Quantity and Units

QUANTITIES AND UNITS

Physics Quantities

 What is physics quantity ?

Physics quantity is something that is measurable, generally has its own unit, and it is stated in numbers or value. 

 For examples:

The measurement result of the length of a table is 100 cm.

     100 represent the value and cm represents the unit and the measuring 

     instrument of the length is roll meter.

Fundamental Quantity

What is base quantity?

Base Quantity (BQ) or Fundamental Quantity is the quantity whose unit has been previously determined and it is not derived from another quantity.

Table 1.1. : Seven Fundamental Quantities

 

No.	Base Quantities	Symbol Quantities	International Units	Measuring Instruments
1.	Length (height, distance, displacement, width, diameter)	l	meter (m)	Ruler, roll meter, vernier caliper, micrometer caliper
2.	Mass	m	kilogram (kg)	Balance scale
3.	Time	t	second (s)	Stopwatch
4.	Temperature	T	Kelvin (K)	Thermometer
5.	Electric current	i	Ampere (A)	Amperemeter
6.	Luminous intensity	I	Candela (Cd)	Photometer
7.	Amount of substance	n	mole (mol)	TDS meter

 Derived Quantity

What is derived quantity?

Derived Quantity (DQ) is a quantity whose unit is derived from a base quantity. Most of derived quantities have the units as a combination of base units.

 Table 1.2. : Several examples of the derived quantities

No.	Derived Quantities	International Units	Derived from
1.	Area (A)	Square meter (m2)	A = length x width (BQ)     = meter x meter = m2 (units BQ)
2.	Volume (V)	Cubic meter (m3)	V = length x width x height (BQ)     = meter x meter x meter = m3 (units BQ)
3.	Speed (v), Velocity (v)	Meter per second (m/s)	(BQ)     = …………. (BQ) v = ………………………
4.	Acceleration (a)	m/s2	(DQ and BQ) a = ………………………….
5.	Density (r)	Mass per square meter (kg/m3)	(BQ and DQ)     = ……………………
6.	Weight (w), Force (F)	Newton (N)	F = mass x acceleration (BQ and DQ) F = ……………………. 1 N = …………….
7.	Work (W), Energy (E)	Joule (J)	W = Force x displacement (DQ and BQ)      =  …………….      = ………….. 1 joule = ……….. = ……………..
8.	Power (P)	Watt (W)	(DQ and BQ) P = ……………………. 1 Watt = ……………..
9.	Pressure (P)	Pascal (Pa)	(DQ and DQ) P = …………………… 1 Pa = ………………
10.	Momentum (P)	N.s	P = mass x changes velocity (BQ and DQ) P = ……………………….

What is difference between the scalar and vector quantity ?

  How do distance and displacement differ ?

Figure 2.2. : The distance from Sheila’s house to a school is 15 kilometer and 
                           the displacement from Sheila’s house to school is 10 kilometer 
                           to Northwest.

 Scalar Quantity is a quantity which only has a magnitude. For examples: distance, length, time, mass, temperature, area, volume, speed, density, energy, etc.  

Whereas, a quantity that is specified by both a magnitude and a direction is a Vector Quantity. For examples: displacement, velocity, acceleration, force, weight, etc.

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Exercise on Chapter Quantities and Unit

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