Question

According to work energy theorem or work energy principle, work done by net force in displacing a body is always equal to change in K.E. of the body. Similarly , if some work is drawn from the body, K.E. of the body decreases by the same amount. In either process, we assume that potential energy of the body does not change.
Read the above passage and answer the following question:
(i) A car weighing 500 kg is running at a speed of `72 km//h`. In how much distance will it stope by applying a braking force of 4000N?
(ii) What are the implication of this theorem in day to day life ?

Answer 1

(i) Here, `m=500kg, u=72km//h=(72xx1000)/(60xx60)m//s=20m//s`
`s=?, upsilon=0, F=-4000N`
As work done `=` change in K.E. of the car,
`:. Fxxs=(1)/(2)m(upsilon^(2)-u^(2))`
`-4000xxs=(1)/(2)xx500(0-20^(2))=-250xx400`
`s=(-250xx400)/(-4000)=25m`
(ii) In day to day life, K.E. acquired corresponds to output and workk done corresponds to work put in through out efforts. The theorem implies that under ideal conditions, our achievement `//` output will be equal to work put in by us. We should plug all loop holes involving loss of energy to get the maximum output.