NCERT Class 9 Motion Notes

 In this article, you will get Class 9 Motion notes. These notes are very important for the board exams.

Motion & Rest

A body is said to be in motion, if its position changes with respect to its surrounding with the passage of time.

A body is said to be at rest, if its position does not change with respect to its surroundings with the passage of time.

Motion & Rest are relative

The states of rest and motion are not absolute. They are relative in nature and depend upon the observer. A body may appear to be at rest to one observer and in motion to the another observer.

For example; to a person standing on ground, earth appears to be at rest but for a person on moon, earth appears to be in motion.

Distance and Displacement

Distance is the length of the actual path travelled by a body, and displacement is the shortest distance between two points.

Both distance and displacement are measured in metres and both are actually the lengths.

There are some differences between distance and displacement. They are;

1. Distance is the length of total path travelled where as displacement is the shortest distance between the initial point and final point.

2. Distance cannot be negative but displacement can be positive, negative or even zero.

3. Distance is a scalar quantity but displacement is a vector quantity.

When is distance equal to displacement

Distance and displacement are equal to each other when a body is travelling along a straight line and never changes its direction of motion.

Speed and Velocity

Speed is defined as the distance travelled per unit time and velocity is defined as the displacement covered per unit time.

That is Speed = Distance covered/time taken, and 

Velocity = Displacement covered/time taken.

Both are used to check how quickly does a body travel.

Both speed and velocity are measured in m/s.

There are some differences between speed and velocity. While speed is a scalar quantity, velocity is a vector quantity. Speed is always positive but velocity can be negative also.

Acceleration

It is defined as the rate of change of velocity with time. If the velocity of a body changes with time, it is said to be having acceleration.

Mathematically Acceleration = change in velocity/time taken.

The units of acceleration are m/s2.

Acceleration can positive as well as negative. If it is negative, it is known as retardation.