The drag force 
acting on a sphere moving through a fluid is given by the expression
where 
is a constant, 
is the density of the fluid and 
is the speed of the sphere.
A ball of weight 
falls vertically from rest in air. The drag force acting on the ball is given by the expression above. The ball reaches a constant (terminal) speed of 
. Assume that the upthrust acting on the ball is negligible and that the density of the air is uniform. For the instant when the ball is travelling at a speed of 
, determine
the drag force on the ball,
the acceleration of the ball.
.
.
