Physical quantities are divided into two categories:

**Scalars quantities:**They have only the magnitude. For example, mass, length, time, temperature etc. The scalars quantities can be added, subtracted, multiplied and divided just as the ordinary numbers.**Vectors quantities:**They have both magnitude and direction. for example velocity, force, electric field, torque etc.

A vector can be defined as a line segment having a specific direction and a specific length. The length is the magnitude of the vector and with an arrow indicating the direction. The direction is from the tail of the vector to its head.

*Figure:5.a*

A vector is denoted by the letter in bold (for example vector p is written as **p**) or it can be represented by an arrow placed over a letter (for example vector is written as .

*Position and displacement vectors:*

A position vector describes the position of a point in a coordinate system.

*Figure:5.b*

If P is the position of an object at time t, then OP is the position vector of the object at that time. It is denoted by .

The displacement vector describes the position of a point with reference to a point other than the origin of the coordinate system.

*Figure:5.c*

An object moves from a point P to point Q. Let is the position vector at a time $t_{1}$ and is the position vector at a time $t_{2}$.

is the displacement vector directed from point P to Q, corresponding to the motion of the object from P to Q.

**Parallel vectors and antiparallel vectors:**

Parallel vectors act along the same direction and they are collinear. The angle between them is $0^{0}$.

*Figure:5.d*

Antiparallel vectors act in the opposite direction and they are collinear. The angle between them is $180^{0}.$

*Figure:5.e*

**Equality of vectors:**

Two vectors are said to be equal if they have the same magnitude and direction.

*Figure:5.f*

**Unit vector:**

Unit vector has a unit magnitude and points in a particular direction. Any vector

can be written as the product of unit vector in that direction and magnitude of the given vector.

Unit vector is expressed as .

We can write, = $p$

A unit vector has no dimensions. Unit vectors along the positive X, Y and Z -axes of the coordinate system can be expressed as

such that .

*Figure:5.g*