1. The problem statement, all variables and given/known data Planet Z is 13700 km in diameter. The free-fall acceleration on Planet Z is 8.79 m/s2. What is the mass of Planet Z? 2. Relevant equations Gm1m2/r2 3. The attempt at a solution I am not sure how this problem is suppose to be worked at all, any of these kind I am having a problem connecting things together. I thought I would use potential energy which would be -G*M1*M2/r but that won't work because I am not given another weight. So my questions are....... Where do I start? What equations do I have to use?