Kinds of motion

Drop an eraser and watch it fall: it moves along a straight line. So does an orange falling from a tree, the students marching in the Republic Day march-past, a heavy box being pushed, a car on a straight road, or a lift going up and down. Motion along a straight line is called linear motion.

Now tie an eraser (or a potato) to a thread and whirl it round. It goes round and round, like a merry-go-round, always the same distance from your hand. Motion along a circular path is called circular motion. The diya in the hand of a dancer spinning in a Ghoomar dance moves this way too.

Hang the eraser from the thread with a steady hand, pull it a little to one side and let go. It swings to and fro, like a swing. Motion to and fro about a fixed position is called oscillatory motion. Another example from the book: press one end of a thin metal strip (about 50 cm long) to a table, push the free end down a little and let go. The free end moves up and down: oscillatory motion.

Some motions repeat. If an object repeats its path after a fixed interval of time, its motion is periodic. An object in circular motion goes round the same circle again and again. An object in oscillatory motion repeats its to-and-fro swing. So circular motion and oscillatory motion are both periodic.