Time, Rate and Speed

3.1 Measuring Time Time is the duration of an event. Olden methods give inaccurate measurements of time. However, man has now developed more accurate ways of measuring time.

Units of time S.I. unit of time is the second, and represented by the symbol s. Other units of time: minute (min), hour (h), millisecond (ms) Time can also be expressed in terms of days, weeks, months and years.

Instruments to measure time To measure a short period of time accurately, we use a stopwatch. There are two types: mechanical and electronic.

Instruments to measure time Mechanical stopwatch oThe mechanical stopwatch has a minute scale and a second scale. oThe smallest division on the mechanical stopwatch is 0.1 s. oThus, the mechanical stopwatch is accurate to 0.1 s. A mechanical stopwatch

Instruments to measure time Mechanical stopwatch 1. Press the operating button to start the stopwatch. Press the operating button again to stop the timing. 2. Press this button to reset the stopwatch. The minute scale The second scale The second hand indicates 13.8 s The minute hand falls on the white area between 0 and 1 min of the minute scale. This shows that the second hand should be read using the second scale between 0 and 30 s.

Instruments to measure time Electronic stopwatch oThe electronic stopwatch is more accurate than the mechanical stopwatch. oIt can measure time intervals accurate to 0.01 s. oIt can also record the time interval in hours, unlike the mechanical stopwatch.

Instruments to measure time Electronic stopwatch 1. Press the “Start/Stop” button to start the stopwatch. Press the button again to stop the timing. 2. Press this button to reset the stopwatch. Seconds Hours Minutes

3.2 What is Rate? A rate is an expression of how one quantity changes with time. Rates are useful for making fair comparisons. Phone charges, internet access plans, bank loan interest rates and inflation rate are some examples of rates in real life.

3.3 Speed Speed is the rate at which distance is covered over time. Average speed is used to simplify calculations since speed may vary over the duration of a journey.

3.3 Speed Average speed is the rate at which total distance travelled varies with the total time taken. Average speed = Total distance Total time

3.3 Speed S.I. unit of speed is metre per second (m/s). Other common units of speed: centimetre per second (cm/s) and kilometre per hour (km/h).

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