# Gravitational potential energy of a cylinder to a particle

1. Dec 29, 2015

### Gian Lukmana

Hi everyone I'm kinda new here, your support will really be appreciated ! :D

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

Let's say the cylinder has radius R, and height T.

2. Relevant equations
U = ∫GmdM/x

3. The attempt at a solution
My attempt is shown in the picture, I took a tiny element of the cylinder with vertical distance y, and horizontal distance r from the particle, then I used integral. am I doing this right ? I think there's something odd with the "y" there. Should I range the y until the upper surface of the cylinder only?

2. Dec 29, 2015

### phinds

Why would you need calculus for this? The volume of a cylinder can be looked up so if you have the density of the material, you can get the mass of the cylinder and the center of mass is trivially easy to get so if you have the mass of the particle, just plug everything into the gravity formula. Am I missing something about what you are trying to do?

3. Dec 29, 2015

### Staff: Mentor

The gravitational force due to a cylinder is not the same as that due to a sphere or point mass of the same mass. Geometry matters.

One approach would be to begin by finding the force exerted by a thin disk at a given distance, then treat the cylinder as a stack of such disks of thickness dy.

4. Dec 29, 2015

### Gian Lukmana

Yeah, I heard that we can't treat other shapes as a sphere, so we can't just blindly use the normal formula. To confirm, does this mean y should be the distance of the particle and the upper surface of the cylinder ?

5. Dec 29, 2015

### Staff: Mentor

I think it's up to you to specify your coordinate system and significant measurements, but it would probably make sense to characterize the setup by specifying distance of the test mass from the near end of the cylinder.

6. Dec 29, 2015

### Gian Lukmana

alright, got it now, thank you so much !

7. Dec 30, 2015

### phinds

OOPS. Guess I didn't think that one through.