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Motion
Chapter Overview
Rest and motion are relative concepts. In this chapter, we explore different types of physical movement, learn to differentiate between uniform and non-uniform motion, and compare fundamental concepts like Mass and Weight.
2.1 Rest and Motion
- State of Rest: A body is at rest if it does not change its position with respect to its immediate surroundings over time.
- State of Motion: A body is in motion if it continuously changes its position with respect to its stationary surroundings over time.
Relative Nature of Motion
An object can be at rest and in motion at the same time, depending on the observer. For example, a passenger sitting in a moving train is at rest relative to the other passengers inside, but in motion relative to a person standing outside on the platform.
2.2 Types of Motion
1. Translatory Motion
In translatory motion, all parts of a rigid body move identically through the same distance in the same time. It has two subtypes:
- Rectilinear Motion: Motion along a straight line path. For example, a car moving straight on a highway.
- Curvilinear Motion: Motion along a curved path. For example, a train negotiating a curved track.
2. Rotatory Motion
A body exhibits rotatory motion if it moves around a fixed, central geometric axis without changing its overall position in space. For example, the spinning blades of an electric fan or a potter's wheel.
3. Oscillatory and Vibratory Motion
This is a rhythmic "to and fro" continuous movement of a body about its stable mean position.
- Oscillatory Motion: The entire body moves to and fro. For example, the swinging pendulum of a grandfather clock.
- Vibratory Motion: Only a specific part of the body oscillates rapidly while the rest remains fixed. For example, the plucked strings of a guitar vibrating to produce sound.
AI Image Prompt: A visual chart showing three distinct panels. Panel 1: A red toy car driving on a straight track (Translatory). Panel 2: A metallic spinning top rotating on its axis (Rotatory). Panel 3: A golden pendulum swinging back and forth with motion blur trails (Oscillatory).
2.3 Mass and Weight
In everyday language, people often confuse mass and weight. However, in Physics, they are completely different quantities.
| Mass | Weight |
|---|---|
| The amount of matter contained in a body. | The gravitational pull or force with which the Earth attracts a body towards its centre. |
| Measured using a physical beam balance. | Measured using a spring balance. |
| S.I. unit is the kilogram ($kg$). | S.I. unit is the Newton ($N$) (since it is a force). |
| Mass remains constant everywhere in the universe. | Weight changes from place to place depending on the strength of gravity ($g$). |
Relation between Mass and Weight
Weight ($W$) is mathematically calculated by multiplying the Mass ($m$) of the object by the acceleration due to gravity ($g$).
$W = m \times g$
On the surface of the Earth, the average value of $g$ is approximately $9.8\text{ m/s}^2$ or $10\text{ m/s}^2$. Therefore, an object having a mass of $1\text{ kg}$ will weigh approximately $10\text{ Newtons}$ on Earth.
Q1. Why does the weight of an astronaut become one-sixth on the Moon compared to Earth, but their mass remains the same?
Ans: Mass is the amount of matter in the astronaut's body, which never physically changes. However, weight depends on the gravitational pull of the planetary body. Since the gravity on the Moon is one-sixth of that on Earth, the downward pull on the astronaut (their weight) is reduced to one-sixth.