# Orbital motion

1. Mar 6, 2013

### Robertoalva

1. At what altitude will a satellite complete a circular orbit of the Earth in 4.4 hours?

2. Relevant equations

T^2 = ((4π^2)(r^3))/GM

3. The attempt at a solution

Used the equation and plugged all the information, and solved, but for a weird reason it comes wrong, maybe I should convert the hours to minutes?

2. Mar 6, 2013

### SteamKing

Staff Emeritus
What units did you use for the quantities in your formula?

G = 6.674*10^-11 m^3/kg*s^2

so time may have to be in seconds rather than minutes.

I don't understand why folks think that the units will take care of themselves when making calculations.

3. Mar 6, 2013

### Robertoalva

cuberoot((((15840seconds)^2)(6.67384E-11 N m^2 Kg^-2)(5.974E24Kg))/(4pi^2)) = r

is that correct?

Last edited: Mar 6, 2013
4. Mar 6, 2013

### lep11

That formula looks good.

Last edited: Mar 6, 2013
5. Mar 6, 2013

### haruspex

That gives an answer in metres. What distance is that calculating, exactly? The question asks for altitude.

6. Mar 6, 2013

### Robertoalva

I do it, but, the answer that it gives me is totally different from the answer that my practice problem gives

7. Mar 6, 2013

### Robertoalva

the answer that this equation gives me is something like,13632 and the real answer should be 7260 or something approximated

8. Mar 6, 2013

### lep11

That formula gives you the distance r between the center of the Earth and the satellite. To get the altitude of the satellite you have to subtract the Earth radius from r which gives you the correct answer.

9. Mar 6, 2013

### Robertoalva

thank you!!!

10. Mar 6, 2013

### lep11

You're welcome.

11. Mar 6, 2013

### haruspex

No, I meant what distance does that number represent. I was trying to point out that you were not calculating altitude. Anyway, looks like you have it now.