CBSE Class 9 Science Chapter 8 Revision Notes
Chapter 8: Motion Revision Notes

When an object moves over time, it is said to be in motion.

When a body moves uniformly, it covers the same distance in the same amount of time.

When a body travels an unequal distance in the same amount of time. It moves in a nonuniform manner.

In explaining motion, both distance and time are relevant.

Even if you don’t see anything, you can tell when a motion has occurred.

When two objects move in a plane (either in the same direction or in the opposite direction), each has a relative motion to the other.
For example, if a person is sitting on a train and watching a tree, the tree is steady but is assumed to be moving in relation to the train.
Displacement vs. Distance
 The distance travelled by an object.
 It only has magnitude and no direction.
 Displacement refers to how far and how far an object has moved from its initial position. i.e. the straight distance between the starting and ending places.
Speed
 The distance travelled by an object in a certain amount of time is called speed.
 Distance divided by time equals speed.
 The metric unit of speed is the metre per second (m/s).
Different Speeds
 Constant speed means that the speed does not alter.
 Changing speeds: 5 kilometres on a bicycle. Take off and pick up speed, slow down up hill, speed up down hill, and come to a complete stop at the stop sign. It took you 15 minutes to complete the journey (.25 h)
 Total speed divided by total time equals average speed.
 Speed at any given time is referred to as instantaneous speed.
Velocity
 Velocity is a term that encompasses both speed and direction.
 The storm is moving at a speed of 20 kilometers per hour.
 Assume that two trains are travelling at the same speed but in opposite directions, resulting in different velocities.
 A race car travelling around an oval track may maintain a constant speed yet have varying velocities at different points.
Acceleration
 Acceleration is defined as a change in velocity over a period of time.
 The direction is indicated by the (+ or ) sign. The + symbol indicates acceleration, while the – sign indicates deceleration.
Equation of uniform (constant) acceleration
a = v/t a = v/t a = v/t
 Galileo Galilei was an Italian physicist and astronomer who lived from 1564 to 1642. Formulated laws of motion for freefalling objects.
 Any object traveling freely under the effect of gravity alone is referred to as a freely falling object.
 It is independent of the object’s initial motion.
 Regardless of the beginning velocity, the acceleration of an item in free fall is downward.
 The acceleration of free fall (gravitational acceleration) is g = 9.80 m/s
 With rising altitude, g lowers.
 g is affected by latitude, height, and depth below the earth’s surface.
 9.80 metres per second square is the average on the surface of the Earth
 The acceleration owing to gravity will be represented by the italics g.
 Not to be confused with the letter g, which stands for grams.
 Falling items can be regarded freely falling if there is no air resistance. Objects of various forms accelerate at varying rates (stone vs feather)
 Both upward and downward acceleration is possible.
 The journey follows a symmetrical pattern.
 The rate of acceleration is constant.
 At identical heights, the magnitude of the velocities is the same.
 As time passes, the images get closer together.
 When the ball moves upward, the acceleration and velocity are in opposite directions.
 The rate of acceleration is consistent (violet arrows maintain the same length)
 In an upward motion, the velocity is decreasing (red arrows are getting shorter)
 Negative acceleration and positive velocity
 At maximum height, velocity is zero.
 The time it takes to get upward and back is always the same.
GRAPHICAL REPRESENTATION OF MOTION
Graph of Distance and Time
 DistanceTime graphs depict how an object’s position changes over time.
 Nonlinear fluctuations imply nonuniform motion, while linear variations imply uniform motion.
 We gain speed because of the slope.
Graph of Distance vs. Time
 Because the slope is constant, OA implies uniform motion with constant speed.
 Because the slope is zero, AB denotes that the body is at rest.
 The motion from B to C is nonuniform.
Graph of Velocity and Time
 The change in velocity with respect to time is represented by a velocitytime graph.
 Acceleration is provided by the slope.
 The displacement is determined by the area under the curve.
 Constant velocity is implied by a line parallel to the xaxis.
EQUATIONS OF MOTION
Three equations can be used to describe the motion of an item travelling with uniform acceleration, namely
v= u + at
v2 – u2 = 2as
s = (1/2)at2 + ut
UNIFORM CIRCULAR MOTION
 Uniform circular motion occurs when an object moves in a circular route at a constant speed.
 As the direction shifts, so does the velocity.
 The rate of acceleration is constant.