Short notes-1 of chapter 2 based on physical quantities and measurement of class 6.

 *Physical quantities-

-A quantity thú㎲at can be measured is called a physical quantity.

-example mass, length, time, temperature, area, volume, force, weight, displacement, velocity, etc.

-physical quantity can be divided into two types:

 •scalar quantity-a physical quantity which can be  expressed only in terms of magnitude.

•example mass, length, time, temperature, area, volume etc.

 •vector quantity- a physical quantity which requires the magnitude as well as the direction to express them..

 •eg, force, weight, displacement, velocity etc.,

Physical quantity=magnitude×unit

                                = n×u

*Measurement-

-measurement is a comparison of an unknown quantity with a known fixed quantity of the same kind.

-measurement is basically a process of comparison.

-unit-unit is the quantity of a constant magnitude which is used to measure the magnitude of other quantities of the same nature.

- magnitude- numerical value.

- eg. Mass of a man=65 kg

    65 is a numerical value called as magnitude.

    Kg  is the unit of mass.

-eg height of a man=1.65m

      1.65 is a numerical value called magnitude.

       m is the unit of height.

 -measurement is expressed as:

Measurement=n×u

   n= magnitude

  u= unit

*Choice of unit:

1. It should be of convenient size.

2. It must be universally accepted.

*System of units:

1. CGS system/British system of unit-

Length- centimetre- cm

Mass -kilogram- kg

Time - second - s

2. FPS system/French system of unit-

Length-foot- ft

Mass - pound - lb

Time - second - s

3. MKS system/metric system of unit-

Length- metre- m

Mass- kilogram- kg

Time- second- s

 *SI system of units-

In  1960, the scientist accepted a set of units for measuring the basic physical quantities length, mass, time and temperature to be called as standard international units inshort si units.

Length-metre- m

Mass-kilogram- kg

Time- second- s

Temperature- Kelvin- K

Luminous intensity-candela-cd

Electric current- ampere- A

Amount of substance- mole- mol

Complementary fundamental units:

Angle- radian- rd

Solid angle- steradian-  st-rd

Multiples are used to measure a large quantity/prefixes used for big measurements.

 deca  10¹  da

 hecto 10²  h

kilo     10³   k

mega   10⁶    M     

Giga     10⁹     G

tera       10¹²    T

peta      10¹⁵     P

Submultiples are used  for small measurements.

deci  10-¹    d

centi  10-²    c

milli   10-³    m

micro  10-⁶     μ

nano    10-⁹   n

pico.      10-¹²    p

femto     10-¹⁵     f

*Convention while writing SI units:

1. Symbols used for units ,always written in small letters.eg

kg and not Kg

m and not M

s and not S.

2. Units named after scientist are written in lower letters in words.eg.

Unit of current- ampere and not Ampere.

Unit of temperature-kelvin and not Kelvin.

3. Symbol for a unit named after scientist always written with a capital letter.eg.

Symbol of Fahrenheit- F and not f

Symbol of Ampere- A and not a

Symbol of Newton- N and  not n

Symbol of Kelvin - K and not k

4. The unit and its symbols are never written in plural.

5. In words the unit is written in small letters. For example, metre and not Metre.

6.a space is left between the symbols of two units when they are used as product. For example, the symbol of unit Newton×second is N s.

7. negative powers are used for the compound units formed by dividing one unit by the other. For example, the unit of speed is metre/second (m/s) or ms-¹.

*Measurement of length:

-length is the distance between two points.

-SI unit of length is metre.

-symbol is m.

-def.

   a meter was initially defined as the distance between two points on a rod of platinum iridium alloy kept at zero degree celsius in the the international bureau of weights and measures at sevres near Paris.

Or

1 metre can also be defined in terms of speed of light.

The distance travelled by light in air in 1/299792458 of a second.

1km=10³ m    (1 k = 10³)

        =1000m.

