# The scalar physical quantity and vector physical quantity

## Physical quantities

The physical quantity is any quantity that can be determined and has a unit of measurement in our life , each physical quantity is measured in a special measurement unit , such as the mass , the time , the speed , the energy and the velocity .

The physical quantity are divided into the scalar physical quantity and the vector physical quantity .

The scalar physical quantity is the physical quantity that has magnitude only and has no direction , to describe the scalar physical quantity , It is enough to know its magnitude only by giving its numeric and its measuring unit , such as we say the body mass equals 10 kilogram .

The length and the time are the scalar physical quantities because they have magnitude only and have no direction .

All the scalars are subject to the algebraic mathematical operations related to the numbers and specially they are added and subtracted if they have the same measurement units .

The vector physical quantity is the physical quantity that has the magnitude and the direction , to describe the vector physical quantity , It is necessary to know its magnitude ( its numeric value and measuring unit ) and also its direction .

The acceleration and the force are the vector quantities because they have the magnitude and the direction .

All the vectors are subject to the mathematical operations called the vectors algebra , The vectors have a great importance in different fields of physics , applied science like the engineering .

Understanding various physical phenomena such as the gravity , the movement of liquids and the geometrical establishments depends basically on the main properties of vectors .

### Distance and displacement

The displacement is the length of the shortest straight line between two positions ( the primary position and the final position ) , It is determined by the magnitude and the direction , It is a vector quantity , It is measured by meter or kilometer .

The distance is the actual length of the path that of a moving object covers from the start point to the end point , It is determined by magnitude only , It is a scalar quantity , It is measured by meter or kilometer .

The displacement equals the distance ( numerically ) when the object moves in a direct straight line in one direction , The two equal displacements have the same magnitude and the same direction .

The direction is determined from the starting point of the movement towards the final point , The displacement equals zero , when the start point is the end point .

#### The velocity and the speed

The velocity is the rate of change of displacement , or it is the displacement covered in one second ( unit time ) , The velocity is a vector quantity and to determine it accurately , It is necessary to identify its magnitude and direction  .

The predator ( Cheetah ) is one of the fastest wild animals , its speed is about 27 m/s , but if we want to represent its velocity , we say that  its velocity is 27 m/s is a certain direction , we say that the cheetah’s velocity equals 27 m/s in the west direction .

The velocity is the speed in a given direction when the body moves in a straight line in one direction , The velocity is measured by meter/second or kilometer /hour .

The speed is the distance covered in a unit time , It is determined by magnitude only , It is a scalar quantity , It is measured by meter/second or kilometer /hour .

##### The acceleration

The acceleration is the change of an object’s speed in one second , we use the acceleration unit ( m/s² ) when the distance is measured in meter and the time is measured in seconds or ( km/h²) when the distance is measured in kilometer and the time is measured in hours .

When a car covers equal distances at equal periods of time , It is said that the object moves with uniform speed but its speed changes ( decreasing or increasing ) by equal values at equal periods of time , it is said that it moves by uniform ( regular ) acceleration .

Uniform acceleration is the change ( increase or decrease ) of the object’s speed by equal values through equal periods of time .

When the car’s speed increases by time ( its initial speed < its final speed ) , the movement is described as the accelerating motion or positive acceleration .

When the car’s speed decreases by time ( its initial speed > its final speed ) , the movement is described as decelerating motion or negative acceleration .