Chapter – 7

Motion

In this post we have given the detailed notes of class 9 Science Chapter 7 (Motion) in English. These notes are useful for the students who are going to appear in class 9 board exams.

Printed Books Are Available Now!

Chapter 7: Motion

7.1 Describing Motion 🚶‍♀️

Reference Point: 📍

• To describe the location of an object, we need a reference point, also known as the origin.  
• For example, if we say a school is 2 km north of the railway station, the railway station is the reference point.  

Distance: 📏

• Distance is the total path length covered by an object.  
• It is a scalar quantity, meaning it only has magnitude and no direction.  
• Example: If an object moves from point A to point B and then back to point A, the distance traveled is the total length of the path covered. 왔다갔다

Displacement: ➡️

• Displacement is the shortest distance between the initial and final position of an object.  
• It is a vector quantity, meaning it has both magnitude and direction.  
• Example: If an object moves from point A to point B and then back to point A, its displacement is zero because the initial and final positions are the same. 왔다갔다

Uniform Motion: 🚗

• An object is said to be in uniform motion if it covers equal distances in equal intervals of time.  
• Example: A car moving at a constant speed of 60 km/h. 🚗

Non-Uniform Motion: 🚕

• An object is said to be in non-uniform motion if it covers unequal distances in equal intervals of time.  
• Example: A car accelerating or decelerating. 🚕

7.2 Measuring the Rate of Motion ⏱️

Speed: 🚄

• Speed is the distance traveled by an object per unit of time.  
• It is a scalar quantity.  
• SI unit: meters per second (m/s)  
• Other units: centimeters per second (cm/s), kilometers per hour (km/h)  

Average Speed: 🚄

• Average speed is the total distance traveled divided by the total time taken.  
• Formula: Average speed = Total distance traveled / Total time taken  

Velocity: 🚄

• Velocity is the speed of an object in a given direction.  
• It is a vector quantity.  
• SI unit: meters per second (m/s)  

Average Velocity: 🚄

• Average velocity is the displacement divided by the total time taken.  
• Formula: Average velocity = Displacement / Total time taken  
• For uniformly accelerated motion, average velocity is the arithmetic mean of initial and final velocity.  
• Formula: Average velocity = (Initial velocity + Final velocity) / 2  

7.3 Rate of Change of Velocity ⏩

Acceleration: 🚀

• Acceleration is the rate of change of velocity per unit of time.  
• It is a vector quantity.  
• SI unit: meters per second squared (m/s²)  
• Formula: Acceleration = (Final velocity – Initial velocity) / Time taken  
• Positive acceleration: Acceleration in the direction of velocity.  
• Negative acceleration (retardation): Acceleration opposite to the direction of velocity.  

Uniform Acceleration: 🚀

• An object has uniform acceleration if its velocity changes by equal amounts in equal intervals of time.  
• Example: Motion of a freely falling object. 🍏

Non-Uniform Acceleration: 🚀

• An object has non-uniform acceleration if its velocity changes by unequal amounts in equal intervals of time.  
• Example: A car increasing its speed by varying amounts in each interval of time. 🚗

7.4 Graphical Representation of Motion 📈

Distance-Time Graphs: 📈

• Time is taken on the x-axis and distance on the y-axis.  
• For uniform motion, the graph is a straight line.  
• For non-uniform motion, the graph is a curved line.  
• The slope of the graph represents the speed of the object.  

Velocity-Time Graphs: 📈

• Time is taken on the x-axis and velocity on the y-axis.  
• For uniform motion, the graph is a straight line parallel to the time axis.  
• For uniformly accelerated motion, the graph is a straight line with a slope.  
• The area under the velocity-time graph represents the displacement of the object.  

7.5 Equations of Motion 🧮

Equations:

• v = u + at  
• s = ut + (1/2)at²  
• 2as = v² – u²  
• Where:
  • u = initial velocity  
  • v = final velocity  
  • a = acceleration  
  • t = time  
  • s = distance traveled  

7.6 Uniform Circular Motion 💫

Uniform Circular Motion:

• An object moving in a circular path with uniform speed is said to be in uniform circular motion.  
• The velocity of the object is constantly changing because its direction is changing.  
• Examples: Motion of the moon around the earth, a satellite in circular orbit, a cyclist on a circular track at constant speed. 🌕 🛰️ 🚴

Important Points:

• The object in uniform circular motion is accelerating even though its speed is constant.  
• The acceleration is directed towards the center of the circle and is called centripetal acceleration.  

Examples:

• An athlete running on a circular track. 🏃‍♀️
• A stone tied to a thread and whirled around. ගල්
• An athlete throwing a hammer or discus. 🔨

We hope that class 9 Science Chapter 7 (Motion) notes in English helped you. If you have any query about class 9 Science Chapter 7 (Motion) notes in English or about any other notes of class 9 Science in English, so you can comment below. We will reach you as soon as possible…