# Homework Help: Gravitational Force as a vector

1. Apr 7, 2010

1. The problem statement, all variables and given/known data

r = <0,16,0> m
m1 = 69kg
m2 = 6e24kg (earth)

I need to compute the gravitational force on the object (m1) as a vector

2. Relevant equations

F = GMm/r^2

3. The attempt at a solution

I get 1.08e14, which is incorrect. I have a feeling I am calculating a magnitude instead of a vector, I'm just not sure what I need to do differently.

2. Apr 7, 2010

### Mintea

A vector is a magnitude with a direction attached; there's no difference in calculation. Maybe you needed to specify the direction of the gravitational force?

3. Apr 7, 2010

### collinsmark

Whoa, hold on. Something is not right.

The "F = GMm/r^2" equation you mentioned is only valid if the object is at the surface of the Earth or at a greater distance away from the Earth's center that that, if you wish to use the entire mass "m2 = 6e24kg (earth)."

But the way that you worked the equation, you calculated the force on an object 16 m away from the center of the Earth, and that the entire mass of the Earth is confined within a 16 m (radius) ball or smaller ball.

As a first course of action, I'm guessing that might need to take the Earth's radius into account, if the object is ouside the Earth's surface. Could you be more clear on what coordinate system you are using when you state, "r = <0,16,0> m"? I.e, where in relationship to the center of the Earth is r = <0, 0, 0> m?

4. Apr 7, 2010