question_answer

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

A) -0.5J                            

B) -1.25J    

C) 1.25J               

D) 0.5J

Correct Answer: B

Solution :

Gravitational force is a conservative force apply law of energy conservation \[{{\operatorname{k}}_{A}}+{{P}_{A}}={{k}_{B}}+{{P}_{B}}\] \[\frac{1}{2}\times \frac{100\times {{\left( 5 \right)}^{2}}}{1000}+0=0+\frac{100}{1000}\times 10\times H\] \[H=\frac{25}{20}\operatorname{m}\] \[{{\operatorname{W}}_{gravity}}\,\,={{F}_{gravity}}\,\,H\,\,Cos18{{0}^{o}}\] Since \[\vec{F}\,\,\operatorname{and}\,\,\vec{H}\]are opposite in direction \[=-mgH\] \[=-\frac{100}{1000}\times 10\times \frac{25}{20}=-1.25\operatorname{J}\]