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Dynamics Rotational Motion - Definition, FAQs

Dynamics Rotational Motion - Definition, FAQs

Edited By Team Careers360 | Updated on May 30, 2022 05:32 PM IST

Have you ever enjoyed spinning the wheel, have you ever seen music CDs rotating in audio system? These are nothing but an example of particle performing circular motion having Rotational dynamics.

In physics, we generally study about linear motion and Rotational motion. As the name suggests rotation, it means body is rotating and having some angular speed. In this chapter we will study about Dynamics of Rotational motion with a suitable example. We will see what is an angular motion? and learn some formulae. In the end we will try to understand mechanics of Rotational motion and find out work done in rotational motion.

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Rotational dynamics/what is rotational motion/rotary motion definition

Definition: In rotational dynamics, particle is moving in circular or curved path having:

  • Torque(?)
  • Angular acceleration(∝)
  • Moment of Inertia(I)
  • Angular displacement(?)

Rotational motion

Rotational motion images

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Rotational motion dynamics

  • Moment of Inertia(I): It is a physical property of any body, by which it resists any rotational change exerted by external torque. It is denoted by (I). The moment of Inertia is given by

I= MR2 It depends on mass, higher the mass higher will be the moment of inertia.

  • Torque: It is a vector quantity, product of perpendicular distance from axis of rotation and force applied to it. Torque has a twisting effect on the body.

Numerically, it is given by

?= ( r×F ).

  • Angular momentum: It is a measure of difficulty possessed by a rotatory body to come to rest.

Numerically it is given by:


Angular motion examples: Orbit of earth around the sun, rotation of tire

Work done in Rotational motion (work –Energy principle)

Work done in rotational motion is defined by the product of torque applied and change in angular displacement.

NCERT Physics Notes :

Work-Energy principle

Let consider small angle ∆? be the angular displacement under the effect of torque ?. Then linear displacement will be

∆r=r∆ ?,

Therefore the work done is given as,

W= ?∆ ?

Let say number of force acting, so net torque will be

(total)= (?1 + ?2 + ……)∆ ?

As we know ∆ ? is very small for all the torque thus net work done be zero.

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Frequently Asked Question (FAQs)

1. Define rotational motion with an example.

Rotational motion can be defined as an object moving in a circular path or rotating along a fixed axis( axis of rotation). Some examples of rotational motion are:

  • Ball rolling down a plane

  • Blade of ceiling fan

  • Rotation of the Earth around the Sun

2. Discuss the difference between translational motion and rotational motion.

The main difference between translational motion and rotational motion is that in translational motion change in relative speed is always zero but in case of rolling motion it is not equal to zero.

Case 1 when body is performing pure motion, then 

V=rw, where w is angular velocity and relative speed is not zero

Case2 when body is sliding, then it is performing translational motion hence in this case relative velocity is zero.

3. Define dynamic inertia definition.

In the uniform motion of an object, the inertia observed is called dynamic inertia.

4. Define dynamics.

The branch of mechanics that deals with the motion of objects under the action of forces.


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