Question

A man weighing 50 kg is standing in a lift. Find the thrust on the floor of the lift due to to his weight when
A) ascents with an uniform acceleration of 1.2m/s?
B) the lift is accelerating downwards at 3m/s?
C) the lift is accelerating with a uniform velocity of 3m/s.

Answer 1

Dear student

(a) when lift is accelerating upward with a uniform a=1.2 m/s², let the thrust force is R. Since the weight acts downward, thrust acts upward and lift is moving upward the equation of motion is
$$\begin{gathered} R-mg=ma \\ R=mg+ma \\ R=50\times 9.8+50\times 1.2 \\ R=550N \end{gathered}$$
(b)​when lift is accelerating downward with a uniform a=3 m/s², let the thrust force is R. Since the weight acts downward, thrust acts upward and lift is moving downward the equation of motion is
$$\begin{gathered} mg-R=ma \\ R=m\left(g-a\right) \\ R=50\left(9.8-3\right) \\ R=50\times 6.8 \\ R=340N \end{gathered}$$
(c) if lift is moving with uniform velocity then a=0 and
R=mg=50*9.8
R=490 N