When an object changes its position with change in time, it is said to be in motion. There are many types of motion; like linear motion, circular motion, periodic motion, random motion, etc. To understand the rate of motion, we need to measure two quantities, i.e. time and distance. Distance is measured by using meter scale, while time is measured by watches. Before the introduction of standard units of measurement, people used various arbitrary units of measurement; like armâ€™s length, hand-span, cubit, foot-span, etc. Use of arbitrary units of measurement created lot of confusion. It was Napoleon who first implemented standard units of measurement in practice. Time measuring devices have evolved from simple sand-clocks to atomic clocks of the modern era. Even today, many people in rural areas tell time by looking at the position of the sun and the stars.

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