Question

Rain drops fall with terminal velocity because
(A) Buoyancy
(B) Viscosity
(C) Low weight
(D) Surface tension

Answer 1

Hint: in the absence of all other external forces, all bodies accelerate to the ground at the same rate. Most internal forces are always acting.

Complete answer:
Generally, if we ignore all external forces, a rain drop accelerates and continues to accelerate until it it’s the ground due to the gravitational force of the earth attracting the raindrop. An effect of this attraction is the weight of the raindrop.
Surface tension is already turned on by now (i.e. it is acting on the molecules of the water) as it is an internal force. The very existence of a drop means that the surface tension is acting on the molecules of water. Recall that surface tension is due to the intermolecular forces between molecules which tend to give the shape of an uncontained liquid a spherical nature.
Now, if we consider that the atmosphere exerts forces on the raindrop. One is the buoyant force and the other is the viscous force. Buoyancy is dependent on the density of a body only and hence cannot allow a body to attain terminal velocity.
The viscous force is the force which is proportional to velocity and hence allows substance to attain terminal velocity when submerged in a fluid of different nature.

Hence, the correct option is B.

Note: For clarity, the viscous force allows substances to attain terminal velocity because, since it is dependent on velocity, when an object is accelerating under gravity, the velocity increases, hence the viscous force (acting upward opposite gravity) also increases. The speed of the object continues to increase (and so does the viscous force) until the viscous force becomes equal to the gravitational force. Hence, the net force is zero and the object will no longer accelerate and instead continues at the velocity at which the equality was attained until it hits the ground.