# Motion in a circle

 P: 582 If the motion of the point mass is such that its direction is perpendicular to the direction of the acceleration and its speed is $$v=\sqrt{ar}$$ where r is the distance between the heavy body and the point mass, then the point mass will describe a circle of radius r around the heavy body. http://en.wikipedia.org/wiki/Circular_motion If the force between the two bodies does not have a 1/r^2 dependency, then tiny deviations from circular motion may cause the orbit to be unstable.