question_answer

A particle of mass 100 g is thrown vertically upwards with a speed of 5 m/s. The work done by the force, of gravity during the time the particle goes up is

A) \[-0.5\,\,J\]                        

B) \[-1.25\,\,J\]

C)  \[1.25\,\,J\]                      

D)  \[0.5\,\,J\]

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\[=m\overset{\to }{\mathop{\mathbf{g}}}\,\cdot \overset{\to }{\mathop{\mathbf{h}}}\,\]                 \[=0.1\times g\times \frac{25}{2\times 9.8}\cos {{180}^{o}}\] [Angle between force and displacement is \[{{180}^{o}}]\] \[\therefore \]  \[W=-0.1\times \frac{25}{2}=-1.25\,\,J\]