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 \[h=\frac{{{u}^{2}}}{2\,\,g}=\frac{25}{2\times 9.8}\]        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\[{{180}^{o}}\]] \[\therefore \]  \[W=-0.1\times \frac{25}{2}=-1.25\,\,J\]