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PHY40-Lesson1 

Units and Dimensions

1. Identiy Basic and Derived physical quantities.

2. Introduce the units of certain physical quantities.

3. Introduce Prefixes and ways of conversion.

4. Identify Dimensional Analysis.

Physics is the study of Nature, Matter, and Energy. 

To start with physics we have to know what we call by Physical Quantities 

Physical Quantities 

Basic 

Derived

Table 1 SI Base Units 

Base Quantity → Base Unit → Symbol 

Length → meter → m

Mass → kilogram → kg 

Time → second → s

Temperature → kelvin → K 

Amount of a substance → mole → mol 

Electric current → ampere → A

Luminous intensity → candela → cd 

Examples; 

The length of a football field is 120 m. 

The physics book has a mass of 1 kg. 

The electric current flowing into the house is around 2 A.

Derived quantities are derived from basic quantities through combining them in different ways such as by multiplying and dividing. 

Some examples on derived units are 

Derived Quantity → Unit

Velocity → [m/s]  

Area → [m²] [m x m]

Volume → m3

Density → Kg/m3

Force → Kg x m/s2 or N (Newton)

Acceleration → m/s2

Pressure → N/m2

Energy → J (Joules)

Power → J/s or Watt

The length of a football field is 120 m 

The height of khalid is 167 cm

The distance from Abu Dhabi to Al ain is 150 km Unit

Dimensional Analysis 

Physics is described by equations such as 

Force = mass x acceleration 

momentum = mass x velocity 

velocity = displacement / time 

Both sides of the equation should have the same unit. This is what we call by Dimensional Analysis.

Check whether the following formulas are dimensionally consistent? 

d=distance [m]

v= velocity [m/5]

t= time [s] 

a= acceleration [m/5a] 

F= Force [kgom/5a]