Physics MCQ Practice 7 — Science Foundation Course for Class 11 & 12

1. A body of mass 5 kg is moving with velocity 6 m/s. Its momentum is:

(a) 30 kg m/s (b) 11 kg m/s (c) 1.2 kg m/s (d) 0.83 kg m/s

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Correct Answer: (a) 30 kg m/s

Explanation:
Momentum p = mv = 5 × 6 = 30 kg m/s

2. The surface tension of water is 0.072 N/m. The excess pressure inside a soap bubble of radius 1 cm is:

(a) 14.4 Pa (b) 28.8 Pa (c) 7.2 Pa (d) 144 Pa

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Correct Answer: (b) 28.8 Pa

Explanation:
Excess pressure = 4T/r = 4×0.072 / 0.01 = 28.8 Pa

3. The equivalent resistance between A and B in a circuit with two 4 Ω resistors in parallel and that combination in series with 4 Ω is:

(a) 4 Ω (b) 6 Ω (c) 8 Ω (d) 12 Ω

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Correct Answer: (b) 6 Ω

Explanation:
Parallel = 2 Ω, total = 2 + 4 = 6 Ω

4. The first overtone frequency of a closed pipe is 300 Hz. The length of the pipe (v=340 m/s) is:

(a) 0.283 m (b) 0.567 m (c) 1.13 m (d) 0.85 m

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Correct Answer: (a) 0.283 m

Explanation:
First overtone = 3v/(4L) = 300 → L = 3×340/(4×300) = 0.283 m

5. The magnetic field at the centre of a current carrying circular loop of radius r is B. If radius is doubled, magnetic field becomes:

(a) B/2 (b) B/4 (c) 2B (d) 4B

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Correct Answer: (b) B/4

Explanation:
B = μ₀I/(2r) → r doubled → B becomes B/4

6. The power of a lens is +2.5 D. Its focal length is:

(a) 40 cm (b) 25 cm (c) -40 cm (d) 0.4 m

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Correct Answer: (a) 40 cm

Explanation:
f (in m) = 1/P = 1/2.5 = 0.4 m = 40 cm (convex lens)

7. In photoelectric effect, the stopping potential depends on:

(a) Intensity of light (b) Frequency of light (c) Both (d) None

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Correct Answer: (b) Frequency of light

Explanation:
Stopping potential depends on frequency (K_max = hf - φ)

8. The half-life of a radioactive element is 10 days. The fraction that remains after 40 days is:

(a) 1/2 (b) 1/4 (c) 1/8 (d) 1/16

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Correct Answer: (d) 1/16

Explanation:
40 days = 4 half-lives → (1/2)^4 = 1/16

9. The gravitational force between two bodies is F. If distance is doubled, force becomes:

(a) F/2 (b) F/4 (c) 2F (d) 4F

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Correct Answer: (b) F/4

Explanation:
F ∝ 1/r² → r doubled → F becomes F/4

10. The SI unit of magnetic flux is:

(a) Tesla (b) Weber (c) Henry (d) Gauss

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Correct Answer: (b) Weber

Explanation:
Magnetic flux Φ = B · A, unit is Weber (Wb)