1cm = 10-²m.     ( 1c = 10-²)   

         = 1/10² m

         = 1/100 m

         = 0.01 m

1mm= 10-³ m.        (1m = 10-³) milli

         =1/1000 m

         = 0.001 m

1 ft=12 inch

1yard= 3foot

          = 3× 12 inch

          = 36 inch

Relationship between ft,inch and centimetres:

1 inch= 2.54 cm

1ft= 12 inch

     =12× 1 inch

    = 12× 2.54 cm

    = 30.48 cm

1 yard= 3 ft

            =3×1 ft

            =3× 30.48cm

            = 91.44 cm

            ≈0.91 cm

Devices for measuring length:

1. A metre ruler-

•It is 1 metre long and is divided into hundred equal parts.

•measures length correct up to 1 mm 0.001m.

•metre ruler measure the length of straight objects.

•measurement of length of an object with a metre ruler-

 -the metre ruler is placed with its markings close to the object.

-the zero mark on the ruler is made to one side with one end of the object.

-the position of the other end of the object is read on the ruler.

-reading gives the length of the object.

Parallax error-error in measurement of length due to thickness of a metre ruler.

* The correct reading is obtained when the eye is kept in front of and in line with the reading to be taken.

•vernier callipers is used to measure length upto 0.1 mm

•screw gauge is used to measure length correctly upto 0.01 mm

2. Use of measuring tape to measure a length

•measuring tape is a flexible ruler of a ribbon of plastic, fibre glass or metal strip with lines as markings in centimetre and millimetre in it.

•to measure the length of a curved line the tape is spread along the length of the curved line.

•Surveyors use a tape of length 100 metre.

*Measurement of mass:

-the mass of a body is the quantity of matter contained in it.

•SI unit of mass is kilogram.

•symbol is kg.

-def.

In 1889,1 kilogram was defined as the mass of a cylinder of platinum iridium alloy kept at the international bureau of weights and measures at sevres near Paris.

Or,

The mass of one litre of water at 4 degree celsius is taken as one kilogram.

-Bigger units of mass:

1 quintal=100 kg

1 metric tonne= 10 quintal

                          = 10×1 quintal

                          =10×100kg

                          =1000kg

-smaller units of mass:

1kg =10³ g       (1kilo =10³)

      = 1000 g

1g=10-³ kg

1mg=10-⁶ kg 

        =10-³ × 10-³kg 

        =10-³ × 1g    (1g= 10-³kg)

       = 10-³ g

Relationship between gram, kilogram and pound:

1g=10-³kg

1kg=1000g

1lb=453.59 g

1g= 0.00220 lb

     ≈0.002 lb

1 lb= 0.45359 kg

       ≈0.45 kg

Devices for measuring the mass:

1. The beam balance-

-The beam has a support just at its middle, having a pointer.

-two identical pans are suspended  at the ends of the beam by means of strings of equal length.

-to measure the mass of an object, standard weights are used.

-the body whose mass is to be measured is placed on the left pan.

-the the standard weights are placed on the right pan.

-they are adjusted so that the beam is again horizontal on holding the balance up.

-the total of the standard weights gives the mass of the given body.

2. The electronic balance-

-for the precise and accurate measurement of mass of an object electronic balance is used.

•an electronic balance has the following three parts:

1.The structure-

 It is  the load bearing part which transfers the load of the object to the load cell.

2. The load cell-

-It converts the load into electrical signals.

3.The signal conditioner-

it is the electronic parts which processes the electrical signal and displays the mass.

Advantages of using electronic balance :

1. No prior setting  or any separate weight box is required.

2. Automatically measures the mass ,precisely  & accurately.

3. Reading is displayed in the digital form on the screen.

*Grocers balance is used by shopkeepers for measuring mass of vegetables, pulses, rice etc.,

*A physical balance is used by goldsmith for accurate and precise measurement of costly items.such as gold,silver, diamond etc.,