# Homework Help: Calculate the constant in Newton's law of gravity

1. Feb 9, 2010

### aloshi

Phobos, a moon of Mars, moving around planten in a circular orbit of radius $$9,5\cdot 10^6$$. phobos period of revolution / orbital time is 7 h and 39 min. calculate the mass of March with the help of the given data and the constant in Newton's law of gravity.

it is easy to calculate the mass in March, but to calculate the constant in Newton's law of gravity, I do not know really how to use it? using the mass, I could be determined for plants Mars. there must be another formula which can calculate the mass in March, but to absorb the constant in Newton's law of gravity. but how? can anyone help me to think how?

2. Feb 9, 2010

### tiny-tim

Hi aloshi!

Newton's law of gravity is force = GMm/r2.

G is the universal gravitational constant (nothing to do with g).

3. Feb 9, 2010

### aloshi

Re: gravitation

but the task Says determine the universal gravitational constant, sow i dont new how to doit

4. Feb 10, 2010

### tiny-tim

Hi aloshi

(btw, it's "Mars", not "March" )
No, it says …
which I think means "calculate the mass … with the help of the constant in Newton's law of gravity", not "calculate the mass … and calculate the constant in Newton's law of gravity".