What do you mean by weight of a body? Use examples to distinguish between real weight and apparent weight of a body.
Weight of a body is the gravitational pull of the earth on the body.
A spring balance measures weight of an object. The object exerts force on the spring of the balance, which is equal to the pull of gravity on the object. This is called apparent weight (as it seems on the scale) of the object and may be different under different circumstances.
Consider an object suspended by a spring in a lift. Let the mass of the object is ‘m’. When the lift is at rest, the resultant force acting on the object is zero and hence the acceleration, too. In this case, if is the force of gravity acting on the object and is the tension in the spring, then applying Newton’s second law of motion;
The acceleration is zero, therefore,
In this case, the scale shows the real weight of the object. However, the weight will seem varying if the lift is moving up or down. Let us consider both cases in detail.
(1) Suppose the lift is moving up with acceleration . This means the upward tension in the spring is more than the downward weight of the body. Or > by an amount ‘ma’ where ‘m’ is the mass of the object. So
The apparent weight of the object would seem more than its real weight by an amount ‘ma’.
(2) Suppose the object is moving down with acceleration. This means the upward tension in the spring is less than the real weight of the object. In this case,
Tension in the spring, i-e, the reading on the scale, is less than the real weight by an amount ‘ma’. This means the apparent weight (as it seems on the scale) is less than the real weight of the object.
(3) Special case: If the cable of the lift breaks, the lift is falling freely under gravity. Then = .
The apparent weight on the scale would be seen as zer