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 m/s². What would be the reading on the scale? (g=10 m/s²) 

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

Answer 1

The Correct Option is D

To find the reading on the weighing scale when the lift is moving upwards with a uniform acceleration, we need to analyze the forces acting on the man. Here's how we solve the problem:

1. Initially, when the lift is at rest or moves with constant velocity, the reading on the weighing scale equals the man's weight, which is due to gravity. The force due to gravity is calculated as:

   F_{\text{gravity}} = m \cdot g = 80 \, \text{kg} \times 10 \, \text{m/s}^2 = 800 \, \text{N}

2. When the lift moves upwards with an acceleration (a), an additional force is applied due to this acceleration. The apparent weight (reading on the scale) is the sum of the gravitational force and the force due to the acceleration of the lift.

   F_{\text{total}} = m(g + a)

3. Substitute the given values into the equation to find the total force:

   F_{\text{total}} = 80 \, \text{kg} \times (10 \, \text{m/s}^2 + 5 \, \text{m/s}^2) = 80 \, \text{kg} \times 15 \, \text{m/s}^2 = 1200 \, \text{N}

4. Therefore, the reading on the scale when the lift is accelerating upwards is 1200 N.

This analysis shows how we consider both the gravitational force and the force due to the acceleration of the lift. The correct answer is 1200 N.