1. The problem statement, all variables and given/known data Calculate the formula for the force of gravity in a universe where Kepler's 2nd law holds and a planet revolves around the sun with polar eq r = 1 + sin (theta). Theta = pi/2 is where velocity and position vector are perpendicular. 2. Relevant equations Universal gravitation law. Perihelion = point at which velocity and position vectors are perpendicular = 2. 3. The attempt at a solution I don't understand something. How does the shape of the orbit of the planet around the sun affect the formula for force of gravity? Wouldn't the solution simply be Newton's Universal Gravitation formula, the radius being a function of theta? I guess what I'm looking for is an explanation as to why my assumption isn't correct, which hopefully will give me a hint as to how best to approach this problem.