Short notes-2 of chapter -1( physical quantities and Measurement) of class-7

 ☆ density

• density of a substance is defined as mass per unit volume of that substance.

• density= mass/ volume.

• equal masses of different substances have different volumes.

• equal volumes of different substances have different masses.

• to distinguish between different substances we used the term called density.

• density is a scalar quantity.

• SI unit is kg/m³  or  kgm-³.

• CGS unit is g/cm³  or  gcm-³.

• relationship between SI unit and CGS unit of density:     1kgm/m³=1000g/1000000cm³

                                    =1/1000 gcm-³

                                     = 10-³ gcm-³

 1gcm-³ = 1000 kg m-³

• generally density  decreases with increase in temperature and increases with decrease in temperature. Because all substances expand on heating and contracts on cooling.

• water contract on on heating from 0 degree Celsius to 4 degree Celsius. This abnormal behaviour of water is known as Anomalous expansion of water.

•

The density of water is maximum at 4 degree Celsius it decreases when it is cooled from food degree Celsius to degree Celsius or it is heated above 4 degree Celsius.

☆Determination of density of regular solids:

•measures mass - by using beam balance & volume of a body  by formula method 

and then uses formula, 

    Density= mass/ volume.

☆ determination of density of an irregular solid:

• measure the mass of the given solid by using Beam balance.

• volume of an irregular solid is measured by liquid displacement method.

• after finding mass and volume.

We uses formula,

 density=mass÷ volume.

☆ determination of density of a liquid:

• take a empty beaker & measure its mass as M1.

• take a measuring cylinder and add a liquid into it to a certain level say a 20 ml.i.e.,20cm³

• transfer the liquid into the empty beaker and measures its mass again as M2.

• the difference between M2 and 1 will give the mass of the liquid in gram.

M=M2 - M1

• find the density of liquid using the the formula

Density=mass/ volume.

              =(M2 - M1) / V

• different substances have different densities.

• barometer used to measure atmospheric pressure makes use of Mercury because of its high density of 13.6 g/cm³.

☆ speed:

• the distance covered or travelled by a body in unit time is called the speed of the body.

•speed = distance travelled/ time taken

• SI unit of speed is m/s or ms-¹.

• CGS unit of speed is centimetre per second i.e., cm/s.

• other units of speed are as follows:

1km/h =1000m/3600s

            =5/18  ms-¹

            =0.27777 ms-¹

            =0.28 ms-¹

1ms-¹ = 3.6 kmh-¹

• 18 kmh-¹ = 1ms-¹

•1 km/minute = 1000m/60minutes

                          = 100m/6minutes

                          =16.67 ms-¹

•1ms-¹ = 100 cms-¹

•1m/minute =100cm/60s

                      =1.6666 cms-¹

                      =1.67 cms-¹.