# Gravitational potential energy experiment

1. Oct 4, 2014

### Jimmy87

1. The problem statement, all variables and given/known data

To explain the forces at work when a marble is released from a height and then eventually comes to rest. We did this experiment where two lengths of hose pipe were taped together. The hosepipe was then bent into a semicircular shape (forming a track) and secured onto a clamp stand. We then had to release marbles from varying distances from the bottom of the hose pipe ramp and measure the time taken for the marble to come to rest.
2. Relevant equations

3. The attempt at a solution
I know the marble has GPE equal to mgh when released and picks up KE as it rolls down the track. I am struggling to apply some rigorous physics as to why it stops. Air resistance is an obvious one. The marble seems to grip the track so I would argue the friction is static friction. If there is no kinetic friction then am I correct in saying friction doesn't oppose the motion of the marble? Is the main opposing force the bumpiness of the hose pipe so the marble encounters a large number of small collision and each collision provides an opposing force to slow down the marble?

2. Oct 4, 2014

### Staff: Mentor

I'm having trouble visualising the track. "Semicircular", you say? A marble released on one end of such a track would roll back-and-forth multiple times, reaching ever-decreasing heights.

There certainly is friction all the time draining energy from the marble's motion.

3. Oct 5, 2014

### Jimmy87

Yes, you have visualised it exactly as it is. I posted a thread recently about static friction and rolling motion (https://www.physicsforums.com/threads/static-friction-of-a-cars-tyre.771649/#post-4862431). It seems that if there is only static friction then friction does not oppose motion (so long as there is little depression of the rolling object), it in fact provides the motion. In the marble experiment the marble seems to grip the hose pipe therefore there is no kinetic friction, only static. I thought only kinetic friction slows down motion and if the marble experiences static friction then surely friction would not be an opposing force in this situation?

4. Oct 5, 2014

### Staff: Mentor

Of rolling friction there'd be plenty. The flexible hose depresses under the ball, so the ball is constantly 'climbing a hill', a bow-wave of polythene ahead of it. Try letting a lot of air out of your bike tyre and compare the effort needed to peddle with that when the tyres are pumped hard so they stay more circular.