According to Newton's equation, gravity's force is F=(GmM) / r^2. Those are the gravitational masses. The mass in F = ma is the inertial mass. The fact that the

According to Newton's equation, gravity's force is F=(GmM) / r^2. Those are the gravitational masses. The mass in F = ma is the inertial mass. The fact that they are the same is remarkable. If it weren't for that, different mass objects would feel different accelerations due to gravity.