MOTION | SPEED NOTES | CBSE | SCIENCE | CLASS 9

Rest And Motion

Motion: An object is said to be in motion when its position changes with time.
Rest: An object is said to be at rest when its position does not change with respect to a reference point with time.
A specific point with respect to which we describe the location of an object is called a reference point.
The terms Rest and Motion are relative.

Distance and Displacement

Distance: The total length of path covered by an object is said to be the distance travelled by it.
Displacement: The length of a straight path that joins the initial and final positions of an object is known as the displacement.

Distance	Displacement
Distance is defined as the total length of the path traveled by an object to go from one point to another.	Displacement is defined as the length of the straight path that joins the initial and final positions of an object.
Since distance has only magnitude and its direction cannot be specified always, it is a scalar quantity.	Since displacement has magnitude and it is specified in a direction from initial position to final position, it is a vector quantity.
Distance can only have positive values	Displacement can have both positive and negative values
Distance depends on the length of the path travelled.	Displacement depends only on the initial and final point regardless of the path travelled.

Speed And Velocity

Speed

Speed: The distance travelled by an object in unit time is referred to as speed.
Its S.I unit is m/s.
In general speed refers to average speed.

Average speed: For non-uniform motion, the average speed of an object is obtained by dividing the total distance travelled by an object by the total time taken.

Average speed, V = Total distance / Total time

V = s/t

Uniform And Non-Uniform motion

Uniform motion or non accelerated motion: When an object covers equal distances in equal intervals of time, it is said to be in uniform motion. Uniform motion is a non-accelerated motion.
Non-uniform motion or accelerated motion: Motions where objects cover unequal distances in equal intervals of time. Uniform motion is an accelerated motion

Note:

For a uniform motion, the average speed of an object is equal to its instantaneous speed throughout the path.

For a Non - uniform motion, the average speed of an object is not equal to its instantaneous speed throughout the path.

For a Non - uniform motion, speed of an object is not same (changes) throughout the motion.

Velocity

Velocity: Velocity of an object is defined as the rate of change in its net displacement.

For a uniform motion in a straight path, the average velocity is equal to its instantaneous velocity throughout the path.

Velocity of an object is equal to the instantaneous velocity of an object.

Differences Between Speed and Velocity

SPEED	VELOCITY
It is defined as the rate of change of distance.	It is defined as the rate of change of net displacement.
It is a scalar quantity	It is a vector quantity.
It can never be negative or zero.	It can be negative,zero or positive.
Speed is velocity without direction.	Velocity is directed speed.
Speed may or may not be equal to velocity.	A body may possess different velocities but the same speed.
Speed never decreases with time. For a moving body,	Velocity can decrease with time. For a moving body , it can be zero.
Speed in SI is measured in ms-1	Velocity in SI, is measured in ms-1

Uniform And Non-Uniform motion

Uniform motion or non accelerated motion: When an object covers equal distances in equal intervals of time, it is said to be in uniform motion. Uniform motion is a non-accelerated motion.
Non-uniform motion or accelerated motion: Motions where objects cover unequal distances in equal intervals of time. Uniform motion is an accelerated motion.