Question

S.I. unit of moment of force is
(A) Dyne-cm
(B) Erg-kg
(C) Newton-metre
(D) Newton-kg

Answer 1

First of all, define the term force and write its formula and then define the term moment of force and then write its formula. Write the S.I. unit of each term involved in that particular formula. After that substituting the S.I. unit of each term back in the formula, you will get the S.I. unit of the required term. Here, moment of force is the cause of rotation of the body about a specific point on its axis.

Complete step by step solution: Force is defined as the rate of change of momentum. For non-variable masses, force (represented by F) is also defined as the product of mass (represented by m) and acceleration (represented by a).
Force, ${\text{F = m}} \times {\text{a}}$
S.I. unit of mass (m) is kg
S.I. unit of acceleration (a) is ${\text{m/}}{{\text{s}}^{\text{2}}}$
Thus, S.I. unit of force is ${\text{kgm/}}{{\text{s}}^{\text{2}}}$ or Newton (N)
And moment of force is the cause of rotation of the body about a specific point on its axis.
Formula for Moment of force = $Force \times perpendicular\ distance$
S.I. unit of force is Newton (represented by N)
S.I. unit of distance is metre (represented by m)
So, S.I. unit of moment of force is Newton-metre (represented by N-m).

Therefore, option (C) is the correct choice.

Note: Keep in mind that the term ‘force’ and the term ‘moment of force’ is different from each other. They are not the same. S.I. The unit of force is Newton (represented by N) and the cgs unit of force is dyne. The S.I. The unit of moment of force is Newton-metre (represented by N-m) and cgs unit of moment of force is dyne-cm.