Circular Motion of a roller coaster

[xkittenx]

Alrighty.. I was just wondering if you'd be able to help me out with a quick little problem I have. For physics we had to build a roller coaster, i am in the process of calculating the work done by the frictional forces throughout the course. Here's the question; In loopdy loops, when the marble is on the side of the loop, where does the force of friction point? what about when the marble is at the top of the loop? I know where the normal force and gravity point, but am unsure as to whether friction applies in these case. Thanx :)

 

[Doc Al]

friction opposes slipping

The friction would always be opposed to the motion of the marble. So its direction will always be tangential to the track at any point.

Of course, the magnitude of the friction force is another thing. I don't see any easy way to calculate it, especially for a rolling object like a marble. (If the marble is rolling without slipping, there will be no sliding friction.)