# Gravity Pull on an object

1. Jun 5, 2009

### talaroue

1. The problem statement, all variables and given/known data
Planet Z is 13700 km in diameter. The free-fall acceleration on Planet Z is 8.79 m/s2.
What is the mass of Planet Z?

2. Relevant equations

Gm1m2/r2

3. The attempt at a solution

I am not sure how this problem is suppose to be worked at all, any of these kind I am having a problem connecting things together. I thought I would use potential energy which would be -G*M1*M2/r but that won't work because I am not given another weight. So my questions are.......

Where do I start?

What equations do I have to use?

2. Jun 5, 2009

### LowlyPion

You have the equation. Simply recognize that

G*M*m/r2 = F

But you know that F = m*a

... so the acceleration part of that expression should be ...

a = G*M/r2

They have given you a and r and you can look up G, so ... solve for M.

3. Jun 5, 2009

### talaroue

FML. You are a genious I was missing the relationship that my equation was equal to force. Now that brings up another question when dealing with gravity will that equation always equal the force?

4. Jun 5, 2009

### talaroue

so i solved for M...and got M=aR^2/g with numbers is 8.79 m/s^2*1.37x10^7 m/ 6.67x10^-11 which comes out to be 2.47x 10^25 correct? It is saying my anwser is worng. I have done it 5 times and it keeps coming up with the same anwser on my calculator.

EDIT: I was rushing and forgot to make it radius instead of Diameter.

5. Jun 5, 2009

### Cyosis

You didn't square the radius either in your post, a typo or a genuine mistake?

6. Jun 5, 2009

### talaroue

Typo as well as forgetting to turn it into the radius