Any moving object has MOMENTUM. Momentum tells you how hard it is to stop a moving object. The higher the momentum, the harder to stop. You calculcate momentum by doing momentum = mass x velocity.
I got two identical tins from the kitchen. The more solid the contents, the better the experiment because if the liquid inside can slosh around, it can affect the movement of the tin.
I put tin A at the top of the slope and let it roll down the slope into tin B. Only tin A was moving before they collided so only tin A had momentum.
When tin A hit tin B, they both roled on a short way but at half the speed at which tin A was going originally. This is because only tin A brought momentum into the collision and now it has to share the momentum with tin B. Bceuase they are identical tins, each should get half the momentum and thus be going at half the velocity. After the collision, there is still the same total momentum as there was before but it now shared outl. Keeping the amount of momentum the same before and after is called CONSERVATION OF MOMENTUM. (It doesn't last long because this rule of Physics only works until friction on the carpet acts on the tins)
Then I set up two ramps at the same height to create a head on collision. Tin A is coming towards us and so we say it has POSITIVE momentum. Tin B is going away so we say it has NEGATIVE momentum. Momentum has a direction so we say that it is a vector.
Now let's say tin A has momentum +10 and tin B has momentum -10. The total momentum means adding them together. +10 + (-10) = 0. So if momentum is conserved, there will be the same total momentum after the collision: ZERO total momentum. What happened was that the tins stopped dead when they hit each other so it turned out to be true.
If we did this with tinsof different mass they would not stop dead because momentum = mass x velocity so one tin would have more momentum than the other. Try it.
