NCERT Solutions for Class 9 Science Chapter 8 Force and Laws of Motion
NCERT Solutions for Class 9 Science Chapter 8 Force and Laws of Motion covers balanced and unbalanced forces, Newton's first, second, and third laws of motion, inertia, mass, linear momentum, and conservation of momentum.
Overview of Topics & Subtopics (Click to Jump)
1. Balanced and Unbalanced Forces
Short Note:
• Force: A push or pull upon an object resulting from its interaction with another object. SI Unit: Newton (N).
• Balanced Forces: When the resultant of all forces acting on a body is zero. They do not change the state of rest or motion of an object, but can change its shape.
• Unbalanced Forces: When the resultant of all forces acting on a body is non-zero. They cause a change in speed or direction of motion (i.e., produce acceleration).
2. First Law of Motion & Inertia
Short Note:
• Newton's First Law of Motion: An object continues in its state of rest or uniform motion along a straight line unless acted upon by an unbalanced external force. Also known as the Law of Inertia.
• Inertia: The natural tendency of an object to resist changes in its state of rest or uniform motion.
- Inertia of Rest: Resistance to move from rest (e.g., falling backward when a bus starts suddenly).
- Inertia of Motion: Resistance to stop moving (e.g., falling forward when a moving bus stops suddenly).
• Mass and Inertia: Mass is the quantitative measure of inertia. Heavier objects have higher inertia.
3. Second Law of Motion & Momentum
Short Note:
• Linear Momentum (p): Product of mass (m) and velocity (v) of an object. p = m × v. It is a vector quantity with SI unit kg·m/s.
• Newton's Second Law of Motion: The rate of change of momentum of an object is directly proportional to the applied unbalanced force in the direction of the force.
- Mathematical Formula: F = m × a (Force = Mass × Acceleration).
- 1 Newton is the force that produces an acceleration of 1 m/s² in a mass of 1 kg.
4. Third Law of Motion & Conservation of Momentum
Short Note:
• Newton's Third Law of Motion: To every action, there is an equal and opposite reaction, and they act on two different bodies simultaneously.
- Example: Recoil of a gun, walking on the ground, rocket propulsion.
• Law of Conservation of Momentum: In an isolated system (where no external unbalanced force acts), the total momentum before collision equals the total momentum after collision.
- Formula: m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂
Intext Questions - Page Number: 85
(a) A stone of the same size has more mass and hence more inertia than a rubber ball.
(b) A train has much greater mass and hence more inertia than a bicycle.
(c) A five-rupees coin has more mass and hence more inertia than a one-rupee coin.
• When the bus brakes: The lower part of our body comes to rest with the bus, but the upper part continues in motion due to the inertia of motion, causing us to fall forward.
• When the bus accelerates: The lower part moves forward with the bus, but the upper part tends to stay at rest due to the inertia of rest, causing us to fall backward.
Textbook Chapter End Exercise Questions
1. It was already in motion initially.
2. The magnitude and direction of the velocity remain completely unchanged.
3. Resistance/frictional forces are zero or completely balanced.
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.