Govt. Exams
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Topics in JEE Physics
A force of 10 N acts on a mass for 5 seconds, changing its momentum by:
Correct Answer:
C. 50 kg·m/s
EXPLANATION
By impulse-momentum theorem, Δp = F·Δt = 10 × 5 = 50 kg·m/s
A projectile is launched at 45° with initial velocity 20 m/s. Taking g = 10 m/s², the range is:
Correct Answer:
B. 40 m
EXPLANATION
Range = u²sin(2θ)/g = (400 × sin(90°))/10 = 400/10 = 40 m
A stone is dropped from height h. If air resistance is ignored, what fraction of its total energy is kinetic at height h/2?
Correct Answer:
B. 1/2
EXPLANATION
At h/2, PE = mg(h/2), Total energy = mgh. KE = mgh - mg(h/2) = mg(h/2). Fraction = (h/2)/h = 1/2
Two objects of equal mass collide elastically and exchange velocities. This is possible only if:
Correct Answer:
A. One object was at rest before collision
EXPLANATION
In elastic collision between equal masses, if one is at rest, they exchange velocities completely. This satisfies both momentum and energy conservation.
The coefficient of static friction between a block and surface is 0.5. The maximum angle of incline at which the block remains stationary is approximately:
Correct Answer:
A. 26.6°
EXPLANATION
At maximum angle, tan(θ) = μs = 0.5, so θ = tan⁻¹(0.5) ≈ 26.6°
A car of mass 1000 kg moving at 20 m/s applies brakes with deceleration of 5 m/s². What is the stopping distance?
Correct Answer:
A. 40 m
EXPLANATION
Using v² = u² - 2as, where v = 0, u = 20 m/s, a = 5 m/s². Therefore, s = u²/(2a) = 400/10 = 40 m
A body of mass 5 kg moves with velocity 4 m/s. A force acts on it for 2 seconds and velocity becomes 10 m/s. What is the impulse?
Correct Answer:
B. 30 N·s
EXPLANATION
Impulse = Δ(momentum) = m(v₂ - v₁) = 5(10 - 4) = 30 N·s
Three blocks of masses m, 2m, and 3m are placed on a horizontal surface. A horizontal force F is applied to the first block. What is the acceleration of the system?
Correct Answer:
A. F/6m
EXPLANATION
Total mass = m + 2m + 3m = 6m. Using F = Ma: a = F/(6m)
A rotating disc has angular velocity 10 rad/s. A point at distance 0.2 m from center has linear velocity of:
Correct Answer:
C. 2 m/s
EXPLANATION
v = ωr = 10 × 0.2 = 2 m/s
A pulley system lifts a mass of 100 kg with an efficiency of 80%. If work output is 8000 J, what is work input?
Correct Answer:
C. 10000 J
EXPLANATION
Efficiency = (Work output/Work input) × 100. 80 = (8000/W) × 100 → W = 10000 J
