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 skillfully held stationary in the sky,
$(d)$ a car moving with a constant velocity of $30\; km/h$ on a rough road,
$(e)$ a high-speed electron in space far from all material objects,and free of electric and magnetic fields.

(N/A) Zero net force: The rain drop is falling with a constant speed,meaning its acceleration is zero. According to Newton's second law of motion,the net force acting on the rain drop is zero.
$(b)$ Zero net force: The weight of the cork acts downward and is balanced by the buoyant force exerted by the water in the upward direction. Thus,the net force is zero.
$(c)$ Zero net force: The kite is stationary in the sky,meaning it has no acceleration. According to Newton's first law of motion,the net force acting on the kite is zero.
$(d)$ Zero net force: The car is moving with a constant velocity,so its acceleration is zero. According to Newton's second law of motion,the net force acting on the car is zero.
$(e)$ Zero net force: The high-speed electron is far from all material objects and external fields,so no force acts upon it.