## Frame Of Reference-

 A frame of reference is a frame in which an observer sits and makes observations.

• A Cartesian Coordinate System is attached with the frame of reference.
• A clock is positioned in this system to measure time.
• An observer is always considered to be present at the origin O.

At any time, the position of a particle is described with the help of its three position coordinates (x, y, z).

### Position Vector-

Consider the particle is present at any point P in the space whose coordinates are (x, y, z).

The position vector of point P is a vector drawn from the origin O of the coordinate system to point P.

It is given by-

## Types of Frame of Reference-

Frame of reference can be of two types-

1. Inertial Frame of Reference
2. Non-Inertial Frame of Reference

### 1. Inertial Frame of Reference-

• A frame of reference which is either at rest or moving with constant velocity is known as inertial frame of reference.
• Newton’s laws are valid in this frame.

### 2. Non-Inertial Frame of Reference-

• A frame of reference moving with some acceleration is known as non-inertial frame of reference.
• Newton’s laws are not valid in this frame.

## Rest and Motion-

 A body is said to be at rest if it does not change its position with time. A body is said to be in motion if it continually change its position with time.

• Rest and motion are relative terms.
• When a particle moves, its position coordinates change with time.
• At one time, one or two or all three position coordinates may change.

Accordingly, we have following three types of motion-

1. Motion in one dimension
2. Motion in two dimensions
3. Motion in three dimensions

## 1. Motion in One Dimension-

 The motion of a particle is said to be in one dimension if only one out of the three coordinates specifying the position of the particle change with time.

• In such a motion, the particle moves along a straight line.
• One dimensional motion is sometimes known as rectilinear or linear motion.

### Examples-

Examples of motion in 1 dimension are-

• A car moving on a straight road
• A ball thrown vertically up
• A stone dropped into a well
• A ball dropped from a certain height above the ground

## 2. Motion in Two Dimensions-

 The motion of a particle is said to be in two dimensions if any two out of the three coordinates specifying the position of the particle change with time.

In such a motion, the particle moves in a plane.

### Examples-

Examples of motion in two dimensions are-

• Projectile motion
• Circular motion
• A carom coin rebounding smoothly from the side of the board
• Motion of a boat in a river
• Motion of an insect on a floor

## 3. Motion in Three Dimensions-

 The motion of a particle is said to be in three dimensions if all the three coordinates specifying the position of the particle change with time.

In such a motion, the particle moves in a space.

### Examples-

Examples of motion in three dimensions are-

• Random motion of a gas molecule
• A kite flying in the sky
• A bird flying in the sky

## Terms Regarding Motion-

We discuss the following parameters in detail while studying the motion of any particle-

1. Distance
2. Displacement
3. Speed
4. Velocity
5. Acceleration

## Problem-01:

“Rest and Motion are relative terms.” Justify this statement.

## Solution-

• It is rightly mentioned that the rest and motion are relative terms.
• This is because they depend on the observer’s frame of reference.
• A body at rest with respect to one observer might be in motion with respect to another observer.
• A body in motion with respect to one observer might be at rest with respect to another observer.

Consider the following examples-

### Example-01:

• A person sitting in a moving train is at rest with respect to fellow passengers.
• While person sitting in a moving train is in motion with respect to a person standing on the platform.

### Example-02:

Consider the following scenario-

• Two persons A and B are standing in a truck moving with velocity v.
• Person C is standing on the ground.

Here,

• A and B are at rest with respect to each other.
• A and B moves with velocity V in the positive X direction with respect to C.
• C moves with velocity V in the negative X direction with respect to A and B.

## Problem-02:

Is it possible to achieve state of absolute rest or absolute motion?

## Solution-

• A body at rest with respect to all other bodies is called in state of absolute rest.
• A body moving with respect to all other bodies is called in state of absolute motion.

• It is impossible to achieve the state of absolute rest.
• This is because all heavenly bodies are moving with respect to each other.

• It is impossible to achieve the state of absolute motion.
• This is because there exists no reference point which is absolutely fixed in the space.

To gain better understanding about Rest and Motion,

Watch this Video Lecture

Next Article- Distance and Displacement

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