Physics Master - Quick Revision
Physics Master - Quick Revision
One-Liners
- Speed = distance / time; Velocity = displacement / time (vector).
- Newton’s 1st: inertia; 2nd: F = ma; 3rd: action-reaction equal-opposite.
- g ≈ 9.8 m/s² downward; weight = mg.
- Work = F·s·cosθ; Power = work / time; 1 hp = 746 W.
- KE = ½mv²; PE = mgh; Total ME = KE + PE (conserved if only gravity acts).
- Impulse = FΔt = Δp; unit kg·m/s.
- Pressure = F / A; 1 atm = 1.01×10⁵ Pa = 76 cm Hg.
- Archimedes: up-thrust = weight of fluid displaced; float when ρbody < ρfluid.
- Boyle: PV = const (T const); Charles: V/T = const (P const).
- PV = nRT; R = 8.314 J/mol·K; 0°C = 273 K.
- Wave speed v = fλ; sound ≈ 343 m/s in air (20°C).
- echo distance = (v × t)/2; t = heard delay.
- Resistance R = ρl/A; 1 Ω = 1 V/A.
- Ohm’s law: V = IR; Power in resistor: P = I²R = V²/R.
- Series: Req = ΣR; Parallel: 1/Req = Σ(1/R).
- Flemings: left-hand motor, right-hand generator.
- Transformer: Vp/Vs = Np/Ns = Is/Ip; η = Ps/Pp.
- Refractive index n = c/v; Snell: n₁sinθ₁ = n₂sinθ₂.
- Mirror: 1/f = 1/v + 1/u; f = R/2; lens convention same.
- e = 1.6×10⁻¹⁹ C; h = 6.63×10⁻³⁴ J·s; c = 3×10⁸ m/s.
| Formula | Use |
|---|
| v = u + at | Uniform acceleration; find final velocity. |
| s = ut + ½at² | Displacement under constant acceleration. |
| F = G m₁m₂ / r² | Universal gravitation; weight on other planets. |
| τ = r × F = Iα | Torque & angular acceleration (rigid body). |
| ρ = m/V | Density; float/sink predictions. |
| η = (F/A)/(dv/dx) | Coefficient of viscosity; fluid flow. |
| v = √(T/μ) | Speed of transverse wave on string (μ = mass/length). |
| β = 10 log(I/I₀) | Sound level in decibels (I₀ = 10⁻¹² W/m²). |
| Φ = BA cosθ | Magnetic flux; Faraday’s law EMI: ε = –dΦ/dt. |
| 1/λ = R(1/n₁² – 1/n₂²) | Rydberg formula; hydrogen spectral lines. |
Memory Tricks
- RGB to VIBGYOR: “Richard Of York Gave Battle In Vain” – spectrum order.
- Fleming rules: Left-hand thrust (motor) → “Motor-bike left home”; Right-hand grip (generator) → “Right returns current”.
- Ohm’s law pie-chart: draw circle, write V=IR inside; cover the one you need → triangle gives relation.
- Lens sign convention: “REAL is Positive” (for real image & convex focal length); virtual & concave negative.
- Planets order: “My Very Educated Mother Just Served Us Noodles” – Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune.
Common Errors
| Error | Correct |
|---|
| Using g = 10 m/s² when question gives 9.8 m/s². | Stick to the value supplied; if not, default 9.8. |
| Confusing average speed with average velocity in round-trip problems. | Velocity can be zero if displacement is zero; speed is never zero. |
| Adding resistors in parallel as R₁ + R₂. | Use 1/R = 1/R₁ + 1/R₂; equivalent R is smaller. |
| Writing image distance +ve for virtual images in mirrors. | Apply sign convention: virtual image → v negative. |
| Taking angle of incidence from normal in Snell’s but drawing it from surface. | Always measure angles between ray and normal. |
5 Quick MCQs
Question 1
A 2 kg stone is dropped from rest; find speed after falling 20 m (g = 10 m/s²).
A) 10 m/s B) 14.1 m/s C) 20 m/s D) 40 m/sAnswer
C) 20 m/s (v² = 2gh)Question 2
An echo is heard after 1 s. If speed of sound is 340 m/s, distance to the wall is:
A) 85 m B) 170 m C) 340 m D) 680 mAnswer
B) 170 m (distance = v × t / 2)Question 3
Power of a 2 kW heater operating 30 min/day for 10 days (in kWh) is:
A) 60 B) 10 C) 20 D) 600Answer
B) 10 kWh (2 kW × 0.5 h × 10)Question 4
Refractive index of glass is 1.5. Speed of light in glass is nearest:
A) 1×10⁸ m/s B) 2×10⁸ m/s C) 2.5×10⁸ m/s D) 3×10⁸ m/sAnswer
B) 2×10⁸ m/s (v = c/n)Question 5
A convex lens has f = 20 cm. Object placed at 30 cm; image position is:
A) 60 cm B) 15 cm C) 12 cm D) 30 cmAnswer
A) 60 cm (1/v = 1/f – 1/u)
BETA
