Centripetal force in orbit, which equation to use?

[libraryismyhome]

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F = Gm1m2/r^2

F = mv^2/r

If there is a question about comparing the orbit radius, if I use the second equation, it does not give me the right answer. Why is this happening?

 

[Rabolisk]

Because you are using it wrong. You have to make sure everything else is held constant. What problem are you talking about?

 

[libraryismyhome]

Because you are using it wrong. You have to make sure everything else is held constant. What problem are you talking about?

. Two identical communication satellites are orbiting the Earth as shown below. Satellite A is twice the distance from the center of the Earth as Satellite B.

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What is the ratio of the centripetal force acting on Satellite A to that acting on Satellite B?

 

[Rabolisk]

Velocity does not stay constant.

 

[libraryismyhome]

Velocity does not stay constant.

how do I know that? The orbit does not seem elliptical..

 

[Rabolisk]

I meant that the velocity of A is not the same as the velocity of B, not that velocity isn't constant during centripetal motion.

 

[golgiapparatus88]

F = Gm1m2/r^2

F = mv^2/r

If there is a question about comparing the orbit radius, if I use the second equation, it does not give me the right answer. Why is this happening?

Centripetal force is created by the force of gravity from the planet. Since it's the only force on the satellite, you can just use the inverse square law

 

[TheRealAngeleno]

Does Fc = mv^2 / r only apply when you're at the surface of the earth? So then for this problem we would just use F = G m1m2 / r^2

1/Ra^2 / 1/Rb^2 = Rb^2 / Ra^2 = 1 / 4

Is this correct? Thanks.