- #1

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## Homework Statement

Calculate the gravitational force between a point mass and a circular platform.

## Homework Equations

G=m1*m2/r^2

## The Attempt at a Solution

http://i.imgur.com/dfZf9GK.jpg

The actual solution is different. They integrated by the angle between a/r (alpha) but i do not understand the cos(alpha) at the start, check this out:

http://i.imgur.com/nwxhCEb.jpg

Can anyone explain this please? Thanks.