A 1.5kg rock is released from rest at the surface of a calm lake. If the resistance offereed by the water as ther rock falls is directly porportional to the rock's velocity, the rocks acceleration is a=g-C(d) v/m, where g is the acceleration fo gravity, C(d) is a constant drage coefficient, v is the rock's velocity, and the m is the mass of the rock. Letting C(d)=4.1kg/s, determeinder the rock's velocity after 1.8s.