Physics Laws - Quick Revision
Physics Laws - Quick Revision
Key Points (One-Liners)
- Newton’s 1st Law: A body stays at rest or in uniform motion unless a net force acts.
- Newton’s 2nd Law: F = ma; acceleration ∝ force, ∝ 1/mass.
- Newton’s 3rd Law: Every action has an equal & opposite reaction.
- Law of Gravitation: F = G m₁m₂ / r²; acts along line joining centres.
- Kepler’s 1st Law: Planets orbit the Sun in ellipses, Sun at one focus.
- Kepler’s 2nd Law: Line planet-Sun sweeps equal areas in equal times (conservation of angular momentum).
- Kepler’s 3rd Law: T² ∝ r³; square of time period ∝ cube of mean orbit radius.
- Ohm’s Law: V = IR; current through a conductor ∝ voltage at constant temperature.
- Joule’s Law: H = I²Rt; heat produced ∝ square of current.
- Faraday’s 1st Law: Mass liberated at electrode ∝ charge passed (m = ZIt).
- Lenz Law: Induced current opposes the change producing it (conservation of energy).
- Boyle’s Law: PV = constant (T fixed); pressure ∝ 1/volume.
- Charles’s Law: V/T = constant (P fixed); volume ∝ absolute temperature.
- Archimedes Principle: Up-thrust = weight of fluid displaced; acts vertically upward.
- Snell’s Law: n₁ sin i = n₂ sin r; ratio of sines is constant for given pair of media.
- Law of Reflection: ∠i = ∠r; incident ray, reflected ray & normal lie in same plane.
| Formula/Rule |
Application |
| F = ma |
Calculate force or acceleration in linear motion. |
| F = G m₁m₂ / r² |
Find gravitational force between two masses. |
| V = IR |
Solve circuits: voltage, current or resistance. |
| H = I²Rt |
Compute heat produced by current in a resistor. |
| P = ρgh |
Gauge pressure at depth h in a fluid. |
| v = f λ |
Relate wave speed, frequency & wavelength. |
| s = ut + ½at² |
Kinematics—distance under uniform acceleration. |
| F = k x |
Hooke’s law—force exerted by a stretched spring. |
| η = (output / input) × 100 % |
Efficiency of any machine/heat engine. |
Memory Tricks
- “VIR” – Ohm’s Law triangle: cover V → I×R, cover I → V/R, cover R → V/I.
- “F = ma” – say “FEMA” (sounds like disaster agency) for force-motion link.
- Kepler’s 3rd: “Two squares make a cube” (T² ∝ r³).
- Lenz: “Oppose the change” – think of stubborn person pushing back.
- Archimedes: “Buoyant = Weight of Water Whacked away.”
Common Mistakes
| Mistake |
Correct Approach |
| Using g = 10 m/s² when Q specifies 9.8 |
Read constant value; default 9.8 unless told 10. |
| Forgetting vector nature in F = ma |
Direction matters; use signs consistently. |
| Confusing Lenz with Fleming’s Right-hand rule |
Lenz gives direction of induced current (oppose change); Fleming gives motor/ generator force. |
| Taking pressure in cm of Hg & leaving in mm |
Convert to same unit (Pa or N/m²) before substituting. |
| Mixing up T² ∝ r³ with T ∝ r |
Square the time, cube the radius—write proportionality clearly. |
Last Minute Tips
- Glance at units—most options eliminate if unit mismatch.
- Write data with symbols first; prevents number-copy errors.
- For lens/mirror sums, draw quick ray diagram—sign convention follows automatically.
- In circuit Qs, check if resistors are in series or parallel before adding.
- If two choices look same, re-read Q—often “not” or “except” is hidden.
Quick Practice (5 MCQs)
1. A 2 kg block accelerates at 4 m/s². The net force is—
A) 2 N B) 6 N C) 8 N D) 16 N
**Ans:** C) 8 N (F = ma = 2×4)
2. Planet-X orbits its star at 4 AU. If Earth (1 AU) takes 1 yr, Planet-X’s year is—
A) 2 yr B) 4 yr C) 8 yr D) 16 yr
**Ans:** C) 8 yr (T² ∝ r³ → T ∝ 8)
3. A 5 Ω wire carries 2 A for 10 s. Heat produced is—
A) 20 J B) 50 J C) 100 J D) 200 J
**Ans:** D) 200 J (H = I²Rt = 4×5×10)
4. When a bar magnet is pushed into a coil, the induced current will—
A) attract the magnet B) repel the magnet C) remain zero D) heat the coil only
**Ans:** B) repel the magnet (Lenz law)
5. An object floats with 30 % volume outside a liquid. Relative density of object is—
A) 0.3 B) 0.7 C) 1.0 D) 1.3
**Ans:** B) 0.7 (ρ_body / ρ_liquid = 0.7)
BETA
