73. A mysterious force acts on all particles along a particular line and always points towards a particular point

P on the line. The magnitude of the force on a particle increases as the cube of the distance from that point;

that is Foor³, if the distance from P to the position of the particle is r. Let b be the proportionality constant,

and write the magnitude of the force as F = br³. Find the potential energy of a particle subjected to this force

when the particle is at a distance D from P, assuming the potential energy to be zero when the particle is at P.