# Homework Help: Gravitational field strength

1. Sep 16, 2010

### thereddevils

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

Write an expression for the gravitational field strength E at a point distance r from a planet of mass M. (In my diagram, there are two planets with mass, m and distance 2a apart)

2. Relevant equations

3. The attempt at a solution

The resultant gravitational field strength is towards the left. Using the parallelogram method to solve for the resultant vector and by the cosine rule,

$$|E|^2=(\frac{GM}{R^2})^2+(\frac{GM}{R^2})^2-2(\frac{GM}{R^2})(\frac{GM}{R^2})\cos 120$$

$$|E|=\frac{GM\sqrt{3}}{(r^2+a^2)^2}$$

$$E=\frac{2GMr}{(a^2+r^2)^{\frac{3}{2}}}$$

#### Attached Files:

• ###### gravitational field strength.bmp
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2. Sep 16, 2010

### Mindscrape

I don't know what a parallelogram method is. You know from symmetry that the y components will cancel, so all you have to do is look at the two x components, which will both add to give

2cosØ(GM)/(a^2+r^2)^(1/2), and cosØ=r/(r^2+a^2)

Last edited: Sep 17, 2010
3. Sep 17, 2010