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Practical Units of Length, Mass and Time
1. Length
1 fermi = 1 fm = 10⁻¹⁵ m
1 X-ray unit = 1XU = 10⁻¹³ m
1 angstrom = 1Å = 10⁻¹⁰ m = 10⁻⁸ cm = 10⁻⁷ mm = 0.1 μm
1 micron = 1 μm = 10⁻⁶ m
1 astronomical unit = 1 A.U. = 1.49 × 10¹¹ m ≈ 1.5 × 10¹¹ m ≈ 10⁸ km
1 Light year = 1 ly = 9.46 × 10¹⁵ m
1 Parsec = 1 pc = 3.26 light years
2. Mass
Chandra Shekhar unit = 1 CSU = 1.4 × mass of the sun = 2.8 × 10³⁰ kg
Metric tonne = 1 Metric tonne = 1000 kg
Quintal = 1 Quintal = 100 kg
Atomic mass unit (amu) = 1 amu = 1.67 × 10⁻²⁷ kg
3. Time
Year: Time taken by Earth to complete 1 revolution around the Sun in its orbit.
Lunar month: Time taken by the Moon to complete 1 revolution around Earth. 1 L.M. = 27.3 days
Solar day: Time taken by Earth to complete one rotation about its axis with respect to the Sun. 1 Solar year = 365.25 average solar days
Siderial day: Time taken by Earth to complete one rotation about its axis with respect to a distant star. 1 Solar year = 366.25 Siderial days = 365.25 average solar days
Shake: An obsolete but practical unit of time. 1 Shake = 10⁻⁸ sec
FAQs on Practical Units of Length, Mass and Time
Q1: What is a light year?
A1: A light year is the distance that light travels in one year, approximately 9.46 × 10¹⁵ meters.
Q2: What is an astronomical unit (A.U.)?
A2: One astronomical unit is the average distance from Earth to the Sun, approximately 1.49 × 10¹¹ m.
Q3: How is an atomic mass unit (amu) defined?
A3: One atomic mass unit (amu) is defined as 1.67 × 10⁻²⁷ kg, roughly the mass of a proton or neutron.
MCQs on Practical Units of Length, Mass and Time
Q1: Which of the following is the largest unit of distance?
(A) Angstrom
(B) Parsec
(C) Light year
(D) Astronomical Unit Answer: (B) Parsec Explanation: A parsec is approximately 3.26 light years, making it the largest unit of distance in astronomy.
Q2: How many kilograms are there in one metric tonne?
(A) 10 kg
(B) 100 kg
(C) 1000 kg
(D) 10,000 kg Answer: (C) 1000 kg Explanation: One metric tonne is equal to 1000 kg.
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