# Rest & Motion | Frame of Reference

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

### Types of Frame of Reference-

There are two types of 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 & Motion-

### Rest

A body is said to be at rest if it does not change its position with time with respect to an observer.

### Motion

A body is said to be in motion if it continually change its position with time with respect to an observer.

Rest and Motion are relative terms.

## Types of Motion-

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 one 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 two 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

## 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 scenario shown where-

• Two kids A and B are going for a picnic in a car moving forward with velocity v.
• Kid C is sadly 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 forward direction with respect to C.
• C moves with velocity v in the backward 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.

Talking about absolute rest,

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

Talking about absolute motion,

• 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.

Read the next article on-

Distance & Displacement

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