Number 10.4 , 10.5. Please help me

that the forces pass through it.) 10.4. An object of mass m released at a very large distance from Earth falls toward Earth's cent culate the time it will take (a) to reach balfway to Earth's center, and (b) to reach from balfway to Earth's center. Compare the two time intervals. Assume Earth to be a point mass -GMm Ir for r> R, show that as a mass m is moved from the sur 10.5. Using the expression U(r) face of Earth to a height h the change in the potential energy is =mgh. %3D of mars m m in a certain force field given by F = - klr is moving toward the center of %3D