Planetary Motion, calculation of orbital period

[Stevo_evo_22]

Hi everyone,



I'm really confused with a particular question:

Homework Statement

A space shuttle orbits the Earth at 6720 km from its centre. The gravitational field strength is 8.9N/kg. Calculate the shuttle's orbital period in minutes...

Homework Equations

g=4pi^2 x R /T^2

The Attempt at a Solution

The thing is, I know how to solve it using g=4pi^2 x R /T^2, but I want to know why I can't just use g=r^3/T^2 (the constant for all objects orbiting a particular mass)...if I have r and g, why can't i just use this equation?

Thanks!

Steve

 

[Dick]

Because r^3/T^2 is a constant. But that constant isn't the same as g. It doesn't even have the right dimensions.

 

[Stevo_evo_22]

Because r^3/T^2 is a constant. But that constant isn't the same as g. It doesn't even have the right dimensions.

ahh ok...thanks :)