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WHY WE USE 'ROTATIONS' EVERYWHERE? 

        

            In this world, rotations are more common than the linear motion.  The planets revolves and spins; the electron circles; the wheels roll; the motor and the engine runs; even opening and closing a door is a partial rotation.

     Force required :  Newton says that if a body is given a push, it will only move in a straight line.  Hence, to keep the body in rotation, a force must act and it should act continuously.  A force must act from the moving body towards the center ceaselessly.  It is centripetal force.
     The sun keeps all the nine planets revolving around itself because of gravitational pull which acts as centripetal force.
     The electron keeps circling the nucleus because of electrical force of attraction.  The electrical motor works because of magnetic Lorentz force.

      The moment:  Next, take a DVD.  Put one ink dot near the center and another ink dot near the edge.  Rotate it.  You can observe that the dot near the center moves fast and dot at the edge moves less fast.  Hence, distance from the center or axis of rotation plays very important role in rotatory motion.  This 'distance' is always multiplied with quantities like mass, velocity, momentum, force, inertia in rotating particles and bodies.  Multiplying with 'distance' is known as finding 'moment' in physics.
     Consider a large carriage door.  If you push it at the edge you can easily close and open.  But, if you push it near the joint (axis of rotation), it is even difficult to move the door a bit.  That is why we find the  'moment of force' to get the real effect of the force.

    Stability:  Rotating wheels keeps its axis of rotation stable.  Examples: spinning top, two wheeler, cycle, the toy-fidget spinner.  When they are in motion, they cannot be brought to the ground       easily(gyroscope effect)

    Energy store:  A rotating giant wheel cannot be stopped 'all of a sudden' due to rotational inertia.  It will continue to rotate for some time.  So a rotating heavy wheel act like mechanical battery storing mechanical energy.  This concept is widely used in automobiles.

    Compactness:  Rotation is a compact motion.  It can be contained in  an atom; in a motor assembly; in a car; in a DVD player; in a cyclotron; in a gyroscope and so on.  Today, people even go for a nano molecular motors.  Hence go for rotation always.

   Note:  Electricity is generated mainly using rotations.  Because rotations are easy to execute.   

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