# Homework Help: Calculating radius in circular motion without frequency or force?

1. Oct 13, 2012

### InertialRef

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

Scientists want to place a 4400.0 kg satellite in orbit around Mars. They plan to have the satellite orbit at a speed of 2645.0 m/s in a perfectly circular orbit. Here is some information that may help solve this problem:

mmars = 6.4191 x 1023 kg
rmars = 3.397 x 106 m
G = 6.67428 x 10-11 N-m2/kg2

What radius should the satellite move at in its orbit? (Measured from the center of Mars.)

2. Relevant equations

v = 2∏r/T

3. The attempt at a solution

This question makes no sense. Is there any way to calculate the period of the motion using a different equation? Otherwise, there are two unknowns, irrespective of which equation is used. Even if I attempt to use two different equations that are solved for T and equate them, there is always an unknown such as frequency or acceleration present along with radius.

2. Oct 13, 2012

### cepheid

Staff Emeritus
Welcome to PF,

If the object is moving in a *circle*, then just as a kinematic requirement, what kind of force must be acting on it? What physical force in this problem is acting as that force (that keeps it moving in a circle)?

3. Oct 13, 2012

### InertialRef

Ahh, universal law of gravitation. Okay, I think I might have a handle on this. Thank you.