Momentum is the idea of energy of motion (well momentum squared is). At non-relativistic speeds momentum is an objects velocity (its speed and direction) that you the observer sees multiplied by its mass. It is a conserved quantity, so if an object collides with another then the momentum in (in every direction) must equal the momentum out. This conservation also applies also to relativistic things like particles and conservation can be used to reconstruct the mass of missing particles that are not directly seen in particle detectors.

So, yeah momentum is the idea of how much “movement” something has got. Momentum is the thing that would knock you off your feet if you get hit by something.

I guess you’re thinking of the equation:

p = m v

If so, it makes sense right? That equation tells you that a heavier thing (bigger ‘m’) has more momentum. That seems right because a bus hitting in to you would certainly give you more of a knock than a cat. And the equation tells you that a faster thing (bigger ‘v’) also has more momentum. Again, that’s sensible because a person walking in to you knocks you much less than a person running in to you.

So that equation is a way of mathematically writing what you already know, that you don’t want to be hit by fast heavy things, but light slow things are not so bad!

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