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PHYSICS-IA From two diametrically opposite points A and B on earth, moon is observed. The angle e subtended at the moon by the two directions of observation is 154'. If radius of earth is 0.638 x 10' m, find the distance of P.2.2 the moon from the earth. Distance between points of observation = Diameter of earth i.e., %3D Sol. b = 2r b = 20.638x10' = 1.276x10 m ; Angle subtented at moon 0 = 154' = 1.9 = 0 = 1.9x1.745x102 = 3.3210 rad %3D %3D %3D 1.276 x107 Distance of moon, D=-=D= = 3.84 x 10m . %3D 3.32x10-2 Distant A man wishes to estimate the distance of a nearby tower from him. He stands at a point A in front of the tower C and spots a very distant object O in line with AC. He then walks perpendicular to AC up to B, a distance of 100 m, and looks at O and C again. Since O is very distant, the direction B0 is practically the same as AO; but he finds the line of sight of C shifted from the original line of sight by an angle 0 = 40 (0 is known as 'parallax') estimate the distance of the tower C from his original position A. P.2.3 object O Sol. Parallax angle 0= 40; AB = ACtan 0 A- %3D 100 m AC = AB/ tan 0 = 100m/ tan 40=100m/0.8391=119m. %3D %3D %3D motot 2.5 ESTIMATION OF VERY SMALL DISTANCES/SIZE OF MOLECULE EO Tunnelling microscopes are used to measure Very small sizes like size of molecule can be measured by special microscope called Tunnelling microscope. The sizes of molecules can be estimated by a simple method explained as below. To determine molecular size of oleic acid, diluted solution of oleic acid in alcohol of concentration 'C' is prepared. Lycopodium powder is spread on the surface of water taken in a trough. 'n' drops of prepared solution each of volume V are put in the water. The drops spread into a thin, large and roughly circular film of molecular thickness on water surface. The diameter of thin film is measured which gives the area 'A' of the film. The thickness 't' of the film is nothing but molecular size as it is mono-molecular layer. very small sizes volume of the film nVC :. Size of molecule = thickness of film i.e., t = A %3D area of the film We use certain special length units for short and large lengths. These are 2.5.1 Measurment of Mass Mass is a basic property of matter. It does not depend on the temperature, pressure or location of the object in space. The SI unit of mass is kilogram (kg). The kilogram is an inconvenient unit in dealing with atoms and molecules. In this case, there is a standard unit of mass, called the unified atomic mass unit (u), R which has been established for expressing the mass of atoms as 1 unified atomic mass unit is (1/12) of the mass of an atom of carbon-12 isotope (2C) including the mass of electrons. lu = 1.6 10-27kg Re %3D Mass of commonly available objects can be determined by common balance. Large masses are determined by gravitational methods, while small masses are measured by mass spectroscopes, which work on the principle of motion of charged particles in electric and magnetic fields. Ti Tr. to 18 UNI ELITE SERIES for Sri Chaitanya Jr. ICON Students