1. The problem statement, all variables and given/known data The question is presented as picture. 2. Relevant equations We do not have the radius of the cavity and the position of the m. How can we find the force acted on it? 3. The attempt at a solution r=1/(M-m)*Ʃmr=1/(M-m)*4/3ρR'^3*r where m is the mass of cavity ,R' is the radius of the cavity and r is the position of center of mass. But I can't find R'.