When an object moves through a fluid , it experiences a retarding force due to turbulence . For a sphere of radius r moving with a speed of v in a fluid of density , p , the retarding force is given by

$\displaystyle

F=kpr^2v^2

$

By relating the retarding force to the transfer of momentum between the fluid and the sphere , explain why the force F is directly proportional to $\displaystyle pr^2v^2$