Laws of Motion for NEET Physics
Master Newton's laws, inertia, momentum, friction, and dynamics of systems — the core of mechanics, explained for NEET aspirants.
🔍 Detailed concept explanation
⚡ 1. Newton's First Law (Law of Inertia)
Every body continues in its state of rest or uniform motion unless acted upon by an external net force. Inertia is the property of mass to resist change in motion. Mass is a measure of inertia.
📈 2. Newton's Second Law
The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force. Mathematically: F = dp/dt = ma (for constant mass). This is the fundamental equation of motion.
🔄 3. Newton's Third Law
To every action, there is an equal and opposite reaction. Forces always occur in pairs acting on two different bodies. Action-reaction pairs do not cancel each other because they act on different objects.
📦 4. Free Body Diagram (FBD)
A diagram showing all forces acting on a body. Crucial for solving problems: isolate the body, identify all forces (gravity, normal, tension, friction, applied forces), and resolve into components.
🧱 5. Friction
Force that opposes relative motion. Static friction (fs ≤ μsN) prevents motion, kinetic friction (fk = μkN) acts during motion. μs > μk. Angle of friction = tan⁻¹(μ).
⛓️ 6. Dynamics of connected systems
For multiple masses connected by strings/pulleys, draw FBD for each mass, apply F = ma, and solve simultaneous equations. Tension in massless strings is constant over ideal pulleys.
📋 Complete formula sheet
(vector form: ΣF = ma)
Impulse I = Δp = FavgΔt
fk = μkN
| Quantity | Formula / Relation |
|---|---|
| Weight | W = mg |
| Normal force on incline | N = mg cosθ |
| Acceleration on rough incline | a = g(sinθ – μkcosθ) |
| Angle of repose | θ = tan⁻¹(μs) |
| Minimum force to move block | Fmin = μsmg / √(1+μs²) (pulling at optimal angle) |
✏️ Solved NEET-level examples
Example 1 (Second law / FBD)
A 5 kg block is pulled on a horizontal surface with a force of 20 N at 30° above horizontal. If μk = 0.2, find acceleration. (g = 10 m/s²)
Example 2 (Connected masses)
Two masses 4 kg and 2 kg are connected by a string over a frictionless pulley. Find acceleration and tension. (g = 10 m/s²)
Example 3 (Friction on incline)
A 2 kg block slides down a 37° incline with μk = 0.3. Find acceleration and time to slide 4 m from rest. (g = 10 m/s², sin37=0.6, cos37=0.8)
📈 Important graphs & key points
- F vs a : straight line slope = mass
- Friction vs applied force : static region linear up to max, then kinetic constant
- Tension in rope over pulley : same on both sides if massless & frictionless
- Spring force : F = –kx (restoring)
⭐ Always draw FBD first. For systems, choose consistent sign convention (direction of acceleration).
⚡ Quick revision box
⚠️ Common mistakes to avoid
- Forgetting that action-reaction pairs act on different bodies, so they don't cancel in same FBD.
- Using f = μsN as actual force before reaching maximum static friction.
- Incorrect normal force on incline (not always mg; if other vertical forces exist, adjust).
- Assuming tension is same in strings when pulley has mass or friction.
- Not resolving forces into components along acceleration direction.
🧠 Exam strategy tips
- Always start with a clean FBD – label all forces.
- Pick a coordinate system (usually along acceleration for one axis, perpendicular for the other).
- For multi-body problems, write separate equations for each body and solve.
- Remember: static friction adjusts up to μsN; if 'just about to move' implies maximum static.
- Practice pulley and wedge problems – they are NEET favorites.
❓ Frequently asked questions
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⚡ Master Laws of Motion – the gateway to solving mechanics problems. Bookmark this page for last‑minute revision.