# Class 9 science notes Chapter Motion

Topic covered : Important Definition, Difference between distance and displacement, Uniform motion, Speed, velocity, acceleration, Equation of motion and much more……… Class 9 science notes Chapter Motion

• Physics is a natural science that involves the study of matter and its motion through space-time.
• An object is said to be in the state of rest, if it does not change its position with respect to surrounding with the passage of time.
• An object is said to be in motion if it change its position with respect to surroundings with the passage of time.
• A point object is one which has no linear dimensions but posses mass. Such as earth is treated as point object during its revolution around sun.
• Mechanics is an important branch of physics, can be broadly divided into Statics and Dynamics. This branch deals with the motion of object in everyday life. It is of two types:
• Statics deals with the bodies at rest under the effect of different forces
• Dynamics deals with bodies in motion.

## Motion

The motion of an object is perceived when its position changes continuously with respect to some stationary object.

For example, motion of air is detected by movement of dust particles, leaves and branches of trees. Similarly, the phenomena of sunrise, sunset and changing of seasons are now understood to be due to rotation of the earth. Atoms, molecules, planets, stars and galaxies are all in motion.

## Distance

It defined as the length of actual path followed by a body between the points between which it moves.

• It is scalar quantity
• It can never be –ve and zero
• Distance can be equal to or greater than displacement

## Displacement

The displacement of an object in motion is the shortest distance between the initial position and final position of the object.

• It is vector quantity.
• It can be –ve, zero or +ve.
• Displacement can be equal to or less than distance.
• SI unit of both distance and displacement is metre (M).

## Difference between distance & displacement

1. Distance is a scalar quantity having magnitude only. Displacement is a vector quantity having both the magnitude as well as direction.
2. Both the distance and displacement have the same units.
3. The distance travelled by an object in motion can never be zero or negative. The displacement of an object can be positive, zero or negative.
4. When final position of an object in motion coincides with its initial position, the displacement is zero, but the distance travelled is not zero.                                                                                                                                                               The reverse is not true, i.e. when distance travelled is zero, the object may be at rest. Its displacement would also be zero.
5. Between two given positions, i. e., initial position and final position, distance can have any value depending on the actual path followed, but displacement has only value, i.e., displacement is the shortest distance between the initial position and final position of the object.
6. Corresponding to a given displacement (between initial position and final position), distance travelled may be larger than the displacement. However, when motion from initial position to final position is along a single straight line, distance travelled is equal to magnitude of displacement in no case; distance travelled can be less than the displacement.
7. Neither the distance nor the displacement gives any information about the nature of motion followed by the object

## UNIFORM MOTION

When an object travels equal distances in equal intervals of time, howsoever small the interval may be, the motion of the object is said to be uniform. For example, suppose a car covers 60 km in first hour, another 60 km in second hour, and again 60 km in the third hour and so on. The motion of the car is uniform motion.

## NON- UNIFORM MOTION

When an object travels unequal distances in equal intervals of time, howsoever small the interval may be, the motion of the object is said to be non- uniform. For example, suppose a car covers 40 km in first hour, 50 km in second hour, 30 km in third hour, 70 km in fourth hour and so on, the motion of the car is non- uniform, as it covers unequal distances in equal intervals of time.

## Speed

The speed of a moving particle is the rate at which it describes its path.

Or

Speed of a body is defined as the distance travelled by the body in unit time. Mathematically, this may be expressed in the following way.

• Speed = distance travelled/ time taken.
• Speed of a body is a scalar quantity.
• Uniform speed: When a body travels equal distances in equal intervals of time, howsoever small the intervals may be, the speed of the body is said to be uniform.
• Non- uniform speed or Variable speed: When a body covers unequal distances in equal intervals of time, however small the intervals may be, the speed of the body is said to be non- uniform or variable.
• An object is said to be moving with a variable speed if it covers equal distances in unequal intervals of time or unequal distances in equal intervals of time.
• When an object is moving with a variable speed, then the average speed for the given motion is defined as the ratio of the total distance travelled by the object to the total time taken.

Average speed = total distance travelled / total time taken.

• If an object has a variable speed such that the speed varies at different instants, then the speed at any instant is called the instantaneous speed.
• Gives no idea about the direction of the motion of the object.
• The speed of an object (like distance) can be zero or positive but never negative.

## Velocity

The velocity of a moving particle is the rate of its displacement or the rate of change of its displacement with time.

Or

Velocity of a body is the distance travelled by the body is unit time in a given direction.

• Velocity of a body is the speed of the body in a particular direction.
• Velocity = displacement / time.
• Velocity is a vector quantity whereas speed is a scalar quantity.
• Therefore, velocity possesses both magnitude and direction and hence, is a vector.
• A particle is said to be moving with uniform velocity when it is moving along a straight line or in a constant direction and covering equal distances in equal intervals of time.

However small these intervals may be. Uniform velocity of a moving object is measured by its displacement per unit time.

• Velocity of a body is said to be variable (or non- uniform) when it covers unequal distances in a particular direction in equal intervals of time or the direction of motion of the body changes.
• If a body moves in such as a way that its speed or its direction or both changes with the time, then the body is said to possess a variable velocity. Variable velocity is measured at any instant by the displacement that the moving object would have in unit time. In other words, we would have to calculate the instantaneous velocity.
• Average Velocity: When velocity of a body is changing at a uniform rate over a period of time, then average velocity of the body for this period is arithmetic mean of initial velocity and final velocity of the body.

The SI unit of speed (and velocity) is metre/second [i.e., m/s] and dimensions are [M0L1T-1].

Average velocity = (initial velocity + final velocity) /2

## ACCELERATION

ACCELERATION of a body is defined as the rate of change of its velocity with time. Thus, acceleration of a body is a measure of change in velocity of the body per unit time.

Acceleration = change in velocity / time taken.

• Here the change in velocity is final velocity – initial velocity
• S.I. unit of acceleration is the m/s2.
• It is a vector quantity.
• When a body is moving with uniform velocity then its acceleration will be zero.
• The negative acceleration is called retardation.

### Uniform acceleration

A body has a uniform acceleration if its velocity changes at a uniform rate. Such as motion of a freely falling body.

### Non uniform ACCELERATION

A body has non-uniform acceleration if its velocity changes at a non-uniform rate.

Retardation (-ve acceleration)

If the velocity of a body increases, then acceleration is +ve and if velocity decreases, then acceleration is –ve.

## Graphical Representation Of Motion

A graph is plotted between two variable quantities. The quantity that is varied at our choice is called independent variable.

• The other quantity, which varies as a result of this change, is called the dependent variable. For example in distance –time graph, time is independent variable and distance is dependent variable.
• Similarly, in velocity-time graph, time is dependent variable and velocity is dependent variable.