# Archive for the ‘11th physics’ category

## Physics 9th | Motion: Displacement

August 31st, 2015

## 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.

June 6th, 2011

## Frame of Reference:

Suppose someone asked you a question “How far is India?” your reaction would be “from where?” Then the person who asked you the question would tell, from here or from United States. So in kinematics some reference is very necessary to measure distances.

Suppose, you are sitting inside a stationary train. At the same time other train is moving. It is very hard to tell which of the trains is moving. If you consider yourself stationary, other train is moving while for the passenger of the other train, your train is moving.

Same is true in case of ground. For a person standing on the ground, you are moving while sitting in the train. While for the passengers sitting in the train, you are stationary.

So motion and rest are related terms depending on the position of the observer.

One Dimensional frame of Reference:

Suppose you have to locate a point (Point P as shown in figure) on a line without touching it. The solution is to mark the line and mark one point as zero (point ‘O’ in figure) and measure the distance of the point ‘P’ from the zero point. In our case the distance of P from the point is 360m.

So we always need a ‘reference point’ from which all the distances would be measured. Same is true with the 2-Dimensional motion. In the case of 2 Dimensional motion, we need a system of coordinate axes to explain the motion.

The definition of the Reference Frame is “A system of coordinate axes which defines the position of a particle or an event in two or three dimensional space is called a Frame of Reference”.