Q90. Which is the SI unit of specific heat capacity?
[MP Police Sub-Inspector – 21 January 2026]
A. Calorie per gram-kelvin
B. Joule per kelvin
C. Joule per kilogram-kelvin
D. Newton per kilogram-kelvin
Answer: C. Joule per kilogram-kelvin
Explanation: Specific heat capacity is energy required to raise unit mass of a substance by one kelvin. Dividing joules by kilograms and kelvins yields J kg⁻¹ K⁻¹. Joules per kelvin alone refers to heat capacity, not specific heat capacity.
Q89. A lens has a power of +5 dioptres. What is its focal length?
[MP Police Sub-Inspector – 20 January 2026]
A. 0.2 m
B. 0.1 m
C. 0.5 m
D. 0.05 m
Answer: A. 0.2 m
Explanation: The dioptre is the unit of optical power, defined as the reciprocal of focal length in metres. Using P = 1/f, focal length equals 1/5 m, or 0.2 m. The positive sign indicates a converging lens.
Q88. What is the SI unit for mechanical work?
[MP Police Sub-Inspector – 18 January 2026]
A. Watt
B. Joule
C. Erg
D. Newton
Answer: B. Joule
Explanation: One joule is the work done when a force of one newton moves its point of application one metre in the force direction. A watt measures the rate of work, an erg is the CGS energy unit, and a newton measures force.
Q87. Which option denotes the unified atomic mass unit?
[MP Police Sub-Inspector – 16 January 2026]
A. Mole
B. Gram
C. u (atomic mass unit)
D. Kilogram
Answer: C. u (atomic mass unit)
Explanation: The unified atomic mass unit, symbol u or dalton, is defined as one-twelfth of the mass of a neutral carbon-12 atom in its ground state. It is convenient for expressing masses of atoms and molecules rather than large laboratory samples.
EXAM YEAR 2025
Q86. Which of the following is the SI unit of pressure?
[SSC CHSL Tier I – 17 November 2025]
A. Ampere
B. Watt
C. Pascal
D. Newton
Answer: C. Pascal
Explanation: The pascal equals one newton acting normally over one square metre. It is the SI derived unit for both pressure and stress. Ampere denotes current, watt represents power, and newton is the SI derived unit of force.
Q85. What is the physical quantity defined as normal force exerted per unit area?
[SSC CHSL Tier I – 14 November 2025]
A. Force density
B. Pressure
C. Weight
D. Work
Answer: B. Pressure
Explanation: Pressure is obtained by dividing the normal force on a surface by the area receiving that force. It has SI unit pascal. A larger area reduces pressure when the applied normal force remains unchanged; this principle explains many practical pressure effects.
Q84. Which formula correctly expresses pressure P in terms of force F and area A?
[SSC CHSL Tier I – 13 November 2025]
A. P = F/A
B. P = F × A
C. P = A/F
D. P = F²/A
Answer: A. P = F/A
Explanation: Pressure measures force distributed across a unit area, so P = F/A. With force in newtons and area in square metres, the result is in pascals. Doubling area while keeping force fixed halves the resulting pressure.
Q83. Complete the unit analogy: watt is to power as pascal is to _____.
[SSC CGL Tier I – 12 September 2025]
A. Energy
B. Temperature
C. Pressure
D. Force
Answer: C. Pressure
Explanation: Each SI derived unit corresponds to a defined physical quantity. Watt names the unit of power, equal to joule per second. Pascal names the unit of pressure, equal to newton per square metre. These quantities are different despite related dimensions.
Q82. The ratio of two sound intensities is commonly expressed in which unit?
[RRB NTPC Undergraduate – 28 August 2025]
A. Phon
B. Pascal
C. Hertz
D. Decibel
Answer: D. Decibel
Explanation: The decibel is a logarithmic unit used to express ratios of quantities such as sound intensities. For intensities I₁ and I₂, the difference in sound intensity level is 10 log₁₀(I₂/I₁) decibels. A hertz measures frequency, not intensity.
Q81. Which of the following is the SI unit of frequency?
[RRB NTPC Graduate – 20 June 2025]
A. Millisecond
B. Pascal
C. Joule
D. Hertz
Answer: D. Hertz
Explanation: Frequency counts periodic cycles per unit time. One hertz denotes one cycle per second and is equivalent to s⁻¹. A millisecond is a duration, pascal measures pressure, and joule measures energy. The hertz is named after Heinrich Hertz.
