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2 Objectives To state the SI unit of time as the second To explain the meaning of speed and average speed To use the speed equation for calculation To calculate rates using appropriate units

3 SI unit for time is the second (s). Other units for time include minute (min), hour (h), day, month, year, decade and century. Quartz clocks and watches are used for accurate time measurements in scientific work. Measurement of Time Time can mean an instant or an interval.

4 Mechanical Stopwatch The mechanical stopwatch can measure time accurate to 0.1 s or 0.2 s. minute scale second scale

5 Electronic Stopwatch The electronic stopwatch gives a higher accuracy than a mechanical stopwatch. It has a digital display of the time and it can measure time intervals accurate to 0.01 s. It can also record time intervals in hours.

6 Let’s learn how to read the stopwatch.

7 To measure a time interval, the stopwatch needs to be started and stopped by hand. This manual operation introduces an error called the human reaction time which can be quite a large fraction of a second. The human reaction time varies from one person to another. Young people usually have shorter human reaction time than older people. For most people, it is about 0.3 s.

8 We live in a world of motion. Movements can be described as being fast or slow. In science, accurate descriptions and explanations of motion require measurements of distance and time. Speed and Average Speed

9 Speed is the distance travelled per unit time. SI unit: m/s or m s -1 Other units include km/h or cm/s Time taken Distance travelled Speed 

10 Speed of some motions 0.3 m/s 300 m/s 22 m/s

11 For example, runners in a 100 m race start from rest and then move at varying speeds until they cross the finishing line. A more relevant description of an object moving at varying speeds is its average speed. For example, runners in a 100 m race start from rest and then move at varying speeds until they cross the finishing line. A more relevant description of an object moving at varying speeds is its average speed. Average speed Most moving objects do not move at constant speeds.

12 Total time taken Total distance travelled Average speed  The average speed is found by dividing the total distance travelled by the total time to travel the distance and can be written as a formula:

13 Example A car travels 24 km in 18 min along the PIE. What is the average speed of the car during that time? Example A car travels 24 km in 18 min along the PIE. What is the average speed of the car during that time?

14 Example The distance of the earth from the sun is km. The speed of light is km/s. If the sun stops giving out light now, how much longer will we be able to see the sun? Example The distance of the earth from the sun is km. The speed of light is km/s. If the sun stops giving out light now, how much longer will we be able to see the sun?

15 Example Carl Lewis holds the world record of 9.92 s for the men’s 100 metre race at the 1988 Seoul Olympics. What is his average speed? Example Carl Lewis holds the world record of 9.92 s for the men’s 100 metre race at the 1988 Seoul Olympics. What is his average speed?

16 Rate is the ratio between two different physical quantities. It describes how a quantity changes with another. Most rates describe how a quantity changes with time, but there are rates which do not involve time. Rate is the ratio between two different physical quantities. It describes how a quantity changes with another. Most rates describe how a quantity changes with time, but there are rates which do not involve time. Rate

17 Pulse rate is an important indicator of one’s state of health. For a healthy person Men: beats/min Women: beats/min Adolescents: beats/min Pulse rate is an important indicator of one’s state of health. For a healthy person Men: beats/min Women: beats/min Adolescents: beats/min

18 Different makes of vehicles have different rates of petrol consumption. Larger vehicles have higher rates of petrol consumption than smaller vehicles. What are the 2 physical quantities in this case? They are volume of petrol consumed (litres) and distance travelled (km).

19 The rate of flow of water from a fireman’s hose must be fast enough in order to put out the fire effectively. What are the 2 physical quantities in this case? They are volume of water (cm 3 ) and time (s).

20 The rate of increase in height is different at different stages of life. What are the 2 physical quantities in this case? They are increase in height (cm) and time (yr).

21 Photographs and pictures -taken from Interactive Science Bk 1