In the topic Dynamic Equilibrium, there is an example which states that a rain drop reaches the earth surface with a constant velocity . How is that possible ?

Isn't the weight = m*g and since the weight of the rain drop is balanced by the bouyant force due to the friction of the air , shouldn't the rain drop be floating in the air ? 

thank you 

 

Asked by srinifnogmi400 | 6th Apr, 2020, 02:13: PM

Expert Answer:

when rain drop falls from height, its velocity increases because of acceleration due to gravity.
 
Friction force due to air resistance opposing the motion of rain drop. Friction force is proportional to velocity of rain drop.
 
Initially gravitational force is greater than friction force, hence rain drop is getting accelerated and its velocity keep on increasing.
 
As velocity of rain drop increases, friction force also increases. At some instant friction force will become equal to gravitational force
and net force acting on the rain drop becomes zero. At this instant onwards, rain drop moves with constant velocity
as per Newton's first law.  At this instant onwards, rain drop has  zero acceleration because net force is zero.
This velocity is called terminal velocity . Rain drop falls to the ground with constant terminal velocity
 
Rain drop will not float in air, because density of rain drop is 1000 times greater than air

Answered by Thiyagarajan K | 6th Apr, 2020, 02:40: PM

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