Motion in a Circle and Oscillatory Motion
Some motions come back again and again — how do we describe them?
Big Idea: Many things in nature move in a way that repeats. A wheel turns round and round; a swing goes to and fro. To describe these repeating motions we use three friendly words: period, frequency, and (for swinging things) amplitude.
By the end you will be able to:
- recognise circular motion (movement around a circle) and oscillatory motion (movement to and fro);
- explain what the period T and the frequency f of a repeating motion mean;
- use the two little formulas f = 1/T and T = 1/f;
- say what the amplitude A of a swing is and give its unit;
- read the units correctly: seconds (s) for T, hertz (Hz) for f, metres (m) for A.
1. What is circular motion?
When a body moves along a circle, we say it is in circular motion (movement around a circle). Its path is not a straight line — it keeps curving so that it stays the same distance from one central point.
You have seen circular motion many times:
- the tip of the second hand on a clock,
- a cabin on a Ferris wheel (like the ones at a city fair),
- a stone whirled round on a string,
- the wheels of a bicycle or a car,
- even the Earth travelling once around the Sun each year.
The special thing about circular motion is that it repeats. After the body has gone all the way round once, it is back where it started, ready to do exactly the same trip again. One full trip round the circle is called one turn (or one revolution).