NCERT Class 9 Science Chapter 4 Solutions: Describing Motion Around Us – Complete Guide

Motion is a fundamental concept in physics that helps us understand how objects move around us. In Class 9 Science Chapter 4 – Describing Motion Around Us, students learn about motion, speed, velocity, acceleration, and real-life applications.

Think It Over

Everything in nature is in motion, from massive astronomical objects to subatomic particles. We have a wide variety of motion in nature, such as flitting butterflies, slithering snakes, hopping hares, galloping horses, tendrils of climbers twinning around a support, closing of flytraps, dancing dust particles in a sunbeam, smoke particles moving in air, rising and falling of ocean tides, and gathering clouds.

Everything in nature is constantly in motion—from the largest galaxies to the smallest particles.

Examples of Motion in Nature:

• Flitting butterflies

• Slithering snakes

• Hopping hares

• Galloping horses

• Tendrils of climbers twining around support

• Closing of flytraps

• Dust particles dancing in sunlight

• Smoke particles moving in air

• Ocean tides rising and falling

• Clouds gathering in the sky

 These examples show that motion is everywhere, making it an essential concept to study.

Chapter 4 Describing Motion Around Us Solutions: Download Now

4.1 Motion in a Straight Line

Motion in a straight line is the simplest form of motion, where an object moves along a straight path.

4.1.1 Describing Position

To describe motion, we first need to define the position of an object.

• Position is measured relative to a reference point (origin)

• It helps us understand where an object is located

Example:
A car 10 meters east of a tree

4.1.2 Distance Travelled and Displacement

Distance

• Total path covered

• Scalar quantity

• Always positive

Displacement

• Shortest distance from initial to final position

• Vector quantity

• Can be zero

Example:
A person walks 20 m forward and returns 20 m

• Distance = 40 m

• Displacement = 0

4.1.3 Average Speed and Average Velocity

Average Speed

Average Speed=Total DistanceTotal TimeAverage\ Speed = \frac{Total\ Distance}{Total\ Time}Average Speed=Total TimeTotal Distance?

• Depends on total distance

• Scalar quantity

Average Velocity

Average Velocity=Total DisplacementTotal TimeAverage\ Velocity = \frac{Total\ Displacement}{Total\ Time}Average Velocity=Total TimeTotal Displacement?

• Depends on displacement

• Vector quantity

Graphs of Motion

• Distance-Time Graph → Shows speed

• Velocity-Time Graph → Shows acceleration

Solved Examples

Example 1:

A car travels 100 m in 10 seconds
Speed = 10 m/s

Example 2:

Object moves 50 m north in 5 sec
Velocity = 10 m/s north

Example 3:

Velocity changes from 10 to 20 m/s in 5 sec
Acceleration = 2 m/s²

NCERT Solutions (Sample)

Q1. Define motion

Change in position with time

Q2. Define speed

Distance per unit time

Q3. Define displacement

Shortest distance between two points

Real-Life Applications

• Vehicles
• Rockets
• Sports
• Planetary motion

Important Tips

Understand concepts clearly
Practice formulas
Focus on numericals
Revise graphs

Conclusion

This chapter builds the foundation for physics by explaining how motion works in our everyday life. By understanding these concepts, students can easily solve problems and perform well in exams.