This PSEB 9th Class Science Notes Chapter 8 Motion will help you in revision during exams.

## PSEB 9th Class Science Notes Chapter 8 Motion

→ A body is said to be in motion when it changes its position with time.

→ At any instant, a body may appear to be in motion to one person but the same object may appear at rest to another person.

→ To describe the position of an object we are required to fix the directive point which is called the origin.

→ The motion of some objects can be controlled while that of some other objects remains uncontrolled and irregular.

→ When an object moves along a straight line path then its motion is called linear motion.

→ Those quantities which can be described completely by their magnitude only are called physical quantities.

→ The shortest distance measured between initial and final position is called displacement of the body.

→ When the final position of the body in motion coincides with its initial position then the value of displacement becomes equal to zero.

→ The displacement of an object may be zero but its distance will not be zero.

→ An odometer is a device that measures the distance covered by automobiles.

→ The rate of change of motion of an object is called its speed. The unit of speed is ms^{-1}.

→ The average speed of an object is obtained by dividing the distance covered by the body by the time taken to cover that distance.

∴ Average speed = Total distance covered/Time taken to cover that distance

→ The rate of change of motion of a body in a particular direction is called its velocity i.e. speed in a particular direction is called velocity.

→ Time interval is measured by digital wristwatch or stopwatch.

→ Speed of sound in air = 340 m/s.

→ Speed of light in air = 3 × 10^{8} m/s.

→ The rate of change of velocity of a body is called its acceleration

- Acceleration = Change in velocity/Time taken
- S.I. unit of acceleration is ms
^{-2}.

→ If the velocity of an object changes by the same rate then its mean (average) velocity, is obtained by taking the arithmetic mean of initial velocity and final velocity.

∴ Average Velocity = \(\frac{\text { Initial velocity + Final velocity }}{2}\)

→ If an object covers equal distances in equal intervals of time however small the interval maybe then its motion is called uniform motion.

→ If a moving object covers unequal distances in equal intervals of time then the motion of the object is called non-uniform motion.

→ Rest: A body is said to be at rest if it does not change its position with respect to its surroundings.

→ Motion: A body is said to be in motion if it changes its position with time with respect to its surroundings.

→ Distance: It is the length of the actual path travelled by a body between its initial position and final position.

→ Displacement: Tire change in position of an object in a given direction is known as displacement. It is measured by the shortest distance moved by a body from the initial position to the final position.

→ Uniform Motion: If an object covers equal distances in equal intervals of time however small the time intervals may be, then the motion of the object is said to be uniform motion.

→ Non-Uniform Motion: If an object covers unequal distances in equal intervals of time then the object is said to be in non-uniform motion.

→ Speed: The distance travelled by a body in a unit of time

Speed = Distance traveled/Total time taken

→ Average Speed: It is the total distance travelled bv an object divided by the time taken to travel that distance.

Average speed = Total distance travelled/Total time taken

→ Uniform Velocity: If a body covers equal distances in equal intervals of time, however small these intervals maybe, in a particular direction its velocity is said to be uniform velocity.

→ Variable Velocity: If a body travels unequal distances in equal intervals of time or changes its direction or both then its velocity is said to be variable velocity.

→ Uniform Circular Motion: When a body moves along a circular path with uniform speed, its motion is called uniform circular motion. Tire motion is uniformly accelerated due to continuous change of direction.

→ Angular Velocity: It is the angle swept by the radius per unit time

ω = \(\frac{\text {θ}}{t}\) rad/s

→ Velocity: It is the displacement produced per unit time.

or

It is the distance travelled per unit time in a given direction.

Velocity = Displacement/Time

→ Uniform Speed: When an object covers equal distances in equal intervals of time, however small these intervals may be, it is said to move with uniform speed.

→ Non-Uniform Speed: When an object covers unequal distances in equal intervals of time, it is said to move with non-uniform speed.