Physical Quantities and Measurement

Physical Quantities

A physical quantity is anything that can be measured. Physical quantities can be divided into base quantities and derived quantities. Base quantities are those quantities which are distinct in nature and cannot be defined by other quantities. Derived quantities are those whose definitions are based on other physical quantities (base quantities).

Some base quantities and their units
QUANTITYSYMBOLS.I. UNIT NAMES.I. UNIT SYMBOL
massmkilogramkg
timetseconds
lengthlmeterm
temperatureTkelvinK
amount of substancenmolemol
Some derived quantities and their units
QUANTITYSYMBOLDERIVATIONDERIVED UNIT
areaAlength x lengthm2
volumeVlength x length x lengthm3
densityρ (pronounced rho)mass/volumekg/m3
velocityvlength/timem/s
concentrationCamount/volumemol/dm3

However some derived quantities have special units which do not show how they were derived.

Some derived quantities with special units
QUANTITYSYMBOLDERIVATIONDERIVED UNITSPECIAL UNIT
pressurePforce/areaN/m2pascal (Pa)
forceFmass x accelerationkgm/s2newton (N)
work doneWforce x distanceNmjoule (J)
powerPwork/timeJ/swatt (W)
electric chargeQcurrent x timeAscoloumb (C)

Fractional units such as m/s, kg/m2 e.t.c can also be expressed using negative indices as ms-1, kgm-2 since 1/x = x-1.

Measurement instruments

Density

Density is a measurement of the mass per unit volume of a substance. An object with a greater mass in a certain volume has high density. An object with less mass in the same amount of volume has a low density. Density is found by dividing the mass of an object by its volume.


ρ = m/V

where ρ is the density, m is the mass, and V is the volume.


The units of density are kg/m3 if the mass is in kg and the volume in m3 or g/cm3 if the mass is in g and the volume in cm3.


QUESTION

  1. If the density of wood is 0.5 g/cm3 what is the mass of
    1. 1 cm3,
    2. 2 cm3,
    3. 10 cm3?
  2. What is the density of a substance of
    1. mass 100 g and volume 10 cm3,
    2. volume 3 m3 and mass 9 kg?
  3. The density of gold is 19 g/cm3.
    1. Find the volume of
    2. 38 g,
    3. 95 g of gold.