JEE Mains Physics 2027 — Laws of Motion: Complete Chapter Notes and Practice MCQs
Laws of Motion is one of the most fundamental and high-weightage chapters in JEE Mains Physics. Appearing in 2-4 questions every year, mastery of Newton’s laws, friction, and circular motion is essential for a good JEE rank. This guide covers complete chapter notes with all key concepts and 10 practice MCQs for JEE Mains 2027.
Newton’s Three Laws of Motion
| Law | Statement | Mathematical Form | Key Concept |
|---|---|---|---|
| First Law (Inertia) | Body at rest stays at rest; body in motion stays in motion unless acted upon by net force | F_net = 0 implies a = 0 | Inertia, pseudo forces |
| Second Law | Rate of change of momentum equals net applied force | F = ma (or F = dp/dt) | Impulse = F*t = delta-p |
| Third Law | Every action has an equal and opposite reaction | F_AB = -F_BA | Action-reaction pairs act on different bodies |
Types of Forces and Free Body Diagrams
Normal Force (N): perpendicular contact force. On horizontal surface: N = mg. On incline: N = mg cos(theta). Tension (T): force in a string. For Atwood machine with masses m1 and m2: T = 2m1m2g/(m1+m2), acceleration a = (m2-m1)g/(m1+m2). Free Body Diagram (FBD): diagram showing all forces on a single body — essential technique for JEE problems.
Friction — Static and Kinetic
| Type | Formula | When Active | JEE Tip |
|---|---|---|---|
| Static Friction | f_s less than or equal to mu_s * N | Body at rest, tending to slide | Can vary from 0 to mu_s*N |
| Kinetic Friction | f_k = mu_k * N | Body in motion | Always constant, mu_k less than mu_s |
| Rolling Friction | Very small | Rolling motion | Usually neglected in JEE |
Motion on Inclined Plane
For a block on incline of angle theta: Normal force N = mg cos(theta). Component along incline = mg sin(theta). Friction force = mu*mg cos(theta). Net acceleration going down the incline = g(sin(theta) – mu*cos(theta)). Angle of repose (theta_r): at this angle, tan(theta_r) = mu. Body just starts sliding at incline greater than theta_r.
Circular Motion and Centripetal Force
- Centripetal acceleration = v^2/r = omega^2 * r (directed towards centre)
- Centripetal force = mv^2/r (provided by friction, tension, gravity, normal force)
- Car on flat circular road: f = mv^2/r, maximum speed = sqrt(mu*g*r)
- Car on banked road: tan(theta) = v^2/(rg) for ideal banking (no friction)
- Vertical circular motion: minimum speed at top = sqrt(gR) (for looping)
- At top of loop: N + mg = mv^2/R; At bottom: N – mg = mv^2/R
Frequently Asked Questions — JEE Mains Laws of Motion 2027
How many questions come from Laws of Motion in JEE Mains?
Laws of Motion typically contributes 2-4 questions in JEE Mains Physics. Friction problems, Atwood machine, and inclined plane with friction are the most commonly tested topics. Free body diagram technique is essential for solving all types.
What is the angle of repose?
Angle of repose is the maximum angle of an inclined plane at which a body placed on it is just on the verge of sliding down. At this angle, tan(theta_r) = mu_s (coefficient of static friction). If the incline is steeper than theta_r, the body slides.
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