This chapter traces how humans measure time and how they describe how fast things move. Early people used repeating natural events — the rising and setting of the Sun, the phases of the Moon, and the seasons — to keep track of time, and invented sundials, water clocks (including the sinking-bowl Ghatika-yantra), hourglasses, and candle clocks for smaller intervals. A major breakthrough came with the simple pendulum: Galileo found that a pendulum of a given length always takes the same time for each swing, and Huygens used this to build the pendulum clock. A pendulum's bob swings between two extreme positions through a mean position; the time taken for one complete oscillation is its time period, which depends on the length of the pendulum but not on the mass of the bob, and stays constant at a place. This constancy of repeating processes underlies all clocks, from pendulum and quartz clocks to extremely precise atomic clocks. The SI unit of time is the second (s), with 60 s = 1 min and 60 min = 1 h. The second half of the chapter introduces speed — the distance covered in unit time — written as speed = total distance / total time, with SI unit metre per second (m/s) and also km/h. The relationships distance = speed times time and time = distance / speed follow, and worked examples show how to use them; the speed found this way is really the average speed. Finally, motion along a straight line (linear motion) is classed as uniform (constant speed, equal distances in equal times) or non-uniform (changing speed), with real journeys usually being non-uniform.
Early devices for measuring timeThe simple pendulum and time periodSI unit of timeSpeed and its formulaUniform and non-uniform linear motion
Key terms
Time period
The time taken by a pendulum to complete one oscillation; it depends on the length of the pendulum but not on the mass of the bob.
Oscillation
One complete to-and-fro swing of a pendulum from one extreme position back to it through the mean position.
Second (s)
The SI unit of time, with 60 seconds making a minute and 60 minutes making an hour.
Speed
The distance covered by an object in unit time, calculated as total distance divided by total time taken.
Average speed
The total distance covered divided by the total time taken, used because objects rarely move at a constant speed.
Uniform linear motion
Motion along a straight line at a constant speed, covering equal distances in equal intervals of time.
Non-uniform linear motion
Motion along a straight line in which the speed keeps changing, covering unequal distances in equal time intervals.
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