# Homework Help: Physics Project

1. Nov 2, 2007

### Crupler22

thanks

Last edited: Nov 2, 2007
2. Nov 2, 2007

### Staff: Mentor

The last part using PE and KE would be the way I'd do it. I'm not sure your math is correct, though (hard for me to read).

3. Nov 2, 2007

### Staff: Mentor

This part's good.

So you found the centripetal force acting on the system when it's at the bottom. Note that this force changes as the system moves up and that it acts perpendicular to the direction of motion.
This doesn't make sense. Since the force and displacement are at right angles to each other, the force certainly doesn't do any work. (The force doing the real work is gravity, which acts downward.)

It's certainly true that energy is conserved, so KE at the bottom equals the PE at the top. But, as I pointed out, your method is incorrect even if it happened to give you a correct answer. But the answer's not correct. Compare it to the answer you'd get by setting KE = PE (not "W" = PE).

4. Nov 2, 2007

### Crupler22

Ooooooooh Ok, I get it. Since the motion of the mass always wants to travel in a path perpendicular to the circular path, because of inertia. And since the force is centripital its acting inward, so their perpendicular. So just curious, if I then did something to make the distance relative to the force it would work? Like since there at right angles use some trig function to set them in the same direction? I know the real equation I was just curious if I could get mine to work. Thanks Doc Al

5. Nov 2, 2007

### Staff: Mentor

Here's what would work that is kind of like what you were doing. Find the force in the direction of motion. There are two forces acting on the system at any point: the string tension and the weight. The tension is always perpendicular to the motion, so that contributes no work. But the weight will have a component in the direction of motion as the system rises up in an arc, so it does do work on the system. But since that component is a function of angle (it's not constant) you can't just multiply by the arc length--you have to integrate.