NCERT Class 9 science Chapter 8 MOTION Notes 

Preview 

In our daily lives, we encounter various objects in motion, from birds soaring to cars cruising. While we often perceive motion when an object's position changes over time, some motions are inferred indirectly, like observing the movement of dust to detect air motion. Curiosities arise regarding phenomena such as sunrise, sunset, and seasonal changes, prompting questions about the Earth's motion. The perception of motion can vary based on the observer's perspective, as illustrated by observations like the apparent movement of trees seen from a moving vehicle. This chapter aims to delve into different types of motion, starting with describing motion along a straight line and employing equations and graphs to express simple motions, followed by discussions on circular motion.

7.1 Describing Motion 

7.1.1 MOTION ALONG A STRAIGHT LINE

Motion Along a Straight Line: Understanding Distance and Displacement

Example of Motion: Consider an object moving along a straight path, starting from point O and reaching positions A, B, and C at different instants.

Understanding Distance and Displacement:

• Distance Covered: The total path length traveled by the object, represented by OA + AC, which equals 95 km in this case.
• Displacement: The shortest distance between the initial and final positions, regardless of the path taken. For instance, the displacement from O to C through A is 35 km.
• Comparison: While the distance reflects the actual path length, the displacement represents the change in position. In this example, the magnitude of displacement is not equal to the distance traveled, showcasing their distinct characteristics.
• Magnitude Equality: Analyzing specific scenarios, such as the motion from O to A, where the distance and displacement are both 60 km, illustrates instances where their magnitudes align.

7.1.2 UNIFORM MOTION AND NONUNIFORM MOTION

Understanding Motion: Uniform and Non-Uniform

1. Uniform Motion:

• Definition: When an object covers equal distances in equal intervals of time along a straight line.
• Example: Object travels 5 m in each consecutive second, indicating uniform motion.
• Implication: Uniform motion suggests consistent speed over time, where objects move at a steady rate.

2. Non-Uniform Motion:

• Description: Objects cover unequal distances in equal intervals of time.
• Examples: Cars navigating crowded streets or individuals jogging in a park illustrate instances of non-uniform motion.
• Observations: Variation in speed among different objects is common, reflecting differences in their rates of motion.

Understanding Speed

1. Speed Measurement:

• Definition: Speed is the rate of motion, measured as the distance traveled per unit time.
• Units: SI unit of speed is meter per second (m/s), with alternative units including centimeter per second (cm/s) and kilometer per hour (km/h).
• Calculation: Average speed is determined by dividing the total distance traveled by the total time taken.

Formula: 

Average speed = Total distance traveled/ Total time taken

 Example: A car covering 100 km in 2 hours has an average speed of 50 km/h, despite variations in speed during the journey.

Understanding Average Speed:

• Calculation Example: For an object traveling 32 m in 6 s, the average speed is calculated as 5.33 m/s.
• Significance: Average speed provides an overall measure of an object's motion, accounting for variations in speed over time.

7.2.1 SPEED WITH DIRECTION

Understanding Velocity

Definition:

• Velocity: Specifies both the speed and direction of an object's motion.
• Comprehensiveness: Provides a more comprehensive understanding of an object's motion compared to speed alone.

Characteristics:

• Uniform or Variable: Velocity can be uniform (constant) or variable (changing).
• Factors Influencing Change: Changes in velocity can occur due to alterations in speed, direction, or both.

Average Velocity:

• Calculation: Similar to average speed, average velocity is determined by dividing the total displacement by the total time taken.
• Uniform Rate of Change: If velocity changes at a uniform rate, average velocity is the arithmetic mean of initial and final velocities.

7.3 Rate of Change of Velocity

Understanding Acceleration

Definition:

• Acceleration: Measures the change in velocity of an object per unit time.

Characteristics:

Uniform vs. Non-uniform Motion:

• Uniform Motion: Velocity remains constant with time, resulting in zero change in velocity.
• Non-uniform Motion: Velocity varies with time, leading to a non-zero change in velocity.

Types of Motion:

• Accelerated Motion: Occurs when velocity changes, either increasing or decreasing.
• Uniform Acceleration: Velocity changes by equal amounts in equal intervals of time, resulting in uniform acceleration.
• Non-uniform Acceleration: Velocity changes at a non-uniform rate, leading to non-uniform acceleration.

