NCERT Science Class 7 - Chapter 9: Motion and Time - Notes

Learning Objectives

Understand slow and fast motion
Learn about speed and its measurement
Understand distance-time graphs
Know about measurement of time

Key Concepts

Speed

Speed: The distance covered by an object in unit time.

Speed = Distance ÷ Time

SI unit of speed: metre per second (m/s). Also expressed as km/h (kilometres per hour).

Conversion: 1 km/h = 5/18 m/s; 1 m/s = 18/5 km/h

Types of Motion

Uniform motion: Object covers equal distances in equal intervals of time. Speed is constant.
Non-uniform motion: Object covers unequal distances in equal intervals of time. Speed changes.

Measurement of Time

Basic unit of time: Second (s). Other units: minute (60 s), hour (3600 s).

Periodic motion: Motion that repeats at regular intervals (pendulum, clock hands).

Simple pendulum: A small weight (bob) suspended from a fixed point by a thread. One oscillation = one complete to-and-fro movement. Time period = time for one oscillation.

Distance-Time Graph

Uniform speed: Distance-time graph is a straight line (linear). Steeper line = higher speed.

Non-uniform speed: Distance-time graph is a curved line.

Object at rest: Horizontal straight line (distance doesn't change).

Odometer and Speedometer

Odometer: Measures total distance travelled by a vehicle.

Speedometer: Shows the speed of a vehicle at any instant.

Summary

Speed is distance divided by time. Uniform motion has constant speed; non-uniform has varying speed. Time is measured using periodic events. A pendulum's time period depends on its length. Distance-time graphs show motion patterns visually.

Important Terms

Speed: Distance per unit time (m/s or km/h)
Uniform motion: Constant speed, straight line graph
Non-uniform motion: Changing speed, curved graph
Oscillation: One complete to-and-fro motion
Time period: Time for one complete oscillation

Quick Revision

Speed = Distance / Time (m/s or km/h)
1 km/h = 5/18 m/s
Uniform motion → straight line on distance-time graph
Pendulum time period depends on length, not mass
Odometer = distance, Speedometer = speed