JEE Physics — Important Questions & Concepts
Mechanics
Q: A ball is thrown vertically upward with velocity 20 m/s. Find max height.
Using v² = u² - 2gh (decelerating under gravity)
At max height v = 0:
0 = (20)² - 2(10)h
h = 400/20 = 20 m
Time to reach max height: v = u - gt → 0 = 20 - 10t → t = 2s
Q: A 5kg block on frictionless surface, force 20N. Acceleration?
F = ma → 20 = 5 × a → a = 4 m/s²
With friction (μ=0.2): F_net = F - μmg = 20 - 0.2×5×10 = 10N
a = 10/5 = 2 m/s²
Q: Work done by variable force F = 3x² + 2 from x=1 to x=3
W = ∫₁³ F·dx = ∫₁³ (3x² + 2)dx = [x³ + 2x]₁³ = (27+6) - (1+2) = 33 - 3 = 30 J
Electricity
Q: Three resistors 2Ω, 3Ω, 6Ω in parallel. Find equivalent resistance.
1/R = 1/2 + 1/3 + 1/6 = 3/6 + 2/6 + 1/6 = 6/6 = 1
R = 1Ω
Quick check: parallel R always less than smallest (smallest = 2Ω → R=1Ω ✓)
Q: Capacitors 4μF and 6μF in series. Find equivalent capacitance.
1/C = 1/4 + 1/6 = 3/12 + 2/12 = 5/12
C = 12/5 = 2.4μF
Note: Series capacitors = less capacitance (opposite to resistors)
Waves & Optics
Q: Speed of light in glass (n=1.5)?
n = c/v → v = c/n = 3×10⁸/1.5 = 2×10⁸ m/s
Q: Young's double slit: d=0.5mm, D=1m, λ=600nm. Fringe width?
β = λD/d = (600×10⁻⁹ × 1) / (0.5×10⁻³)
= 600×10⁻⁹ / 5×10⁻⁴ = 1.2×10⁻³ m = 1.2 mm
Revision Notes
KINEMATICS equations (constant acceleration):
v = u + at
s = ut + ½at²
v² = u² + 2as
s_nth = u + a(2n-1)/2
ENERGY:
KE = ½mv² | PE = mgh | W = Fd·cosθ
Work-energy theorem: W_net = ΔKE
ELECTRICITY:
Series R: R = R₁+R₂+... | Series C: 1/C = 1/C₁+1/C₂+...
Parallel R: 1/R = 1/R₁+1/R₂+... | Parallel C: C = C₁+C₂+...
Ohm's law: V=IR | Power: P=VI=I²R=V²/R
OPTICS:
n = c/v | Snell's law: n₁sinθ₁ = n₂sinθ₂
Mirror: 1/f = 1/v + 1/u | Lens: 1/f = 1/v - 1/u
Fringe width β = λD/d (Young's)
