Describing Motion Around Us Class 9 PPT & Notes | Free Download | USP

Class 9 Science Chapter 4 – Describing Motion Around Us

From a speeding car on the highway to a leaf falling gently from a tree, everything around us is either moving or at rest — and Physics gives us precise tools to describe this motion. In this chapter, "Describing Motion Around Us," Class 9 CBSE Science students learn how to describe the motion of objects using distance, displacement, speed, velocity, acceleration, and motion graphs. This free PPT and notes download covers the complete NCERT 2026–27 syllabus in a simple, exam-focused format so you can revise quickly before tests.

Motion, Rest and Reference Point

An object is said to be in motion if it changes its position with time with respect to a fixed point called a reference point (or origin). If an object does not change its position with time, it is said to be at rest. Motion and rest are relative terms — a passenger sitting inside a moving bus is at rest with respect to the bus but in motion with respect to a person standing on the road.

• Reference point: a fixed point used to describe the position of an object
• An object can be in motion for one observer and at rest for another
• Motion can be along a straight line, circular, or random (zig-zag)

Distance and Displacement

Distance and displacement are two of the most confused terms in this chapter, and a favourite for exam questions.

• Distance: The total path length covered by an object, irrespective of direction. It is a scalar quantity (has only magnitude) and is always positive.
• Displacement: The shortest straight-line distance between the initial and final position of an object, along with direction. It is a vector quantity and can be zero, positive, or negative.
• Displacement can never be greater than distance, but distance can be greater than or equal to displacement.

Speed and Velocity

Once we know how to measure distance and displacement, we can describe how fast an object moves using speed and velocity.

• Speed: Distance covered per unit time (Speed = Distance ÷ Time). It is a scalar quantity, measured in m/s.
• Velocity: Displacement per unit time (Velocity = Displacement ÷ Time). It is a vector quantity, as it has both magnitude and direction.
• Uniform motion: An object covers equal distances in equal intervals of time.
• Non-uniform motion: An object covers unequal distances in equal intervals of time.
• Average speed: Total distance travelled divided by total time taken.

Acceleration

Acceleration describes how quickly the velocity of an object changes with time. It is defined as the rate of change of velocity, calculated as Acceleration = (Final velocity − Initial velocity) ÷ Time taken, and is measured in m/s². When the velocity of an object increases, the acceleration is positive; when velocity decreases, the acceleration is negative, also called retardation or deceleration. A car speeding up at a traffic signal and a ball slowing down as it rolls uphill are both everyday examples of acceleration in action.

Graphical Representation of Motion

Motion graphs are one of the most important tools in this chapter, since numerical and graph-based questions are frequently asked in exams.

• Distance–Time Graph: The slope of this graph gives the speed of the object. A straight line indicates uniform speed, while a curved line indicates non-uniform speed.
• Velocity–Time Graph: The slope of this graph gives the acceleration of the object, and the area enclosed under the graph gives the displacement.
• Equations of Motion: For uniformly accelerated motion, three key equations connect velocity, acceleration, time, and displacement: v = u + at, s = ut + ½at², and v² = u² + 2as.

Distance vs Displacement Comparison Table

Real-Life Applications

The concepts of this chapter are used constantly in everyday life. A car's speedometer shows instantaneous speed, while GPS navigation apps like Google Maps calculate both distance and displacement to suggest the fastest route. Athletes and coaches use speed and acceleration data to improve performance in sprints, while pilots and ship captains rely on velocity and direction calculations for accurate navigation. Even something as simple as judging how long it will take to reach school depends on understanding the relationship between speed, distance, and time covered in this chapter.

Frequently Asked Questions

Q1. What is the difference between distance and displacement?
Distance is the total path length covered by an object and is always positive, while displacement is the shortest straight-line distance between the initial and final position and can be zero, positive, or negative.

Q2. What is the difference between speed and velocity?
Speed is distance covered per unit time and is a scalar quantity, while velocity is displacement per unit time and is a vector quantity with a specific direction.

Q3. What does the slope of a distance-time graph represent?
The slope of a distance-time graph represents the speed of the object — a steeper slope means higher speed.

Q4. What does the area under a velocity-time graph represent?
The area enclosed under a velocity-time graph gives the displacement of the object during that time interval.

Q5. What are the three equations of motion?
The three equations of motion for uniformly accelerated motion are v = u + at, s = ut + ½at², and v² = u² + 2as, where u is initial velocity, v is final velocity, a is acceleration, t is time, and s is displacement.

What's Included in This Free Download

• 85-slide PPT covering the complete chapter with diagrams and real-life examples
• Complete notes based on the new NCERT 2026–27 syllabus
• Practice worksheet with important questions

Prepared by Sumeet Sahu, Unique Study Point, Indore. Free for all Class 9 CBSE students.