An object of mass 5 kg falls from rest through a vertical distance of `20 m` and attains a velocity of 10 m/s. How much work is done by the resistance of the air on the objrct?
`(g=10m//s^(2))`.

Explanation

Applying work-energy theorem, work done by all the forces=change in kinetic energy
or `W_(mg)+W_(air)=(1)/(2) mv^(2)`
`:. W_(air)=(1)/(2)mv^(2)-W_(mg)=(1)/(2)mv^(2)-mgh`
`=(1)/(2)xx5xx(10)^(2)-(5)xx(20)=-750 J`