Q. 4.     Certain force acting on a  20 \; kg  mass changes its velocity from 5\; m\; s ^{-1} to 2\; m\; s ^{-1}. Calculate the work done by the force.

Answers (1)

By the equations of motion, we can write :

                                              v^2\ =\ u^2\ +\ 2as

or                                             s\ =\ \frac{v^2\ -\ u^2}{2a}

or                                                  =\ \frac{2^2\ -\ 5^2}{2a}\ =\ \frac{-21}{2a}\ m

And the work done is :

                                              W\ =\ F.s

                                                      =\ 20a\times \frac{-21}{2a}

or                                                   =\ -210\ J

Thus work done is  - 210 J.

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