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# Rotation in Physics

Last updated date: 17th Apr 2024
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Rotation physics corresponds to the rotational motion included in the kinematics. Rotation physics plays a major role in kinematics in explaining everything around starting from the rolling motion of a ball to the motion of planets in their respective orbits around the sun. Rotation physics mainly focuses on the study of rigid body motion such as rotation of a disc about a fixed axis, the motion of a solid sphere, the concept of torque, etc. Rotation physics gives a deep insight into the concept involved in rotation kinematics.

## What is Called Rotation?

We observe the rotational motion in almost everything around us. Every machine, celestial bodies, most of the fun games in amusement parks, motion of the cricket ball, the way washing machines work, etc. The objects that turn about an axis exhibit rotational motion. All the particles and the centre of mass of the object do not undergo identical motions, but all the particles of the body undergo an identical motion. By definition, it becomes important for us to explore how the different particles of a rigid body move when the body is subjected to rotation.

In rotational kinematics, we will estimate the relation between kinematical parameters of rotation. Let us recall angular equivalents of the linear quantities: position, displacement, velocity, and acceleration which we usually consider during the study of an object that is subjected to circular motion. One should always remember that circular motion and rotational motion are two different aspects of physics and kinematics.

Rolling is an example of this category. Arguably, the foremost important application of rotational physics is within the rolling of wheels and wheels like objects as our world is now crammed with automobiles and other rolling vehicles. The rolling motion of a body may be a combination of both translational and rotational motion of a round-shaped body placed on a surface. When a body is about during a rolling motion, every particle of the body has two velocities – one thanks to its rotational motion and therefore the other thanks to its translational motion (of the centre of mass), and therefore the resulting effect is that the resultant of both velocities in the least particles.

### Rotation Definition Science

Let us try to understand what is called rotation physics and rotation definition science, what characterises rotation. You may notice that in the rotation of a rigid body about a fixed axis or fixed line, every particle of the body moves in a circle, which lies in a plane perpendicular to the axis and has its centre on the axis. The figure shown below illustrates the rotational motion of a rigid body.

Now, consider few particles from a given object, let P1 be a particle of the rigid body at a distance r1from the fixed axis or the fixed-line. The particle executes a circle of the radius r1 with a centre C1 on the fixed-line. The circle lies in a plane perpendicular (or 900) to the axis of rotation. From the figure, it shows that another particle P2 of the rigid body, which is at a distance r2 from the fixed axis or fixed-line. The particle P2 describes a circle of the radius r2 with a centre C2 on the fixed axis. The circle described by the second particle also lies in a plane perpendicular to the fixed axis. We should notice that the circles described by P1 and P2 may lie in different planes, but both planes are perpendicular to the fixed axis. For any particle on the axis like P3 , r = 0. Any such particle remains stationary while the body rotates. This is expected since the axis is fixed.

In some illustrations of rotational motion, however, the axis may not be fixed. A prominent example of this kind of rotation is a spin top spinning in place, as shown in the given figure below. We assume that the spin top does not slip away from the place to place and so does not execute the translational motion.

Now, the axis of such a spinning top moves around the vertical axis through its point of contact with the ground, sweeping out a cone as shown in Figure. This movement of the axis of the spinning top around the vertical axis is known as the precession. The point of contact of the spin-top with the ground is fixed. The axis of rotation of the spin-top at any instant passes through the point of contact.

Another simple illustration of this kind of rotational motion is the oscillating table fan or a pedestal fan where we see the rotation of wings of a fan about one fixed axis. The axis of rotation of such a table fan has an oscillating movement in a horizontal plane about the vertical axis through the point at which the axis is pivoted.

Now, in more general cases of rotational motion, such as the rotation of a spin-top or a pedestal fan, one point of the rigid body is fixed, not the one line. In this case, the axis of rotation will not be fixed, though it always passes through the fixed point. In rotation physics, however, we mostly deal with the simpler and special cases of rotational motion in which one line or the axis is fixed. Thus, the rotational motion will always be about a fixed axis or fixed-line.

The rolling motion of a cylinder down an inclined plane is a combination of rotational motion about a fixed axis and translational motion. Thus, something else in the case of rolling motion which we referred to earlier is rotational motion.  So, according to the rotational motion meaning, the motion of a rigid body which is not pivoted or fixed in some way is either a pure translational motion or a combination of translational motion and rotational motion. The motion of a rigid body which is pivoted or fixed in some way is rotational motion. The rotational motion executed by any object is always about an axis that is fixed (e.g. a ceiling fan) or moving (e.g. an oscillating table fan).

### Did You Know?

• Rotation physics is an important part of classical mechanics. As the advancement took place the consideration of linear motion was getting contradicted. People were often confused with circular motion and rotational motion. After decades of understanding, physicists were able to conclude that rotational motion or rotation is the motion of a particle in a circular motion.

• A two-dimensional object rotates about a centre (or point) of rotation. A three-dimensional object rotates about a line known as an axis. If the axis of rotation is within the body of the object, then the body is said to rotate upon itself, or spin, which refers to the relative speed and perhaps free-movement with angular momentum. A circular motion about an external point is known as an orbit or more precisely an orbital revolution, for example, the motion of the Earth around the Sun.

## FAQs on Rotation in Physics

1. What is the Rotational Force?

Ans: Rotational force is also known as the torque of the object. It is the force required to rotate the object over a fixed axis.

2. How Do You Calculate Rotation in Physics?

Ans: Calculation of rotation in physics is one of the most important parts. The rotation of an object can be determined by means of the analysis of the motion of the object using a set of theorems and laws.