How can I prove momementum is conserved when two carts are pushed apart by a magnet?

Question

We have this lab setup with an airtrack and two carts of different masses. They each have a magnet on them so they repel each other. This is for a high school physics class and we want them to weigh the carts and let them start with zero initial speed. Then they will let them go and we have meters that measure their speeds. So they could do this once and figure out if the two carts end with the same momentum but how can they do a bunch of trials and end up with a %uncertainty? The two momentums should be the same, but for each trial, they might be different. How can they calculate a result that says, yes, momentum is conserved?

Answer 1

The carts start with an initial speed of zero so the total momentum is zero. Since momentum is conserved, you will expect the total momentum after they repel each other to be zero also.
You can't compute a relative error when the expected value is zero. If the class knows how to compute standard deviation, they should use that.