Give the magnitude and direction of the net force acting on:
$(a)$ a drop of rain falling down with a constant speed.
$(b)$ a cork of mass $10 \,g$ floating on water.
$(c)$ a kite skilfully held stationary in the sky.

(N/A) The rain drop is falling with a constant speed,which means its acceleration $a = 0$. According to Newton's second law,the net force $F = ma = m(0) = 0$. Thus,the net force is zero.
$(b)$ $A$ cork floating on water is in equilibrium. The downward gravitational force (weight) is balanced by the upward buoyant force (upthrust). Therefore,the net force acting on the cork is zero.
$(c)$ Since the kite is held stationary,its velocity is zero and its acceleration is zero. According to Newton's first law,if an object is at rest,the net external force acting on it must be zero.