Chapter 4: Describing Motion Around Us
Notes Prepared by N.K. Singha (M.Sc.)
Chapter Overview
Everything around us is in motion. From the movement of planets to a flying bird, a moving car, or even dust particles floating in sunlight—motion is present everywhere. Motion is one of the most fundamental concepts in Physics. This chapter explains how scientists describe motion using distance, displacement, speed, velocity, acceleration, graphs, and mathematical equations.
Learning Objectives
After studying this chapter, you will be able to:
- Understand the meaning of motion.
- Describe the position of an object.
- Differentiate between distance and displacement.
- Explain speed, velocity and acceleration.
- Interpret position-time and velocity-time graphs.
- Solve numerical problems using equations of motion.
- Understand uniform circular motion.
1. Motion
Motion is the change in the position of an object with time with respect to a reference point.
- A moving train
- Flying aeroplane
- Walking person
- Running athlete
- Rotating ceiling fan
- Revolving Earth
If the position does not change with time → Object is at Rest.
Reference Point
A fixed point used for comparing the position of an object is called the Reference Point (Origin).
Without a reference point, motion cannot be described.
Suppose a car is 50 m east of your house.
Here,
Reference Point = House
Position = 50 m East
Position
Position tells us
- How far an object is from the reference point.
- In which direction it is located.
- 40 m East
- 80 m West
Both distance and direction are required.
Motion in a Straight Line (Linear Motion)
When an object moves along a straight path, the motion is called Linear Motion.
- Train on a straight railway track
- Falling stone
- Runner on a straight track
- Lift moving vertically
- Car on a straight road
Distance
Distance is the total length of the actual path travelled by an object.
Properties
- Scalar Quantity
- Has only magnitude
- Never negative
- Depends on actual path
- SI Unit = metre (m)
Distance = Total Path Length
A person walks
30 m East
then
20 m West
Distance = 30 + 20 = 50 m
Displacement
Displacement is the shortest distance between the initial position and final position along with direction.
Properties
- Vector Quantity
- Has magnitude and direction
- Can be positive, negative or zero
- SI Unit = metre (m)
Displacement = Final Position − Initial Position
Walk
30 m East
then
20 m West
Distance = 50 m
Displacement = 10 m East
Difference Between Distance and Displacement
| Distance | Displacement |
|---|---|
| Actual path travelled | Shortest path |
| Scalar | Vector |
| No direction | Direction required |
| Always positive | Positive, Negative or Zero |
| Greater than or equal to displacement | Less than or equal to distance |
Important Result
Distance ≥ Displacement
Distance equals displacement only when the object moves in one direction without turning back.
Special Cases
When Displacement = 0
- One complete round on a circular track.
- Returning to the starting point.
- Moving forward and coming back to the same position.
Walk 100 m forward
Return 100 m back
Distance = 200 m
Displacement = 0
Scalar and Vector Quantities
Needs only magnitude.
Examples
- Distance
- Speed
- Time
- Mass
- Temperature
Needs magnitude and direction.
Examples
- Displacement
- Velocity
- Acceleration
- Force
Important NCERT Facts
- ★ Distance is always greater than or equal to displacement.
- ★ Displacement can never be greater than distance.
- ★ Distance cannot be negative.
- ★ Displacement can be zero.
- ★ Motion always depends upon the reference point.
- ★ Rest and motion are relative concepts.
Real-Life Examples
A student walks
100 m to school
then returns home.
Distance = 200 m
Displacement = 0
A runner runs
400 m on a circular track.
Distance = 400 m
Displacement = 0
Car moves
60 km East
then
40 km East
Distance = 100 km
Displacement = 100 km East
Common Mistakes Made by Students
💡 Memory Trick
Distance → D = Detailed Path Displacement → D = Direct Path
📋 Formula Summary
🏁 Chapter Summary
- Motion means change in position with time.
- Reference point is necessary to describe motion.
- Position is described by distance and direction.
- Distance is the total path travelled.
- Displacement is the shortest distance from start to finish.
- Distance is a scalar quantity.
- Displacement is a vector quantity.
- Distance is always greater than or equal to displacement.
- If an object returns to its starting point, displacement becomes zero while distance is not zero.