SCIENCE CAPSULES

    Imagine a point rotating on a circular path.  Also imagine a normal line drawn from the point and its foot always touching a diameter.  Again imagine radius connecting the point from the center.  As the point rotates ,  the foot of the normal line  will oscillate on the diameter(simple harmonic motion).  And also as the point rotate, the radius connecting the point will be sweeping the angles.  The sine of those angles will make a wavy graph.      Hence rotation, oscillations, and waves are similar motions and can be related.  Remember rotation of a coil in a generator  produces wavy alternating current.    

    Imagine a disc rotating about the axis  passing through its centre.  Also assume that the disc is made up of many particles.  You can observe that the particles at the edge are moving very fast.  But the particles near the axis are moving very slow.  In fact, the linear velocity gradually increases , as we move from axis to the edge along the radius.  This fact should be taken into account when we make rotating devices like DVD player, wheels, gears etc.,     When a whole body moves in a straight line, all the particles in the body moves with the very same velocity.