A car comes to a sliding stop in 5 m. During this process, the force on the car due to road is 100 N and is directed opposite to the motion.
(a) How much work done the road do on the car?
(b) How much work done the car do on the road?

Explanation

Hint : (a) The stopping force and displacement make an angle of `180^(@)` with each other
`W_(r)=F d cos theta`
`F=100 N, d=5m, cos 180^(@)=-1`
(b) An equal and opposite force acts on the road due to the car but road undergoes no displacement.
(a) Work done by the road, `W_(r)=Fd cos theta`
`=100xx5xx(-1)=-500J`
It is this negative work that brings the car to rest in accordance with W - E theorem.
(b) According to Newton.s III law, an equal and opposite force acts on the road due to the car i.e. 100 N.
However the raod undergoes no displacement, so work done on the road by the car is zero.
Note : Work done on body A by B not necessarily equal and opposite to the work done on B by A.