In a p-n junction at equilibrium, the potential difference across the junction is called:

The correct answer is B: Built-in potential. The built-in potential (V₀) develops naturally across a p-n junction due to diffusion of charge carriers. For silicon at 300 K, V₀ ≈ 0.7 V, independent of external voltage.

A thin lens of refractive index 1.5 is placed in a medium of refractive index 1.33. If its focal length in air is 20 cm, what is its focal length in the medium?

The correct answer is C: 50.4 cm. Using lensmaker's formula: focal length changes with relative refractive index. f_m/f_a = [(n_lens/n_medium - 1)/(n_lens/n_air - 1)] = [(1.5/1.33 - 1)/(1.5/1 - 1)] = [0.128/0.5] = 0.256. So f_m = f_a/0.256 = 20/0.256 ≈ 78 cm. Recalculating: relative index in medium = 1.5/1.33 ≈ 1.128. f_m = f_a × (1

A particle in circular motion has angular velocity ω = 2t rad/s. If radius is 5 m, what is tangential acceleration at t = 2s?

The correct answer is B: 10 m/s². Tangential acceleration at = r(dω/dt) = 5 × 2 = 10 m/s² (constant as dω/dt = 2)

A conducting rod of length L is moving with velocity v perpendicular to a uniform magnetic field B. If the rod is in electrostatic equilibrium, what is the induced EMF?

The correct answer is A: ε = BvL. Motional EMF in a rod moving perpendicular to magnetic field: ε = BvL. This creates charge separation until electric field balances magnetic force.

In a potentiometer, if the balancing length for a cell is 60 cm and for standard cell is 50 cm, what is the EMF of the unknown cell? (Standard cell = 1.018V)

The correct answer is B: 1.221 V. E/E_std = L/L_std, so E = 1.018 × (60/50) = 1.018 × 1.2 = 1.2216 ≈ 1.221 V

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Mechanics

Physics questions for JEE Main — Mechanics, Electrostatics, Optics, Modern Physics.

50 Q 9 Topics Take Mock Test

Difficulty: All Easy Medium Hard 41–50 of 50

Topics in JEE Physics

All Mechanics 100 Thermodynamics 100 Electrostatics 100 Current Electricity 100 Magnetism 100 Optics 100 Modern Physics 100 Semiconductors 100 Waves 100

Q.41 Medium Mechanics

A particle moves in a straight line with acceleration a = 3t² m/s². If initial velocity is 2 m/s, find velocity after 2 seconds.

A 10 m/s

B 12 m/s

C 14 m/s

D 16 m/s

Correct Answer: A. 10 m/s

EXPLANATION

v = u + ∫a dt = 2 + ∫₀² 3t² dt = 2 + [t³]₀² = 2 + 8 = 10 m/s

Test

Q.42 Medium Mechanics

A particle has position x = 4t² - 2t + 3. Its velocity at t = 1s is:

A 4 m/s

B 6 m/s

C 8 m/s

D 10 m/s

Correct Answer: B. 6 m/s

EXPLANATION

v = dx/dt = 8t - 2. At t = 1s, v = 8(1) - 2 = 6 m/s

Test

Q.43 Medium Mechanics

Work done by centripetal force in circular motion is:

A Positive

B Negative

C Zero

D Depends on radius

Correct Answer: C. Zero

EXPLANATION

Centripetal force is perpendicular to velocity (displacement), so W = F·s·cosθ = F·s·cos90° = 0

Test

Q.44 Medium Mechanics

A 50 kg person stands in an elevator accelerating upward at 2 m/s². Normal force is: (g = 10 m/s²)

A 400 N

B 500 N

C 600 N

D 700 N

Correct Answer: C. 600 N

EXPLANATION

N - mg = ma. N = m(g+a) = 50(10+2) = 600 N

Test

Q.45 Medium Mechanics

A ball thrown upward returns to same height in 4s. Initial velocity is: (g = 10 m/s²)

A 10 m/s

B 15 m/s

C 20 m/s

D 25 m/s

Correct Answer: C. 20 m/s

EXPLANATION

Time to max height = 2s. u = gt = 10(2) = 20 m/s

Test

Q.46 Medium Mechanics

Two blocks (m₁ = 3 kg, m₂ = 2 kg) are connected by a string on a frictionless surface. Applied force F = 20 N. Tension in string is:

A 8 N

B 10 N

C 12 N

D 15 N

Correct Answer: A. 8 N

EXPLANATION

System acceleration: a = F/(m₁+m₂) = 20/5 = 4 m/s². Tension = m₂a = 2(4) = 8 N

Test

Q.47 Medium Mechanics

A rope can withstand maximum tension of 500 N. A mass of 40 kg is suspended and pulled horizontally. The maximum horizontal force is:

A 300 N

B 400 N

C 450 N

D 500 N

Correct Answer: A. 300 N

EXPLANATION

Tension T = √[(mg)² + F²]. 500 = √[(400)² + F²]. F² = 250000 - 160000 = 90000. F = 300 N

Test

Q.48 Medium Mechanics

A 2 kg mass moving at 3 m/s collides elastically with a 1 kg mass at rest. The velocity of 2 kg mass after collision is:

A 1 m/s

B 2 m/s

C 3 m/s

D 4 m/s

Correct Answer: A. 1 m/s

EXPLANATION

For elastic collision: v₁ = [(m₁-m₂)u₁ + 2m₂u₂]/(m₁+m₂) = [(2-1)(3) + 0]/(3) = 1 m/s

Test

Q.49 Medium Mechanics

A projectile is fired at 45° angle with initial velocity 40 m/s. The range is: (g = 10 m/s²)

A 80 m

B 120 m

C 160 m

D 200 m

Correct Answer: C. 160 m

EXPLANATION

Range = u²sin(2θ)/g = (40²)sin(90°)/10 = 1600/10 = 160 m

Test

Q.50 Medium Mechanics

A block of mass 5 kg is placed on a rough surface (μ = 0.2). The maximum static friction is:

A 5 N

B 8 N

C 10 N

D 12 N

Correct Answer: C. 10 N

EXPLANATION

f_max = μ × N = μ × mg = 0.2 × 5 × 10 = 10 N (assuming g = 10 m/s²)

Test

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