# Ap physics gravitational force on satellites. Any help appreciated

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1. Sep 22, 2013

### astru025

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

A 450 kg satellite orbits at a distance from the Earth's center of about 6.5 Earth radii. What gravitational force does the Earth exert on the satellite?
2. Relevant equations

Gravitational force= G* ((mMe)/(r^2))

3. The attempt at a solution
G times mass times earths mass divided by radius squared.
G= 6.67E-11
m= 450 kg
Me(earths mass)= 5.98E24
I inserted this into the equation but came up with the wrong answer it said. I don't know what I'm doing wrong and am getting very frustrated !

2. Sep 22, 2013

### Staff: Mentor

Make sure your distances are in meters.

Even easier would be to use ratios. By what factor is the weight of the object reduced when the distance is multiplied by 6.5?

(Please don't post the same thing twice!)

3. Sep 22, 2013

### astru025

I made sure my distances were in meters and still cannot come up with the answer.

4. Sep 22, 2013

### Staff: Mentor

5. Sep 22, 2013

### astru025

I came up with 104964.74.
This is how I came up with that: G (6.67E-11 N*m^2/kg^2) times ((450 kg.) * (5.98E24 kg)) / (1.71E12 m)

I got 1.71E12 m by taking 6.5 times 6.37E6 and squaring that.

6. Sep 22, 2013

### Staff: Mentor

Redo that calculation; you are off by a factor of 1000.

7. Sep 22, 2013

### astru025

Thank you so much. Fixed it and came up with a final answer of 103.75 and it was correct! Thanks so much!

8. Sep 22, 2013

### Staff: Mentor

You are very welcome. :thumbs:

9. Sep 22, 2013

### astru025

I have another problem that is really challenging me and I was going to see if you could tell me what I'm doing wrong.
Problem: A 135 kg satellite experiences a gravitational force by the Earth of 960 N. What is the radius of the satellite's orbit?

I used the equation for gravitational force: F=G * ((m*Me) / (r^2)).
I put 135 in for m. 5.98E24 for Me( the earths mass), and for r I put in (x*6.4E6)^2. And I put 960 in for F.
I got an answer of 1.17 and it said it was wrong. And idea why? Thanks so much.

10. Sep 22, 2013

### voko

11. Sep 23, 2013

### Staff: Mentor

Why would you do that?

12. Sep 23, 2013

### astru025

6.4E6 is earths radius and then you square that. I'm not sure why I had that x in there.

13. Sep 23, 2013

### Staff: Mentor

r is the radius of the orbit; that's what you're solving for. So just leave it as "r" and solve for it.

14. Sep 23, 2013

### astru025

Thank you. I came up with 7489.99 and that was the correct answer!

15. Sep 23, 2013

### astru025

Now how do I find the altitude from this same problem?

16. Sep 23, 2013

### Staff: Mentor

Altitude means how far above the earth. Hint: Make use of the earth's radius.

17. Sep 23, 2013

### astru025

I'm so dumb. I subtract the earths radius (6370) from my radius and I got the correct answer! Thanks so much.