A child stands on a weighing machine inside a lift. When the lift is going down with acceleration g 3 , the machine shows a reading 20 N. When the lift goes upwards with acceleration g 3 , the reading would be ( g = gravitational acceleration)

Step-by-step solution

Mass and apparent weight Let the mass of the child be m , and R be the reading on the scale, which equals the normal force exerted on the child. The apparent weight changes when the elevator accelerates. Applying Newton's second law, F = ma : Elevator descending with acceleration a = g 3 The forces are the gravitational force mg downward and the normal force R₁ upward. Since the acceleration is downward, mg - R₁ = ma . Given R₁ = 20 N and a = g 3 , we have: mg - 20 = mg 3 2mg 3 = 20 mg = 30 N The actual weight is 3