Physics 1st Ch 01 Physical World and Measurement

Physics - First Paper Chapter 1: Physical World and Measurement Selected Questions and Answers (HSC English Version) Chapter 1: Physical World and Measurement B. Creative Questions 1. By measuring a copper-sphere, fastened by a thread, inside a liquid contained in a measuring cylinder Monon measured the volume as 80 mL. After taking out the sphere in air he obtained the volume as 60 mL. Again due to inquisitiveness he used lead-sphere of equal mass instead of copper-sphere. In this case the previous reading changed. He informed this to his class teacher and the teacher explained nicely this phenomenon scientifically. (a) What is measurement? (b) Why is vernier scale used in slide callipers? (c) Calculate the mass of the copper sphere in the stimulus. Solution : Given: Reading with sphere submerged = 80 mL; liquid volume alone = 60 mL. VCu = 80 −60 = 20 mL = 20 cm3 Standard density of copper: ρCu = 8.9 g/cm3. m = ρCu VCu = 8.9 × 20 = 178 g mCu = 178 g (d) Explain mathematically the reason for the change of readings in the measuring cylinder of the spheres made of copper and of lead. Answer: (a) Measurement is the process of comparing an unknown physical quantity with a known, inter-

(d) Both spheres have the same mass m = 178 g. The volume of the lead sphere is: VPb = m ρPb = 178 11.3 ≈15.75 cm3 Since ρPb = 11.3 g/cm3 > ρCu = 8.9 g/cm3, we have VPb < VCu. The measuring cylinder reading with the lead sphere: Vreading = Vliquid + VPb = 60 + 15.75 ≈75.8 mL This is less than 80 mL (the copper reading). The reading changed because for equal mass, the denser material occupies less volume, displaces less liquid, and therefore gives a lower reading in the measuring cylinder. In general, for equal masses: higher density ⇒smaller volume ⇒smaller reading. This relationship is summarised as: V = m ρ , so V ∝1 ρ at constant m. 2. In practical class of physics the teacher gave you a piece of metal sphere and asked to measure the radius of curvature by a spherometer. The distance between the two legs of the spherometer that you used was 8 cm. During measuring the mass by oscillation method a mass of 10 g was placed on the right pan and if the difference of the two fixed points was found 30, then answer the following questions. (a) What is vernier constant? (b) Distinguish between fundamental quantity and derived quantity. (c) What is the radius of curvature of the piece of t

(a) Vernier constant (least count of a vernier calliper) is the difference between one main scale division (MSD) and one vernier scale division (VSD): VC = 1 MSD −1 VSD. It is the smallest length that can be unambiguously measured by the instrument. (b) Fundamental quantities are physically independent base quantities that cannot be ex- pressed in terms of any other quantity; e.g., mass (M), length (L), time (T), temperature (Θ). Their units are called fundamental or base units. Derived quantities are obtained by mathematically combining fundamental quantities through the laws of physics; e.g., velocity = length/time, force = mass × acceleration. Their units are called derived units. (c) (d) 3. In order to use in the laboratory the teacher gave Anik a few wires of different cross-sections to verify whether the values were correct or not. By measuring the cross-sections of the wires by a screw gauge Anik found that the values as mentioned on the bodies were not matching. He informed the teacher about the discrepancy. The teacher found that as Anik did not follow the correct procedure of using screw gauge, hence he got discrepancy. By avoiding backlash error Anik measured again and f