Class IX :: Chapter 9:: FORCE AND LAWS OF MOTION
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- Force: A push or pull that can change the state of motion or shape of an object. SI unit is Newton (N).
- Balanced vs. Unbalanced Forces: Balanced forces cause no change in motion; unbalanced forces cause acceleration.
- First Law (Inertia): An object remains at rest or in uniform motion unless acted upon by an external unbalanced force.
- Inertia & Mass: Inertia is the resistance to change in motion. Mass is the quantitative measure of inertia.
- Second Law: F = ma (Force = mass × acceleration). Also defines momentum: p = mv.
- Momentum: p = m × v. It is a vector quantity with direction same as velocity. SI unit: kg m/s.
- Second Law (Momentum form): Force = Rate of change of momentum (F = (mv - mu)/t).
- Third Law: To every action, there is an equal and opposite reaction, acting on two different bodies.
- Conservation of Momentum: In an isolated system (no external force), total momentum before collision = total momentum after collision.
- Practical Applications: Catching a cricket ball, recoil of a gun, rocket propulsion, and swimming are based on these laws.
Basic Level Questions
Chapter Summary
What You Will Learn: Chapter 9, "Force and Laws of Motion," introduces you to the fundamental principles that govern how and why objects move. You will transition from simply describing motion (from Chapter 8) to understanding its causes. The chapter builds a strong conceptual foundation by exploring Newton's Three Laws, the concepts of inertia and momentum, and the powerful principle of momentum conservation.
Core Concepts & Exam Utility: Mastering this chapter is crucial for solving numerical problems on force, acceleration, and collisions, which are staples in board exams and competitive tests like NTSE and Olympiads. You will learn to calculate force from acceleration, determine recoil velocity, solve collision problems using momentum conservation, and apply these laws to real-world scenarios.
How This Website Helps: This platform solidifies your understanding by providing a curated set of high-quality, NCERT-standard Multiple Choice Questions (MCQs). By practicing these targeted questions, you can self-assess your grasp of key definitions (like inertia), formula applications (F=ma, p=mv), and problem-solving skills for numerical challenges. This focused practice is designed to boost your confidence and exam readiness, turning theoretical knowledge into scoring ability.