Question

The principle involved in the construction of beam balance is:
(A) Principle of moments
(B) Principle of inertia
(C) Principle of superposition
(D) Principle of velocity

Answer 1

Hint Balance is based on the torque acting on the two bodies at both corners. In short, we can say that when the amount of total clockwise moment is equal to the total anti-clockwise moment then the body is said to be in a balanced position.

Complete Step-by-step solution
The principle of moments states that if the amount of total clockwise moment is equal to the total anti-clockwise moment then the body is said to be in a balanced position.
The definition of a beam balance is that it is a device that is used to determine the mass of a body under gravity. The beam balance consists of a beam generally made up of iron supported at the center by a sharp edge resting on a support moving inside a vertical pillar. The beam carries a light pointer which moves over a calibrated scale. The beam also has two identical pans which are separated by an equal distance from the center at the two ends of the beam.
If the mass of the two bodies placed on the pan is the same then the gravitational force acting on them is the same. Hence as the force acting on both the pans is the same and both pans are at an equal distance from the axis of rotation so the torque on them will be the same as the magnitude of torque is equal to the product of force and the distance between the point of application of force from the axis of rotation.

Hence the correct answer to the above question is (A) Principle of moments.

Note
Moment and torque are the same things, torque is a vector quantity so the torque vector acting on both the pan is the same then only they will become balanced. The direction of torque can be determined using the right-hand thumb rule on distance vector and force vector.