Universal Gravitation questions

1. Oct 3, 2005

jshaner858

1) if something is 5 earth radii above the earth's surface and you know its mass, how would you figure out force of gravity on it?

2)and given the radius and mass of a planet how would you be able to figure out the acceleration due to gravity on it?

2. Oct 3, 2005

Grogs

$$a_{grav} = \frac{GM}{r^2}$$

where G is the gravitational constant, M is the mass of the planet, and r is the distance from the center of the planet.

3. Oct 3, 2005

jshaner858

that won't work for the first question...the object is a spaceship and there is nowhere in that equation to plug in its mass...

4. Oct 3, 2005

jshaner858

nevermind i figured it out myself...thanks