⚛️Physics
Laws of Motion
Newton's laws, friction, circular motion, free body diagrams, and applications in real-world problems.
Class 11Class 12
What You Will Learn in Laws of Motion
Newton's laws of motion describe the relationship between forces acting on a body and its motion — forming the foundation of classical mechanics.
- 1st Law (Inertia): a body at rest/uniform motion continues unless acted on by net external force.
- 2nd Law: F = ma (net force = mass × acceleration); vector equation.
- 3rd Law: every action has an equal and opposite reaction (forces act on different bodies).
- Friction: f = μN (μ_s > μ_k); friction is a contact force opposing relative motion.
- Free body diagram: isolate one body, mark all forces — weight, normal, tension, friction.
- For connected bodies (Atwood): acceleration a = (m₁-m₂)g/(m₁+m₂); T = 2m₁m₂g/(m₁+m₂).
Key Formulas
F = maf = μNFor incline: a = g(sinθ - μcosθ)Atwood: a = (m₁-m₂)g/(m₁+m₂)
Example
A 5 kg block on a floor with μ=0.3 requires F=μmg=0.3×5×10=15 N to start moving.
Solved Numerical Example
Two masses m₁=3 kg, m₂=2 kg connected by string over frictionless pulley. a = (3-2)×10/(3+2) = 2 m/s². T = 2×3×2×10/(3+2) = 24 N.
Expected Exam Questions — Laws of Motion
Q1.A 2 kg block rests on a surface with μs=0.4 and μk=0.3. Find minimum force to start motion and force to maintain uniform motion. (g=10 m/s²)
Answer: N = mg = 20 N. To start: F ≥ μs×N = 0.4×20 = 8 N. To maintain: F = μk×N = 0.3×20 = 6 N.
Q2.Why does a gun recoil when fired? Which law explains this?
Answer: Newton's 3rd law: when bullet is fired with force F forward, gun receives equal and opposite reaction force backward (recoil). Momentum conservation: m_bullet × v_bullet = m_gun × v_recoil (initial momentum = 0).
🔘 MCQ Practice — Laws of Motion
MCQ 1.A body of mass 5 kg is acted upon by force F = 20N. The acceleration is:
A. 4 m/s² ✓
B. 100 m/s²
C. 0.25 m/s²
D. 2.5 m/s²
✓ Correct Answer: 4 m/s²
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