A liquid drop of 1 gm falls from a height 1 km. It hits the ground with a speed of 50 m/s. What is the work done by unknown resistive force ?
Answers
Work is said to be done in physics only if there is displacement. Potential Energy(storing of energy) -is the ability of the body to do work when left.
For example,you take a ball to a height h then the ball will have maximum potential energy that means when you will leave it will do work.But potential energy is said to be there in body till you didn't leave the body.
Another example,take a spring and leave it like that on your bed then it is not doing work as there is no displacement. But when you squeeze the spring then potential energy of the spring becomes maximum as it has ability to do work when left but potential energy is said to be there when you didn't leave.
One thing if you hold a body and after leaving it doesn't do any work that means on leaving it doesn't cover displacement hence that body has no potential energy.For example when you hold a pen on the surface of the bed then on leaving also it will be at same position hence it has no potential energy.
Kinetic energy is seen when a body is in motion hence doing work.
For example,take the ball to a height h but don't leave then it has maximum potential energy but when you leave the ball then it is in motion as well as doing work resulting in Kinetic Energy being maximum.
Main point-
When Kinetic Energy is gained by a body as you seen above then it loses potential energy (store energy) means that store energy is been used in the form of kinetic energy.
Gain in kinetic energy=Loss in potential energy
This above case is only applicable for isolated system but not in this practical world because there is opposing forces being acting hence
Gain in kinetic energy is not equal to Loss in potential energy.
Hence to know the opposing force which act in potential energy to kinetic energy we need to subtract that is
kinetic energy - potential energy
Formula for potential energy=mass*gravity*height
Formula for kinetic energy=1/2*mass*(velocity)^2
Now come to the questions answer,
As the rain drop is falling hence it is in Kinetic energy losing potential energy
Loss in potential energy=mgh
=1/1000kg*10*1000m
=10J
Gain in kinetic energy=1/2*mv^2
=1/2*1/1000*50*50
=5/4
=1.25J
Hence to know work done by resistive force/opposing force
=kinetic energy - potential energy
=1.25 - 10
= -8.75J