## Tuesday, January 12, 2016

### Physical Quantities of Rotational Dynamics

To understand rotational motion, we need different physical quantities from translatory motion. We have physical quantities like displacement, velocity, acceleration and force are used to understand translatory motion of a body. We can not use same physical quantities to understand rotational motion. To understand why not, let us consider a example. Let us consider a body is in rotational motion about an axis of rotation. It is the line around which every point of the body is rotating.

Let the body completes one rotation. After completion of one rotation, displacement of the body is zero as the body came back to the original position. As the displacement is zero, its velocity is also zero. As the velocity is zero, acceleration which is defined as rate of change of velocity as also zero. As the force is defined as the product of mass and acceleration, force acting on the system is also zero.

We know that the work done is the product of force and displacement. As the force is zero automatically work done is also zero.

Consider the scenario here that the body has zero displacement, velocity, acceleration, force and work done and still the body is having its rotational motion. It clearly indicates that the parameters we have taken are not suitable to understand rotational motion. We need all together different physical quantities.

Hence we need to study about physical quantities like angular displacement, angular velocity, angular acceleration, torque, moment of inertia and rotational kinetic energy to understand rotational dynamics.

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