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post #1 of 11 (permalink) Old 11-21-2007, 03:16 AM Thread Starter
 
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Exclamation Need help with Physics, PLEASE!!

Hey guys,

This is a last resort for me i have looked everywhere and can't seem to find any help so Please if any of you are good at physics i could really do with some of your expertise.

I need to calculate the centripetal force of satellites in orbit:
I know the formula for centipetal force is: Fc= MV^2/R

Now the problem i have is that the question hasn't given me the mass of the satellite so i need to calculate M (to substitute into the equation).

Here is the data i have:
Height: 150 km
R Orbit: 6550 km
Velocity: 7814 m/s
Period: 87 min


So does anyone have a formula that i can use to calculate the mass of the satellite, so then i can substitute it into the centripetal force equation.

Thanks guys for your help.
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post #2 of 11 (permalink) Old 11-21-2007, 03:32 AM
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Re: Need help with Physics, PLEASE!!

I'm no physicist, but does this help at all...

http://www.space-electronics.com/Lit...web%20page.pdf
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post #3 of 11 (permalink) Old 11-21-2007, 05:02 AM
 
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Re: Need help with Physics, PLEASE!!

Actually any satellite of any mass could be in that orbit. The satellite mass term in the equation for gravitational force (Fc= GMm/R^2) cancels with the satellite mass term in the centripetal force equation. That's why you can determine the mass of a body from the period of something orbiting it without knowing the mass of the orbiting body.
My advice... use any mass you like!
Why not the Hubble Space Telescope at 11110 kg
so long as you say where you got the mass value from you should be ok!
Jim
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post #4 of 11 (permalink) Old 11-21-2007, 05:35 AM
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Re: Need help with Physics, PLEASE!!

I remember a similar question back when i was doing Physics, as long as you write down a value for Mass M and say where you got it from or why you used it, you can use any value.
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post #5 of 11 (permalink) Old 11-21-2007, 05:40 AM
 
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Re: Need help with Physics, PLEASE!!

Isn't it Courtney who's the actual rocket scientist here? If I remember correctly the mass only counts when it comes to an actual alteration of orbital height?
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post #6 of 11 (permalink) Old 11-21-2007, 08:01 AM
 
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Re: Need help with Physics, PLEASE!!

Looks like you don't have enough information to determine the satellite's mass. I'd leave it as the variable "M". It could be Sputnik - it could be the moon!
LOL! You could have some fun and toy with this as well, assuming you're in Earth orbit.
http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation
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post #7 of 11 (permalink) Old 11-21-2007, 09:12 AM
 
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Re: Need help with Physics, PLEASE!!

You have to be given a value to solve the equation, otherwise the result wouldn't be the same as the registered answer for the question that was given to you.

Check this link out:

http://www.astronomynotes.com/gravappl/s8.htm

Jimmy
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post #8 of 11 (permalink) Old 11-21-2007, 09:44 AM
 
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Re: Need help with Physics, PLEASE!!

Silly me. It's the same thing.... Shows how fried I am!
:P
Time for Turkey and a long nap!
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post #9 of 11 (permalink) Old 11-21-2007, 11:32 AM
 
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Re: Need help with Physics, PLEASE!!

Quote:
Originally Posted by DavyH View Post
Isn't it Courtney who's the actual rocket scientist here?
Quote:
Originally Posted by jb4674 View Post
You have to be given a value to solve the equation, otherwise the result wouldn't be the same as the registered answer for the question that was given to you.

Check this link out:

http://www.astronomynotes.com/gravappl/s8.htm

Jimmy

Yes, that's me. Sorry, I'm on vacation so I'm just now getting to this. The website above details exactly the physics involved.

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post #10 of 11 (permalink) Old 11-21-2007, 11:54 AM
 
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Re: Need help with Physics, PLEASE!!

Man I remember physics. Last time I looked at anything like that was first year of undergrad - five years ago now? I remember those kinds of problems being pretty fun. A healthy break from biology and chemistry.
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post #11 of 11 (permalink) Old 11-21-2007, 09:41 PM Thread Starter
 
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Re: Need help with Physics, PLEASE!!

Thanks for the links

Thank you guys for your help, this question has been freaking me out, its the last question i have left in my assignment and i just couldn't do it, i might try the equations i found in those links and i'll see what happens.
So thanks again
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