Laws of Motion and Friction
🟢 Lite — Quick Review (1h–1d)
Rapid summary for last-minute revision before your exam.
Laws of Motion and Friction — Key Facts for UPPSC PCS Core concept: Newton’s three laws form the foundation of classical mechanics; friction opposes relative motion between surfaces High-yield point: F = ma (Second Law) is the most frequently applied formula in UPPSC physics questions ⚡ Exam tip: Questions on friction (static vs kinetic) and momentum conservation appear almost every year in UPPSC Prelims
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Laws of Motion and Friction — UPPSC PCS Study Guide
Newton’s First Law (Law of Inertia)
- A body at rest stays at rest, and a body in motion stays in motion unless acted upon by an external force
- Inertia: the tendency of a body to resist changes in its state of motion
- Mass is the measure of inertia
Newton’s Second Law
- Rate of change of momentum is proportional to the applied force: F = ma
- Momentum = mass × velocity
- 1 Newton = force required to accelerate 1 kg mass by 1 m/s²
Newton’s Third Law
- Every action has an equal and opposite reaction
- Action and reaction act on different bodies — they do not cancel each other
Conservation of Momentum
- In absence of external force, total momentum of a system remains constant
- Useful for collision and recoil problems
Friction
- Friction opposes the relative motion of surfaces in contact
- Static friction (fₛ): prevents motion up to a maximum value fₛ(max) = μₛ × N
- Kinetic friction (fₖ): acts during motion = μₖ × N
- Rolling friction is much less than sliding friction
- μₛ > μₖ always
- Applications: brakes, walking, tires
Key Formulas
- F = ma
- f = μN
- Momentum p = mv
- Impulse = Ft = Δp
Common UPPSC Question Patterns
- Numerical on F = ma and friction
- Concept-based questions on Newton’s laws
- Friction direction in inclined planes
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Laws of Motion and Friction — Comprehensive UPPSC PCS Notes
Detailed Theory
Newton’s Laws — Deeper Understanding
- First Law defines force as the cause of change in state of motion
- Second Law is the most fundamental — gives quantitative measure of force
- Third Law explains rocket propulsion, recoil of guns, walking
Momentum and Impulse
- Impulse-J theorem: Impulse = change in momentum
- Area under F-t graph gives impulse
- In elastic collision: momentum and kinetic energy both conserved
- In inelastic collision: only momentum conserved
Friction — Detailed Analysis
- Causes of friction: roughness of surfaces, molecular adhesion, interlocking of surface irregularities
- Coefficient of friction (μ) is dimensionless
- Angle of friction (θ): tan θ = μ
- For inclined plane: component of weight parallel to plane = mg sinθ; normal reaction = mg cosθ
- Body starts sliding when tanθ ≥ μₛ
Circular Motion
- Centripetal force: F = mv²/r
- Centrifugal force is a pseudo-force in rotating frames
- Banking of roads: tanθ = v²/rg — reduces dependence on friction
Free Body Diagrams (FBD)
- Essential for solving problems with multiple forces
- Draw all forces on the body, resolve components, apply Newton’s laws
Problem-Solving Strategy
- Draw FBD for each body
- Apply Newton’s second law along each axis
- Use constraint relations (e.g., same acceleration for connected bodies)
- Check limiting cases (impending motion, just slipping)
Common Mistakes to Avoid
- Confusing action-reaction pairs (they act on different bodies)
- Using kinetic friction coefficient where static is needed
- Forgetting that friction can act up or down an incline depending on direction of impending motion
Previous Year UPPSC Focus
- 2019 Prelims: Question on impulse-momentum theorem
- 2018 Prelims: Friction on inclined plane calculation
- Frequent: Newton’s second law numericals with two blocks connected by string