Question

Which of the following instruments make use of the principle of moments during operation?
(A) Ruler
(B) Vernier callipers
(C) Spring balance
(D) Simple balance

Answer 1

The principle of moment states that for rotational equilibrium to take place, the anticlockwise moment must be equal to the clockwise moment. Moment is the product of force and distance, and if under gravity, the force could be the weight of an object.

Complete answer:
Generally, the principle of moment states that for rotational equilibrium to be attained, the sum of all clockwise moments must be equal to the sum of all anticlockwise moments. The question is to find which of the options given uses this principle as a mode of operation.
Let us investigate the options one after the other.
The ruler – this is simply an instrument for measuring linear length, such as a straight line, or a straight object, such as rectangular, triangular, etc. it has nothing to do with the principle of moment, hence can be eliminated
The Vernier calliper – this is an instrument also used to measure linear length, but unlike the ruler can measure the diameters of a circular object. It is also more accurate than the ruler.
The spring balance – the spring balance is an object for measuring weight. It uses the principle of force, that is, upward force equal downward force.
The simple balance – the simple balance can also measure weight. Usually used by comparison with already known weights. It uses the principle of moment for the comparison
Hence, the correct option is D.

Note:
For clarity, the simple balance is simply a beam supported at the centre. The ends of the arm are attached to a carrier (or container) which holds the weight. On one side, a known weight is placed and on the other is the unknown mass to be measured. When they are unequal, the beam is imbalanced, then more known weights are placed in (or removed from) the container until the beam balances horizontally.