# Homework Help: Circular Motion and Gravity

1. Oct 4, 2009

### jibjab

Hi everyone, I am taking my first physics class ever and I'm lost!! I have a couple of questions that are giving me a lot of trouble. Any assistance you can give me is very much appreciated!

1. The problem statement, all variables and given/known data
A turntable rotates counterclockwise at 85 rpm. A speck of dust on the turntable is at (theta)0.31 rad at t=0 s. What is the angle of the speck at t=7.77 s? Answer should be between 0 and 2(pi) rad.

3. The attempt at a solution
For this, I changed 85 rpms to 1.4 rps. I tried to find the angular velocity by dividing 2(pi)rad/1.42 rps, and my answer was 4.42 rad/s. Then I plugged the info into the equation (theta final)=0.31 + (4.42 rad/s)(7.77 s) and got 34.65.
I divided that answer by 2(pi).
I got 5.51 X 2(pi) rad, following the example in the book. I ended up with a final answer of 183 deg, which was wrong.

1. The problem statement, all variables and given/known data
The passengers in a roller coaster car feel 58% heavier than their true weight as the car goes through a dip with a 36.1 m radius of curvature. What is the car's speed at the bottom of the dip?

3. The attempt at a solution
I really don't understand how you figure this out without their true weight?

1. The problem statement, all variables and given/known data
An earth satellite moves in a circular orbit at a speed of 5247.66 m/s. What is its orbital period?

3. The attempt at a solution

I used the equation T=2(pi)X(square root)r^3/GM <-- (I hope that makes sense)
using G=6.67 X 10^-11, M=5.98 X 10^24 and r=6.37 X 10^24
My answer was 5057.96 s. I tried a couple other ways but they were wrong too.

1. The problem statement, all variables and given/known data
A 182.4 kg block on a 43.1 cm long string swings in a circle on a horizontal, frictionless table at 55 rpm. What is the speed of the block? The tension on the string?

3. The attempt at a solution
This one I just need an equation for, the ones that I've found need either one or the other so I don't know how to go about it?