Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
Laws of Motion explains why objects move the way they do. Newton's three laws, friction, and circular motion are core topics that appear in every competitive exam. NEET typically has 1–2 questions from this chapter annually.
Newton's Laws of Motion
First Law — Law of Inertia
A body continues in its state of rest or uniform motion in a straight line unless acted upon by a net external force.
Inertia: Resistance to change in state of motion. Inertia ∝ mass.
Second Law — Law of Force
Net force = rate of change of momentum
F = ma (when mass is constant)
F = dp/dt (general form, where p = mv)
Impulse = F × t = change in momentum = Δp
Third Law — Action and Reaction
For every action force there is an equal and opposite reaction force acting on a different body.
- Forces always occur in pairs
- Action and reaction forces never cancel each other (they act on different bodies)
Free Body Diagrams
A free body diagram shows all forces acting on a single object. Steps: 1. Isolate the object 2. Draw all forces: weight (mg downward), normal reaction (N), tension (T), friction (f), applied force (F) 3. Apply Newton's second law in each direction
Friction
| Type | When | Formula |
|---|---|---|
| Static friction | Body at rest | f_s ≤ μ_s N |
| Kinetic (sliding) friction | Body moving | f_k = μ_k N |
| Rolling friction | Body rolling | Much less than sliding |
- μ_s > μ_k always
- Angle of friction (λ): tan λ = μ
- Angle of repose: Angle at which body just starts to slide: tan θ = μ_s
Circular Motion
A body in circular motion has centripetal acceleration directed toward the centre.
- Centripetal acceleration: a_c = v²/r = ω²r
- Centripetal force: F_c = mv²/r (directed inward)
Applications
| Situation | Source of centripetal force |
|---|---|
| Car on a flat curve | Friction between tyres and road |
| Car on a banked curve | Component of normal reaction |
| Conical pendulum | Component of tension |
| Satellite in orbit | Gravitational force |
Banking angle for frictionless road: tan θ = v²/rg
Maximum speed on flat road: v_max = √(μrg)
Important Questions
One Mark
- State Newton's first law of motion.
- Define limiting friction.
- What is the SI unit of impulse?
- Define centripetal acceleration.
- Write the formula for centripetal force.
Short Answer (3 marks)
- Explain Newton's second law. Show how it leads to F = ma.
- Distinguish between static and kinetic friction. State factors affecting friction.
- Derive the banking angle formula for a vehicle on a curved road.
Long Answer (5 marks)
- State and explain Newton's three laws with examples. Show how the third law applies to rocket propulsion.
- Explain the motion of a car on (a) a flat curve and (b) a banked curve. Derive the expression for maximum speed in each case.
Common Mistakes
- Applying Newton's third law to forces on the same object — action-reaction pairs act on different objects.
- Forgetting that friction force direction is always opposite to the direction of motion (or intended motion).
- Using centripetal force as an additional outward force — it is an inward force requirement, not a new force.
Board Exam Tips
- Draw free body diagrams for every numerical problem — they eliminate errors.
- Memorise μ_s > μ_k and the implication for starting vs. stopping a sliding object.
- NEET: Expect numerical problems involving friction on inclines and circular motion.