# Physics 9th | Motion: Displacement

## Motion: Displacement

### Motion

1. A body is said to be in motion if it changes its position with time with respect to the observer.

e.g. When a tree T is observed by an observer A sitting on a bench, the tree is at rest. This is because the position of the tree is not changing with respect to the observer A.

Now when the same tree T is observed by an observer B sitting in a train moving with a velocity V, then the tree is moving with respect to the observer B because the position of the tree is changing with respect to the observer B.

So we always need a reference point to explain the motion. In case 1, the reference point is the observer A, while in second case, reference point may be observer B.

### Type of Motion

(i) Linear motion

(ii) Circular motion

(iii) Vibratory motion

#### (i) Linear motion:-

The motion of a body is said to be linear motion If the body moves in a straight line or path.

e.g. Motion of a moving car on a straight road.

Motion of a ball dropped from the roof of a building.

#### (ii) Circular Motion: –

The motion of a body is said to be circular motion if the body moves around a fixed point.

e.g Motion of an electric fan

Motion of a spinning top

#### (iii) Vibratory motion: –

The motion of a body is said to be vibratory motion if the body moves to and fro about a fixed point.

e.g. Motion of a pendulum

Motion of a swing

### Nature of Motion

(i) Uniform motion

(ii) Non- uniform motion.

#### (i) Uniform motion: –

The motion of a body is said to be uniform if the body covers equal distance in equal interval of time.

e.g.

#### (ii) Non- Uniform motion: –

The motion of a body is said to be non-uniform if the body covers unequal distances in equal interval of time.

eg.

### Scalar quantity:

The physical quantity which has only magnitude and no sense of direction is called scalar quantity.

e.g. distance, speed , time, work, etc.

### Vector quantity: –

The physical quantity which has both magnitude as well as direction is called vector quantity.

e.g. displacement, force, momentum, etc.

### Distance: –

The actual length of path covered by a moving body between its initial and final position is called the distance covered by the body. It has no sense of direction.

It is a scalar quantity and its S.I. unit is meter (m)

### Displacement:

The shortest distance between the initial and final position of a moving body is called the displacement of the body and is directed from the initial to the final position

It is a vector quantity & its S.I unit is metre (m)

Distance will be equal to displacement if the body moves in a straight line, otherwise distance will be greater than displacement

i.e. Distance Displacement

### Difference between distance and displacement

 Distance Displacement 1. The actual length of path covered by a moving body between its initial and final position is called the distance covered by the body. 2. It has no sense of direction. 3. Distance is a scalar quantity. 4. It is always positive. 5. It can not be zero for a moving body. 1. The shortest distance between initial and final position of a moving body is called the displacement of the body and is directed from initial to final position 2. Direction is associated with displacement. 3. Displacement is a vector quantity. 4. It can be positive or negative. 5. It can be zero if initial an final position of the body is same.