A light year is a unit of which of the following?
- A.Time
- B.Distance
- C.Speed
- D.Brightness of a star
Correct answer
Explanation
The correct answer is B, distance. A light year is the distance that light, moving at about three lakh kilometres per second, covers in one year, which works out to roughly 9.46 trillion kilometres. It is used for the enormous distances between stars, where kilometres would run to unmanageable numbers.
Option A is the trap built into the name; the word year refers to the time taken by the light, not to the quantity measured. Option C, speed, has its own units such as metres per second. Option D, brightness, is measured by other quantities altogether, such as magnitude in astronomy. Two more distance units belong with this one: the astronomical unit, the mean distance between the Earth and the Sun, and the parsec, which equals about 3.26 light years.
Read the full article: Units and Measurements: SI Units, Dimensions and Instruments
Practice Questions
View allWhat is the SI unit of force?
- A.Joule
- B.Newton
- C.Watt
- D.Pascal
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One newton is the force that gives a mass of one kilogram an acceleration of one metre per second squared, which follows directly from the equation force equals mass times acceleration.
Read the full explanation →How many base units are there in the International System of Units (SI)?
- A.Five
- B.Six
- C.Seven
- D.Nine
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The SI system, adopted in 1960 by the General Conference on Weights and Measures, rests on seven base units: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela.
Read the full explanation →Which one of the following is NOT a base unit of the SI system?
- A.Kelvin
- B.Mole
- C.Newton
- D.Candela
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The newton is a derived unit: it is built from the kilogram, the metre and the second, since force equals mass times acceleration, and it can be written as one kilogram metre per second squared.
Read the full explanation →The candela is the SI unit of which physical quantity?
- A.Luminous intensity
- B.Electric current
- C.Amount of substance
- D.Pressure
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The candela takes its name from the candle, which was once used as a rough standard of light, and it is one of the seven base units of the SI system.
Read the full explanation →The SI unit of pressure is
- A.Newton
- B.Pascal
- C.Hertz
- D.Tesla
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Pressure is force divided by area, so its unit is one newton per square metre, which is given the name pascal after Blaise Pascal.
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