Rotation Motion & Moment Of Inertia-Couple

Rotational motion can be described as a movement of a rigid body on a fixed axis in a circular motion. Couples in rotational motion is a force in a parallel magnitude at the equal and opposite directions, which results in rotation.

In this article, we will go through the definition of rotational motion, couples, moments of inertia and other related topics of rotational motion. It is an integral part of physics. 

But before focusing on the couples in rotational motion, we need a little understanding of the motion.

Let’s start the article with motion.

Motion

Motion can be defined as a change of position of an object or a body with time. There are some of the physical characteristics of motion like velocity, displacement, distance, deceleration, acceleration, speed, which were analysed in motion. It can be two types

1 Linear Motion: when a body moves in a straight line, it is called linear motion.

2 Non-linear Motion: when a body does not move in a straight line, it is called non-linear motion

Motion can be categorised into different types. They are as follows

1. Translational motion
2. Rotational motion
3. Periodic motion
4. Non-periodic motion

Rotational motion

A motion of a body or object about a fixed axis is known as Rotational motion. In rotational motion, a rigid body moves in a way that all the resulting particles move in a circular motion at a common axis. There are various examples of rotational motion in our daily life such as,

1. Earth moves in a rational motion on its axis, which causes the cycle of night and day.
2. The blades of the helicopter or fan move in a rotational motion.
3. The motion of the Ferris wheel in an amusement park.
4. The motion of the gears, wheels of the vehicles, motors, etc., also move in a rotational motion.

The rotational moment on a fixed axis

The rotation motion of a rigid body in a circular motion on its own axis without deforming the object can be said as a rotational motion on a fixed axis. A typical example of rotational motion on a fixed axis is the motion of a CD or compact disc in a CD player, which is run by a device inside the player. 

What are Couples? Definition.

The couples can be defined as a force, where two opposite and equal forces are applied on the object or body at different points simultaneously. The resultant force of the couples is zero, but these forces (opposite and equal forces) try to rotate the body in a similar direction. We can calculate the couple by the formula.

M = F*d

d= Perpendicular distance in metre (m)

F= Force in Newton (N)

Couples forces only produce rotation motion, not the translation motion. Couples in rotational motion is the pure moment where the resultant of the forces is not a 0. Let us look for examples of couples in rotational motion. While driving the car, the hands of a person apply a couple of forces to the steering wheel for rotational motion. While turning the steering wheel, each hand grips points on the wheel on opposite sides of the shaft and when they apply opposite and equal forces with the same magnitude on the shaft, the wheel turns or rotates in the same direction and if the magnitude of the force of the two hands were not the same, then there will be an imbalance that will create the translation. 

Dynamics of Rotational motion

There are mainly two dynamics of rotational motion, which are torque and moment in inertia. 

Torque: It is also known as moment of force or moment. Torque is the tendency of a force to change the rotational motion of an object or a body. The twist or turning force on an object is torque, which can be calculated by multiplying distance and force.

Moment of inertia: It is the inertia of an item in rotation motion. It is also referred to as the rotational inertia, mass moment of inertia, second moment of mass and angular mass. A moment of inertia is the quality of a body’s resistance by an object or a body to rotational change. Furthermore, it is affected by the rotational axis, mass dispersal, as well as other attributes. When there is a larger moment of inertia, more torque (force × distance from the axis of rotation) is required to change the angular acceleration. 

It is measured in kilograms per metre square or Kg/m2 and is represented as I. The formula of moment or inertia is expressed as

I = Mr2

Where,

M = mass of the particle

r = distance from the rotation axis

Conclusion

In couples, during rotational motion, the body moves in a rotation(circular) motion but with the forces of the same magnitude at different points of the body. The examples of couples in rotational motion are opening and closing of the bottle cap, unlocking the padlock, closing and opening of the tap, etc.

In the above article, we must have discussed the topics related to rotational motion and hopefully solved your queries.