Directionality:

• Positive and Negative Acceleration: Acceleration is considered positive when in the direction of velocity and negative when opposite to the velocity direction.
• Unit:
• SI Unit: Acceleration is measured in meters per second squared (m/s²).
• Examples: 
• Freely Falling Body: Represents uniformly accelerated motion.
• Car on a Straight Road: Can exhibit non-uniform acceleration if its speed changes by unequal amounts in equal time intervals.

7.4 Graphical Representation of Motion

7.4.1 DISTANCE–TIME GRAPHS 

1. Distance-Time Graphs

• Representation:
• Purpose: Represents the change in the position of an object with time.
• Axis: Time is plotted along the x-axis, while distance is plotted along the y-axis.

2. Types of Motion Represented:

• Uniform Speed: Results in a straight line graph where distance is directly proportional to time.
• Non-uniform Speed: Shows nonlinear variation of distance with time, indicating changing speed.

3. Uniform Speed:

• Graph Appearance: A straight line on the distance-time graph, indicating constant distance covered per unit time.
• Example: Portion OB on the graph shows uniform increase in distance over time.

4. Determining Speed:

• Method: Use a small portion AB of the graph to form a triangle ABC, where AC represents time interval and BC represents distance covered.

5.Non-uniform Speed:

• Graph Appearance: Nonlinear shape indicates varying speed over time.
• Example: Graph in Fig. 7.4 represents motion with non-uniform speed.
• Note:
• Uniform Velocity: Term used interchangeably with uniform speed if magnitude of displacement equals distance on the y-axis.

7.4.2 VELOCITY-TIME GRAPHS

Velocity-Time Graphs

Representation:

• Purpose: Illustrates the variation in velocity with time for an object.
• Axis: Time is plotted along the x-axis, while velocity is plotted along the y-axis.

Uniform Velocity:

• Graph Appearance: Constant height of the graph, parallel to the x-axis, indicates uniform velocity.
• Example: Fig. 7.5 shows a velocity-time graph for a car moving at a uniform velocity of 40 km/h.

Distance Calculation:

• Method: Area under the velocity-time graph represents displacement.

Uniformly Accelerated Motion:

• Graph Appearance: Straight line graph indicates uniform acceleration, where velocity changes by equal amounts in equal intervals of time.
• Example: Fig. 7.6 depicts a velocity-time graph for a car undergoing uniformly accelerated motion.
• Distance Calculation for Accelerated Motion:

• Method: Area under the velocity-time graph determines distance moved.
• Non-uniformly Accelerated Motion:
• Graph Shape: Velocity-time graphs can vary in shape for non-uniformly accelerated motion.

7.5 Equations of Motion 

Equations of Motion for Uniformly Accelerated Motion

Concept:

Uniform Acceleration: Object moves along a straight line with constant acceleration.

Relationship: Equations relate velocity, acceleration, and distance covered during motion.

Equations:

Graphical Method: Derived by graphically analyzing the velocity-time and displacement-time graphs.

Application:

Predictive Tool: Used to calculate final velocity, displacement, and time taken for an object undergoing uniformly accelerated motion.

7.6 Uniform Circular Motion

Circular Motion and Changing Direction

Concept:

• Uniform Circular Motion: Object moves along a circular path with constant speed.
• Changing Direction: Motion along a closed path requires changing direction, even if speed remains constant.

Examples:

1. Athlete Running on a Track:

• Rectangular, hexagonal, or octagonal tracks require changing direction at corners.
• Circular track: Constant change in direction due to curved path, exemplifying uniform circular motion.

2. Circular Path Activity:

• Experiment with a stone tied to a thread, moving it along a circular path.
• Stone moves tangentially once released, indicating change in direction at every point.

3. Sports Events:

• Athlete throws hammer or discus, imparting circular motion.
• Object continues along tangent upon release, illustrating uniform circular motion.

Understanding:

• Observation: Objects moving along closed paths require changes in direction, even with constant speed.
• Implication: Uniform circular motion involves continuous change in direction, while speed remains constant.
• Examples: Moon orbiting Earth, satellites in circular orbits, cyclists on circular tracks exhibit uniform circular motion.
• Conclusion:
• Circular motion involves constant change in direction, showcasing uniform circular motion, where speed remains constant while direction changes continuously.