Laws of Motion
Physics 11 - Chapter 05
Practice 100+ MCQs on Laws of Motion. Includes Basic, Intermediate, Advanced questions with detailed explanations and Mock Tests.
Prepare Newton's laws, inertia, momentum, impulse, friction, and dynamics of circular motion for
Success in Class 11 Physics & Competitive Exams.
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- Newton's First Law (Law of Inertia): A body at rest remains at rest, and a body in motion continues in uniform motion unless acted upon by an external unbalanced force.
- Newton's Second Law: F = ma (Force = mass × acceleration). Also expressed as F = dp/dt (Force = rate of change of momentum).
- Newton's Third Law: For every action, there is an equal and opposite reaction. Action-reaction pairs act on different bodies.
- Inertia: The natural tendency of a body to resist any change in its state of rest or uniform motion.
- Momentum (p): p = mv (mass × velocity). SI unit: kg·m/s.
- Impulse: J = FΔt = Δp (Change in momentum). SI unit: N·s.
- Law of Conservation of Momentum: If no external force acts on a system, the total momentum remains constant.
- Friction: Force that opposes relative motion between surfaces in contact.
- Static Friction: f ≤ μₛN (up to limiting value)
- Kinetic Friction: f = μₖN (constant during motion)
- Rolling Friction: Least among all types
- Normal Force (N): Perpendicular contact force exerted by a surface on an object.
- Tension: Force transmitted through a string, rope, or cable when pulled.
- Free Body Diagram (FBD): Essential tool showing all forces acting on a single body.
- Apparent Weight:
- Lift at rest/constant velocity: W = mg
- Lift accelerating upward: W = m(g + a)
- Lift accelerating downward: W = m(g - a)
- Lift in free fall: W = 0 (weightlessness)
- Equilibrium: Net force = 0, Net torque = 0. Body either at rest or moving with constant velocity.
- Circular Motion: Requires centripetal force F = mv²/r = mω²r towards center.
- Banking of Roads: tanθ = v²/rg for no friction case, where θ is banking angle.
- Motion on Inclined Plane:
- Component parallel to incline: mg sinθ
- Component perpendicular to incline: mg cosθ
- Connected Systems: Use constraint equations and apply Newton's laws to each mass separately.
Basic Level Questions
Chapter Summary
This chapter introduces us to the fundamental principles that govern the motion of objects - Newton's Three Laws of Motion. These laws form the cornerstone of classical mechanics and provide the framework for understanding how forces affect motion. The first law introduces the concept of inertia, the second law quantifies the relationship between force, mass, and acceleration, while the third law explains the nature of force pairs.
We explore various types of forces including friction (static, kinetic, and rolling), tension in strings, normal force, and gravitational force. The chapter emphasizes the importance of Free Body Diagrams (FBDs) as a crucial problem-solving tool to visualize all forces acting on a body. The concept of momentum and impulse helps us understand collisions and the effect of forces over time, leading to the powerful Law of Conservation of Momentum.
Practical applications covered include motion on inclined planes, connected systems (like Atwood machine), circular motion requiring centripetal force, and the interesting case of apparent weight in accelerating lifts. Mastering this chapter is essential as it provides the foundation for understanding more complex physical phenomena in higher studies of physics and engineering. Remember to always identify the system, draw proper FBDs, apply Newton's laws correctly, and check for conservation principles when solving problems.