1. The problem statement, all variables and given/known data Calculate the gravitational force between a point mass and a circular platform. 2. Relevant equations G=m1*m2/r^2 3. 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.