loses momentum, some other object must gain it.
Conservation of momentum only applies to a closed system.

You are forgetting an important part. The total momentum of a system is conserved. Take this example, imaging an elastic collision where a moving object strikes a stationary object with equal mass and then the stationary object starts moving while the moving object stops moving. What happens? Well intuitively you would say that the stationary object would start moving with the same velocity as the original one, and you would be correct. For each object, their momenta certainly were not conserved before and after, but if you take the sum of their momenta, they would certainly be equal.

Ahh, thank you Simon And Legaldose. It all makes sense now :)