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I'm really confused with a particular question:

**1. 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...

**2. Homework Equations**

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

**3. 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