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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

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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.

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Mechanical Stopwatch

The mechanical stopwatch can measure time accurate to 0.1 s or 0.2 s.

minute scale

second scale

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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 intervalsaccurate to 0.01 s. It canalso record time intervalsin hours.

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Let’s learn how to read the stopwatch.

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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.

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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

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Speed is the distance travelled per unit time.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

Speed

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Speed of some motionsSpeed of some motions

0.3 m/s

300 m/s

22 m/s

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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.

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Total time taken

Total distance travelledAverage 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:

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:

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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?

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Example

The distance of the earth from the sun is 149 000 000 km. The speed of light is 300 000 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 149 000 000 km. The speed of light is 300 000 km/s. If the sun stops giving out light now, how much longer will we be able to see the sun?

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Example

Carl Lewis holds theworld record of 9.92 sfor the men’s 100 metrerace at the 1988 SeoulOlympics. What is hisaverage speed?

Example

Carl Lewis holds theworld record of 9.92 sfor the men’s 100 metrerace at the 1988 SeoulOlympics. What is hisaverage speed?

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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

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Pulse rate is an important indicator of one’s state of health.

For a healthy person

Men:70 - 72 beats/min

Women:78 - 82 beats/min

Adolescents:80 - 90 beats/min

Pulse rate is an important indicator of one’s state of health.

For a healthy person

Men:70 - 72 beats/min

Women:78 - 82 beats/min

Adolescents:80 - 90 beats/min

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Different makes of vehicles have different rates of petrol consumption.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).

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The rate of flow of water from a fireman’s hose must be fast enough in order to put out the fire effectively.

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 (cm3) and time (s).

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The rate of increase inheight is different atdifferent stages of life.

The rate of increase inheight is different atdifferent stages of life.

What are the 2 physical quantities in this case?

They are increase in height (cm) and time (yr).

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Photographs and pictures

-taken from Interactive Science Bk 1