I have always wanted to ask a question like this – but never found the right test for it. Perfect for a blog.
Suppose you are in your car at a stop light behind a truck. The truck is pulling a trailer with a rollable tractor on it.
When the light turns green, the truck starts to accelerate. However, the cable holding the tractor to the trailer breaks and the tractor starts to roll off the trailer. You are right behind the trailer. What do you do?
Do you have an answer yet? You should probably come up with one quickly. That tractor is rolling off now. I will put the answer below.
The best thing to do would be to stop and stay where you are. Although the tractor is rolling backwards with respect to the trailer, it is still moving forward (even if a little). In the case of zero friction, the tractor would stay in the exact same position on the road (which is still in front of you). In the case of a little friction, there will be a slight friction force pushing in the direction of the motion of the truck. This small friction force will slightly increase the momentum (and thus speed) of the tractor.