Reading time using stopwatch

Time is used to indicate when an event occurs, the order in which several events occurs or the rate at which an event happens. Under normal circumstances, time is usually measured using a clock or a wristwatch as shown in Figures 3.19(a) and (b), respectively.

Figure 3.19: A dock and wristwatch

For those experiments that require measurement of time in the laboratory, stopwatches are used. At this level, we shall concentrate on measuring time by means of a stopwatch.

A stopwatch is a device that is held in the hand and is used to show the time elapsed. Thus, the time interval is obtained from the time it is started to the time it is stopped. It is used when time must be measured precisely and with a minimum error. Stopwatches are used for timing laboratory experiments or sporting events like athletics.

There are two types of stopwatches, namely analogue stopwatch and digital stopwatch, as shown in Figure 3.20.

An analogue stopwatch has two scales, the minute scale and second scale. The pointer on each scale enables the time that has elapsed to be read, see Figure 3.20 (a).

Figure 3.20: Stopwatches

A digital stopwatch shown in Figure 3.20 (b) displays the actual time in hours, minutes, and seconds. Digital stopwatches are more accurate than analogue stopwatches. A large digital version of a stopwatch is called a stop clock. Stop clocks are designed for viewing at a distance, such as in sports stadia.

How to operate a stopwatch

The timing functions of a stopwatch are controlled by two buttons, one at the top of the watch and the other to the side of the watch. Pressing the top button starts the timer and pressing it again stops the timer. The elapsed time is then displayed on the screen. The side button usually has two functions, first, to reset the stopwatch to zero and second to record the split times or lap times. When it is pressed while the stopwatch is running, the elapsed time is displayed, with the watch mechanism continuing to run to record the total time elapsed.

Measurement of mass

Mass is among the fundamental physical quantities of measurement. It is defined as the quantity of matter in an object. It measures the amount of matter in an object. The SI unit of mass is the kilogramme, written as kg. Other units of mass based on kilogramme are tonne (t), gramme (g) and milligramme (mg). Their equivalences are as given below. See also Table 3.6.

1 tonne = 1 000 kg.

1 kilogramme = 1 000 g.

1 gramme = 1 000 mg.

Table 3.6: A comparison of the units of mass

Mass should not be confused with weight as is evidenced in day-to-day discussions and conversations. Weight is defined as a measure of the gravitational force acting on an object, or is the measure of how heavy an object is.