Gravitational potential energy experiment

[Jimmy87]

Homework Statement

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.

Homework Equations

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?

 

[NascentOxygen]

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.

 

[Jimmy87]

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.

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?

 

[NascentOxygen]

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.

 

[HalfLostAndSearching]

Thank you for sharing your experiment with me. It is always exciting to see students conducting hands-on experiments to understand scientific concepts.

The experiment you described is a great way to explore the concept of gravitational potential energy. As you correctly noted, when the marble is released from a height, it has a certain amount of potential energy due to its position in the Earth's gravitational field. As it rolls down the track, this potential energy is converted into kinetic energy, which is responsible for the marble's motion.

Now, let's discuss the forces at work when the marble comes to rest. You are correct in saying that air resistance is one of the main opposing forces. Air resistance is a type of friction that occurs between the marble and the air molecules. As the marble moves through the air, it experiences a force in the opposite direction of its motion, which slows it down.

However, there are other forces at play as well. One of them is the friction between the marble and the track. As you mentioned, this friction is likely static friction, which means that it only comes into play when there is no relative motion between the marble and the track. Static friction is responsible for the marble's ability to grip the track and roll down without slipping.

In addition to these forces, the bumpiness of the hose pipe can also play a role in slowing down the marble. As the marble encounters small bumps and imperfections on the track, it experiences small collisions that can also contribute to its deceleration.

Overall, it is a combination of all these forces that ultimately brings the marble to rest. By measuring the time it takes for the marble to come to rest from different starting points, you can investigate how these forces change with distance and how they affect the marble's motion.

I hope this explanation helps you better understand the physics behind your experiment. Keep up the good work in exploring the world of science!