Kinematics of the rotational motion

Rotational movement is widespread in nature, no less (but can be even more) than translation motion. Indeed, the motion of electrons around the nucleus (within the Bohr atomic model) and the earth around the sun, the rotation of a gyroscope, the rotation of numerous details and assemblies in technology and industry, the rotation of a wheel (this genius invention of mankind)—all of these are examples of rotational motion. 

The rotational motion of the IRB around a motionless axis Oz in which all points of the body are moving in parallel planes, making circles with their centers lying on a single straight line coinciding with the z-axis, is referred to as the rotational motion of the IRB. 

When rotating, aU points of the IRB have linear velocities differing in size and direction, depending on the point distance from the axis of rotation. So, for a description of rotational motion we should iutroduce angular kinematic features unique to the whole body angular displacanent, angular velocity and angular acceleration. 

Let us restrict ourselves to the case of IRB rotatiou around an axis whose space position does not change in time. 

Vectors whose directions are aligned with the rotation direction are called axial vectors. Angular displacement dtp is an axial vector, the modulus of which is equal to the ratio of arc dS and radius R and the direction of which coincides with the rotation axis in accordance with the right screw rule. 

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