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? \[(g=10\text{ }m/{{s}^{2}})\] [AIPMT 2003]

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 weighing scale will be equal to R. The equation of motion gives

\[R-Mg=Ma\]

or         \[R=Mg+Ma=M(g+a)\]

\[\therefore \] \[R=80(10+5)=80\times 15=1200\,N\]