Question

SI unit of moments of couple is ….
$
  (a){\text{ Nm}} \\
  {\text{(b) dyne cm}} \\
  (c){\text{ N}}{{\text{m}}^{ - 2}} \\
  (d){\text{ none of these}} \\
$

Answer 1

Hint – In this question the basic definition of moment of a couple, it refers to the two equal and opposite forces acting at two different points separated by some d distance apart such that this force produces a torque which is given as $\tau = Fd$. This will help finding the S.I unit.
Step by step answer:

Moment of couple
A couple is known as the oppositely directed pair of forces which is equal in magnitude and displaced by some perpendicular distance also called as moment.
Because of this oppositely acting pair of forces it starts rotating i.e. a torque is developed about an axis which is normal to the plane of forces.
The simplest kind of moment has two equal and opposite forces whose line of action do not coincide, this is also called a simple couple.
Let the two forces be F and –F displaced by the perpendicular distance d then the torque or moment of couple is given as,
$ \Rightarrow \tau = Fd$
Where, $\tau $ = moment of couple, F = magnitude of the force and d = perpendicular distance between the two parallel, opposite and equal forces.
Now as we know that the S.I unit of force is Newton and S.I unit of distance is meter.
So the S.I unit of moments of a couple is Newton-meter (N-m).
So this is the required answer.
Hence option (A) is the correct answer.

Note – SI units is known as international systems of units, it is a system of physical units based upon the standard measurement of meter, kilogram, second, ampere, kelvin, candela and mole together with a set of prefixes to indicate the multiplication or division by a power of 10. This standard system is adopted everywhere around the world.