question_answer

A particle of mass 100 g is thrown vertically upwards with a speed of 5\[m{{s}^{-1}}\]. The work done by the force of gravity during the time the particle goes up is

A)  \[-\,0.5\text{ }J\]                           

B)  \[-1.25\text{ }J\]

C)  \[1.25\text{ }J\]                              

D)  \[0.5\text{ }J\]

Correct Answer: B

Solution :

                The height (h) traversed by particle while going up is \[h=\frac{{{u}^{2}}}{2g}=\frac{25}{2\times 9.8}\] Work done by gravity force\[=m\overrightarrow{g}\cdot \overrightarrow{h}\] \[=0.1\times g\times \frac{25}{2\times 9.8}\cos 180{}^\circ \] [Angle between force and displacement is\[180{}^\circ \]] \[\therefore \]  \[W=-\,0.1\times \frac{25}{2}=-1.25\,J\]