question_answer 12)

A man weighs 70 kg. He stands on a weighing machine in a lift, which is moving (i) upwards with a uniform speed of . (ii) downwards with a uniform acceleration of . (iii) upwards with a uniform acceleration of . What would be the readings on the scale in each case ? What would be the reading, if the lift mechanism failed and it came down freely under gravity ?  

Answer:

The apparent weight measured by the weighing machine is the measure of the reaction R exerted on the man due to the lift. (i) When the lift moves upward with uniform velocity, reaction of the lift is equal to the weight of the man.  Apparent weight,   (ii) When the lift moves downwards with uniform acceleration,                                     Fig. 5. 126 (a)                     (b)   Resultant downward force,  or    Apparent weight,   . (iii) When the lift moves upwards with uniform  acceleration, Resultant upward force,  or    Apparent weight,     When the lift falls freely under gravity, a = g  Apparent weight, This is the condition of weightlessness.