Title 1. UNITS AND MEASUREMENT.pdf ✅

Oswaal CBSE Question Bank Chapterwise & Topicwise, PHYSICS, Class-XI Short Answer Type Questions-I (2 marks each) Q. 1. Find the number of times the heart of a human being beats in 10 years. Assume that the heart beats once in 0.8 sec. A Ans. In 0.8 s, the human heart makes one beat. ∴ In 1 s the human heart makes = 1 0.8 = 10 8 beats. 1 ∴ In 10 years the human heart makes = 10 8 × 10 × 365 × 24 × 60 × 60 beats = 3.942 × 108 beats. 1 Commonly Made Error Students commits mistake while finding beats in one second and seconds in one year. Answering Tip Students should carefully find the number of beats in one second and then multiply the same with the number of seconds in an year. Q. 2. Derive S.I. unit of Joule (J) in terms of fundamental units. U Ans. Joule is a unit of work. Using the relation, Work = force × displacement = mass × acceleration × displacement = velocity mass× ×displacement time = mass × displacement×displacement time×time = mass × displacement2 × time–2 1 Unit of work, J = kg × m2 × s–2 = kgm2 s–2. 1 Q. 3. Give conversion of the some commonly used large units of length into metre. U Ans. 1 Light year (ly) = 9.46 × 1015 m 1 Astronomical unit (AU) = 1.496 × 1011 m 1 Parallactic second (parsec) = 3.08 × 1016 m 2 Q. 4. Why an optical microscope is not used to measure the size of a molecule ? U Ans. Size of molecule ranges from 10–8 m to 10–10 m. An optical microscope uses visible light of average wavelength 6000 Å, i.e., 6000 × 10–10 m to measure the sizes. Since, size of molecule is smaller than the wavelength of light used, so optical microscope cannot resolve a molecule. 2 Q. 5. Give conventional rules for the rounding off of uncertain digits. U Ans. The conventional rules are : (i) If the insignificant digit to be dropped is more than 5, the preceding digit is increased by 1, but if it is less than five, then preceding digits is not changed, e.g., 1.748 is rounded off to 3 significant figures as 1.75 and 1.742 as 1.74. 1 (ii) If the insignificant digit to be dropped is 5, then this digit is simply dropped if the preceding digit is even but if odd, then the preceding digit is increased by 1. e.g., the number 1.845 rounded off to three significant digits is 1.84 but for number 1.875 it is 1.88. 1 Short Answer Type Questions-II (3 marks each) Q. 1. What is system of units ? Mention some of them. R Ans. In general, a complete set of base units as well as derived units is called system of units. But, it was a practice to name the system of units after three fundamental units of length, mass and time only, e.g., (a) FPS system : In this system length is measured in foot (f), mass in pound (p) and time in second (s). So, this system is called FPS system. This system is also known as British Engineering system of units or simply British system of units. (b) C.G.S. system : In this system length is measured in centimeter (cm), mass in gram (g) and time in second (s), so this system is called C.G.S. system. (c) M.K.S. system : In this system length is measured in metre (m), mass in Kilograms (kg) and time in second (s), so this system is called M.K.S. system or metric system. (1 mark each) Q. 2. What is a prefix ? Give some common prefixes for multiples and submultiples. R Ans. Prefix is used to increase or decrease the value of a fundamental or derived unit as per practical requirements. Multiples Submultiples Exa (E) = 1018 atto (a) = 10–18 Peta (P) = 1015 femto (f) = 10–15 Tera (T) = 1012 pico (p) = 10–12 Giga (G) = 109 nano (n) = 10–9