explain Stokes law
Stokes law give the mathematical equation that Express the settling velocity of a small particle in a fluid medium .  it is derived by consideration of the forces acting on a particular particle as it strikes through a liquid column under the under the interference of gravity.force acting in resistance to the fall is equal to 6πrnv  where r is the radius of a sphere ,n is the  viscosity of liquid and v is the velocity of the fall .
the force acting downward is equal to 4/3πr3(d1-d2)g  where d1 is the density of the spheee,d2 is density of liquidn and g is the gravitational constant. At a constant velocity the upward and downward forces are inbalance. equating the two expression given above and solving for v therefore yields the required velocity expressed by strokes law as v=2/9(d1-d2)gr2/n