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\[5\text{ }m/{{s}^{2}}\]. What would be the reading on the scale?\[(g=10m/{{s}^{2}})\]

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\[M=80\text{ }kg\] Acceleration of lift, \[a=5\text{ }m/{{s}^{2}}\] When lift is moving upwards, the reading of weighting scale will be equal to R. The equation of motion gives \[R-mg=ma\] or     \[R=mg+ma=m(g+a)\] \[R=80(10+5)=80\times 15=1200\text{ }N\]