Question

Why is weight of a body considered as a variable quantity?

Answer 1

Explanation:

The mass, strictly the inertial mass, relates the acceleration of a body to the applied force via Newton's law:

F=ma

So if you apply a force of 1 Newton to a mass of 1kg it will accelerate at 1m/s².This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface.

The weight is the force a body exerts when it is in a gravitational field. The weight depends on the gravitational field. For example the weight of a 1kg mass at the Earth's surface is 9.81 Newtons, while at the surface of Mars it's about 3.5 Newtons.

This is possibly a bit too much info: if so ignore this last paragraph. Although weight specifically means the force exerted in a gravitational field, Einstein told us that sitting stationary in a gravitational field is equivalent to being accelerated in the absence of gravity. The inertial mass defined using Newton's laws is the same as the gravitational mass defined by the force a body exerts in a gravitational field. So if you take a 1kg mass at the Earth's surface, the weight of 9.81 Newtons it exerts is exactly the same as the force you'd need to accelerate the 1kg mass at 9.81m/s²

Answer 2

Weight of the body is the variable quantity and changes with the change in position and location due to the acceleration of the gravity acting on it. Yes they are used at different places and time.