Chapter – 8

Force and Law of Motion

In this post we have given the detailed notes of class 9 Science Chapter 8 (Force and Law of Motion) in English. These notes are useful for the students who are going to appear in class 9 board exams.

Chapter 8: Force and Laws of Motion

What is Force?

• Force is a push or pull that can change the state of motion or rest of an object. ➡️ ⏏️
• We don’t see or feel force itself, but we see its effects. 🙈
• Force can make a stationary object move, and it can also change the speed, direction, or shape of an object. 💨

Examples of Force:

• Pushing a trolley to make it move. 🛒
• Pulling a drawer to open it. 🗄️
• Hitting a ball with a hockey stick to change its direction. 🏑
• Stretching a spring by applying force. ➡️ 〰️ ⬅️
• Changing the shape of a rubber ball by squeezing it. 🏐

Balanced and Unbalanced Forces

• Balanced forces are equal in magnitude and opposite in direction. ⬅️ ➡️
• When balanced forces act on an object, they do not change the object’s state of motion or rest. 🧘
• Unbalanced forces are not equal in magnitude. ⬅️ ➡️
• When unbalanced forces act on an object, they change the object’s state of motion or rest. 🏃

Friction

• Friction is a force that opposes motion between two surfaces in contact. 🚗
• Friction can cause a moving object to slow down and eventually stop. 🛑

Newton’s First Law of Motion

• An object at rest stays at rest, and an object in motion stays in motion with a constant speed and direction, unless acted upon by an unbalanced force. 😴 ➡️ 🏃
• This is also known as the law of inertia. 🧘
• Inertia is the tendency of an object to resist changes in its state of motion or rest. 🪨
• The more massive an object is, the more inertia it has. 🐘

Examples of Inertia:

• When a car suddenly brakes, passengers tend to move forward due to inertia. 🚗 ➡️ 🧑
• When a car suddenly accelerates, passengers tend to move backward due to inertia. 🚗 ⬅️ 🧑
• When a car turns sharply, passengers tend to move to one side due to inertia. 🚗 ↪️ 🧑

Newton’s Second Law of Motion

• The rate of change of momentum of an object is directly proportional to the applied force and occurs in the direction of the force. 💨  
• Momentum is the product of an object’s mass and velocity. 🏋️
• The formula for force is F = ma, where F is force, m is mass, and a is acceleration. 🧮

Newton’s Third Law of Motion

• For every action, there is an equal and opposite reaction. ⬅️ ➡️
• Action and reaction forces act on different objects. 🎭

Examples of Newton’s Third Law:

• When you walk, you push backward on the ground, and the ground pushes forward on you with an equal and opposite force. 🚶
• When a gun is fired, the gun exerts a forward force on the bullet, and the bullet exerts an equal and opposite force on the gun, causing recoil. 🔫
• When a sailor jumps forward out of a boat, the boat moves backward. ⛵ ⬅️

Remember:

• Force is a vector quantity, meaning it has both magnitude and direction. 🧭
• The SI unit of force is the newton (N).
• The SI unit of momentum is kilogram-meter per second (kg m/s).
• Newton’s laws of motion are fundamental principles that govern the motion of objects in the universe. 🌌

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