Question

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

• A. 800 N
• B. 1200 N
• C. Zero
• D. 400 N

Answer 1

Answer: (B)

1200 N

Answer 2

When lift is moving upwards, it weighs more than actual weight of man by a factor of $m a .$
Mass of man $M=80\, kg$

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