**What is SI Units and why do we need it?**

- Scientists all over the world have adopted a common set of units to express measurements for the sake of uniformity.
- This system is called as the International System of Units or SI Units.

**International System of Units- SI**

1.Temperature - kelvin

2. Distance - metre

3. Electric Current - ampere

4. Time - second

5. Amount of substance - mole

6. Mass - kilogram

7. Intensity of light - candela

8. Time - second

**Measurement:**The comparison of unknown quantities with some known quantities is known as measurement. Measurement of a quantity has two parts

1.A number

2.Its unit.

**Fundamental physical quantities:**

- Length
- Mass
- Time

**Length:**The distance between one end and the other desired end is called as length.

**Standard unit of length:**

- The standard unit of length is 'Metre'.
- It is represented by letter ‘m’.
- Very small lengths can be measured in mm and cm.

**Measuring Tools:**

1. Measuring tape

2. Stop clock

3. Measuring jar

4. Balance

5. Metre scale

**Unit of length:**

- 1 km (kilometre) = 1000 m (metre)
- 1 m (metre) = 100 cm (centimetre)
- 1 cm (centimetre) = 10 mm (millimetre)
- 10 d (decimetre) = 1m (metre)
- 1000 mm (millimetre) = 1 m (metre)
- 1000000000 n (nano metre) = 1 m (metre)

**Parallax:**

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight.

**Methods of measuring Length of a curved line:**

the length of the line = (number of segments × length of each segment) + length of the left over part.

**Volume:**

- Volume of Solids:
- Area = Length × Breadth.
- volume of the solid = length × breadth × heigth.
- SI unit for Volume of solid is cubic metre.

**Volume of Liquid :**

- A liquid whose volume is to be found can simply be poured into a graduated container. Graduated cylinders, beakers, pipettes and burettes are available for measuring exact volumes.
- SI unit for Volume of Liquids usually measured in litres.

**Measuring the volume of objects with irregular shape:**Volume of irregular objects can be measured by water displacement method.

**Water displacement method:**

Let us fill a measuring cylinder with water to certain level, say 50 ml.Now tie the stone with a piece of fine thread. Then immerse the stone completely into water.As the stone is immersed, we can observe that water level increases.Because the stone displaces the water to occupy the space inside the measuring cylinder.The displacement would be equal to the space taken up by stone. So the amount of water displaced will be the volume of the stone. This method is called as water displacement method.

**Example:**

Suppose initially the water level was 50 ml. After you immerse a stone the water level rises to 75 ml.

SO,Volume of the water displaced is 75 ml – 50 ml = 25ml

1ml = 1 cubic cm

25ml = 25 cm3.

Hence volume of stone = 25cm3

**Volume of gas:**

- 1 m3 = 1 kilolitre (kl or kL)
- 1cm3 = 1 millilitre (ml or mL)
- 1mm3 = 1 microlitre (μl or μL)
- SI unit for Volume of gases usually measured in litres.

**Mass and Weight:**

**Mass:**

- Mass is the measure of the amount of matter in an object.
- Beam Balance : It is generally used to measure mass.

**The SI unit of mass is kilogram. It is represented by the ‘kg’.**

- 1000 milligram = 1 gram
- 1000 gram = 1 kilogram
- 1000 kilogram = 1 tonne

**Weight:**

- Weight is the gravitational pull experienced by the mass.
- Electronic Balance : It is a device used to find accurate measurements of weight.

**Time:**

- We generally measure the passage of time in terms of hours, minutes and second as the situation demands.
- Clocks are used to measure time.

**Odometer:**It is a device used for indicating distance traveled by an automobile.

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