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Units & Measurement

General Science · Physics

Useful for: Railway RRB, REET, UPTET, BPSC TRE, Assistant Teacher Recruitment (Super TET), Bihar STET, OSSC RHT — Regular High School Teacher, Bihar Assistant Professor

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Overview

Physical quantities are measured in SI units; seven base units build all derived units.

Key points

Seven SI base units: metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount), candela (luminous intensity).
Derived units: newton (force), joule (energy), watt (power), pascal (pressure), volt, ohm.
Prefixes: kilo 10³, mega 10⁶, milli 10⁻³, micro 10⁻⁶, nano 10⁻⁹.
A light year is a unit of distance, not time.
1 ångström = 10⁻¹⁰ m.

Explanation

1 kWh = 3.6 × 10⁶ J (the electricity "unit").
Accuracy is closeness to the true value; precision is closeness of repeated readings.
Vernier callipers and screw gauges measure small lengths precisely.
Dimensions help check formulas.

Step by step

Identify the concept or law involved.
Recall the definition, unit or formula.
Check units and the direction of the effect before choosing.

Examples

5 km = 5,000 m.
1 kWh = 1,000 W × 3,600 s = 3.6 × 10⁶ J.
A light year measures distance.

Traps and advanced points

Trap: Calling a light year a unit of time.
Trap: Mixing mass (kg) and weight (N).
Trap: Wrong prefix powers.

Quick revision

7 base units.
1 kWh = 3.6 MJ.
Light year = distance.

Practice MCQs

Which option correctly answers this General Science check: density is?
1. newton
2. ammeter
3. 3 × 10^8 m/s
4. mass per unit volume
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Answer: (d) mass per unit volume. ρ = m/V.
Which option correctly answers this General Science check: speed of light in vacuum is approximately?
1. sodium
2. vitamin D
3. ohm
4. 3 × 10^8 m/s
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Answer: (d) 3 × 10^8 m/s. This is the standard approximate value used in basic physics.
Which option correctly answers this General Science check: sound cannot travel through?
1. vitamin D
2. mgh
3. ½mv²
4. vacuum
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Answer: (d) vacuum. Mechanical sound waves require a medium.
ALP Basic Science & Engineering: Complete the statement: “SI base unit of length ____.”
1. very small thickness or diameter
2. metre
3. protect a circuit from excessive current
4. engineering drawing
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Answer: (b) metre. Use SI units consistently before calculations.
ALP Basic Science & Engineering: Which statement opening is correctly completed by “metre”?
1. SI base unit of mass
2. mechanical advantage of a machine is
3. SI base unit of length
4. efficiency of a machine is
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Answer: (c) SI base unit of length. Use SI units consistently before calculations.
ALP Basic Science & Engineering: Which statement opening is correctly completed by “kilogram”?
1. PPE stands for
2. SI base unit of mass
3. greenhouse effect is intensified by increased atmospheric
4. SI base unit of length
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Answer: (b) SI base unit of mass. Kilogram is the SI base unit of mass.
ALP Basic Science & Engineering: Complete the statement: “SI base unit of mass ____.”
1. unsafe on live electrical equipment
2. manages hardware/resources and provides services to applications
3. kilogram
4. small lengths/diameters with finer precision
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Answer: (c) kilogram. Kilogram is the SI base unit of mass.
Which option correctly answers this General Science check: pressure is?
1. 3 × 10^8 m/s
2. force per unit area
3. vitamin C
4. chlorophyll
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Answer: (b) force per unit area. P = F/A.
Which option correctly answers this General Science check: instrument used to measure potential difference?
1. voltmeter
2. watt
3. mitochondria
4. potassium
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Answer: (a) voltmeter. A voltmeter is connected in parallel.
Which option correctly answers this General Science check: work done is zero when force is perpendicular to displacement because?
1. mass per unit volume
2. cos 90° = 0
3. NaCl
4. vitamin C
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Answer: (b) cos 90° = 0. W = Fs cosθ.
The chemical formula of washing soda is?
1. CaCO₃
2. Na₂CO₃·10H₂O
3. NaHCO₃
4. NaCl
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Answer: (b) Na₂CO₃·10H₂O. Washing soda is sodium carbonate decahydrate; baking soda is NaHCO₃.
Which option correctly answers this General Science check: vitamin associated with blood clotting?
1. convex mirror
2. vitamin K
3. acidic solution
4. newton
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Answer: (b) vitamin K. Vitamin K is required for synthesis of several clotting factors.
Which option correctly answers this General Science check: mirror used as rear-view mirror in vehicles?
1. convex lens
2. convex mirror
3. vitamin K
4. moisture
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Answer: (b) convex mirror. Convex mirrors provide a wider field of view.
Which option correctly answers this General Science check: vitamin deficiency classically associated with scurvy?
1. chlorophyll
2. gene
3. vitamin C
4. vitamin D
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Answer: (c) vitamin C. Vitamin C is important for collagen synthesis.
Which option correctly answers this General Science check: functional unit of kidney?
1. hertz
2. vitamin C
3. 9.8 m/s²
4. nephron
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Answer: (d) nephron. Nephrons filter blood and help form urine.

Units & Measurement

Overview

Key points

Explanation

Step by step

Examples

Traps and advanced points

Quick revision

Practice MCQs

More topics in this chapter