question_answer

                A man weighs 80 kg. He stands on a weighing scale in a lift which is moving upwards with a uniform acceleration of \[5\,m/{{s}^{2}}\]. What would be the reading on the scale? \[(g=10\,\,m/{{s}^{2}})\]                     

A)                 800 N                    

B)                 1200 N                 

C)                 Zero                      

D)                 400 N

Correct Answer: B

Solution :

                Key Idea: When lift is moving upwards, it weighs more than actual weight of man by a factor of ma.                      Mass of man M = 80 kg                                 Acceleration of lift, \[a=5\,\,m/{{s}^{2}}\]                 When lift is moving upwards, the reading of weighing scale will be equal to R. The equation of motion gives                 R - Mg = Ma or            R = Mg + Ma = M (g + a) \[\therefore \]  \[R=80\,(10+5)\,=80\times 15\,=1200N\]