Physics class 9th chapter motion class notes, Schemes and Mind Maps of Physics

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Motion

Introduction

●A body is said to be in motion if it changes its position w.r.t surrounding and time. ●A fixed point or a fixed object w.r.t which a body changes its position is known as origin or point of reference. ●If a physical quantity has only magnitude then it is known as a scalar quantity. E.g. Mass, time, distance, speed etc ●If a physical quantity has both magnitude and direction then it is known as vector quantity. E.g. Weight, displacement, velocity etc.

Position

➢ Distance

●Can be defined as actual length of the path covered by a body ●A scalar quantity ●SI unit- metre(m).

➢ Displacement

●Can be defined as the shortest distance between initial and final position of a body ●A vector quantity ●SI unit- metre(m) ●Can be positive, negative or zero. ●For a course of motion, the displacement of an object may be zero but the corresponding distance covered is not zero. So, ∴Displacement ≤Distance

Motion in a Straight Line

The motion of objects along a straight line is known as rectilinear motion. In case of rectilinear motion or one- dimensional motion only one coordinate is needed to specify the position of a body.

➢ UNIFORM MOTION

When a body travels equal distances in equal intervals of time, no matter how small the interval may be.

➢ NON-UNIFORM MOTION

When a body covers unequal distances in equal intervals of time, even if the interval is very small. For example a car moving through a crowded market.

Speed

●Speed measures the rate of motion of an object. ●Speed of an object = Distance(s) Time(t) ●A scalar quantity ●SI unit of speed - metre per second (m/s). Other units include (km/h) ●For a moving body, speed is always positive and can never be zero. ●Average speed = Total distance travelled Total time taken

Velocity- Speed with Direction

●Velocity can be defined as the rate of change of displacement of an object. ●Velocity of an object = 𝐷𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑇𝑖𝑚𝑒 ●A vector quantity ●SI unit of velocity - metre per second (m/s) ●Velocity of an object can be positive, negative or zero.

➢ Average Velocity

1. Depends only on the initial and the final position of an object and doesn’t depend on the path taken by the object.

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Body moving with uniform velocity Body moving with non-uniform velocity

➢ Speed/ velocity-time Graph

Speed-time graph or velocity-time graph When the speed/velocity reamins constant When initial velocity is zero and increases uniformly with time Uniform Acceleration When initial velocity is non-zero and increases uniformly with time 4

●Since the velocity of the object changes continuously in a circular motion, due to change in its direction, the object undergoes acceleration. ●The magnitude of its acceleration is given by 𝑣 2 𝑅is always directed towards the centre. ●In a uniform circular motion the acceleration of an object is always directed towards the center of the circle.This acceleration is called centripetal acceleration. 5