Answer of physics short questions 2068 11 1. Answer in brief any six questions: a) If the scalar product of two vectors is equal to the magnitude of their vector product, find the angle between them. Ans: Let A and B be two vectors and X be the angle between them. Then, magnitude of Scalar product= |A.B|= AB cos X magnitude of vector product= |A X B|= AB sin X According to the question, |A.B|= |A X B | or, ABcos X= AB sin X or, tan X= 1 or, tan X= tan 45 or, x= 45 so, the angle between A and B is 45 degree. b) From a high tower, one ball is dropped from the rest and the second ball is simultaneously projected horizontally. Neglecting air resistance, which ball will reach the ground earlier? Ans: Since air resistance is neglected the time taken by both ball to reach the ground is given by: t= (2H/g)^1/2 where H is height of tower and g is acceleration due to gravity. Since for both the balls H and g are same, the time t must be same. And hence they reach the ground at same time. c) Explain why a man carrying a load on his back leans forward? Ans: For a man to be stable the perpendicular line drawn from center of gravity should pass from base of the man. So, the man leans forward in order to shift the center of gravity towards the center and to be more stable. d) What are elastic and inelastic collisions? Give examples of each. Ans: The collisions in which both the momentum and kinetic energy is conserved, it called elastic collision eg. collision of ideal gas molecules. The collisions in which only momentum is conserved but not KE is called inelastic collision. For eg, collision between two rocks. e) If the force of the gravity acts on all bodies in proportion to their masses, why does not a heavy body falls faster than a light body? Ans:The force of gravity is more in larger mass but more force is needed to accelerate the larger mass according to Newtons second law of motion. So, As a result both light and heavy body have same acceleration and fall at a same time. f) What will happen if a capillary tube of insufficient height is dipped into water? Ans; We know, the height of capillary is given by: h= 2Tcos X/dgR ie.h is inversely proportional to R so, the radius of curvature increase as there is less height of capillary. g) Explain why soldiers are ordered to break steps while crossing bridge? Ans: When the soldier dont break steps then due to their marching steps, bridge also vibrates. If the frequency of vibration due to marching of soldier coincide with the natural frequency of bridge, bridge vibrate with maximum displacement due to phenomena of resonance. If this displacement exceed the elastic limit of the bridge, it may collapse. So in order to prevent bridge from collapsing, soldiers are ordered to break steps while crossing bridge. 2. Answer in brief any two questions a) Define absolute temperature. Ans: Temperature expressed in kelvin is called absolute temperature. Different scales can be converted as; (C-0)/(100-0)=(K- 273)/(373-273)= (F- 32)/ (212- 32)= (Scale- lower fixed point)/ (upper fixed point- lower fixed point) b) Explain why dew are formed in clear nights but not in cloudy night. Ans: Dew are formed when the temperature of earth becomes below due point. In the clear nights, earth can loss heat by means of radiation and can attain temperature below dew point. Where as, in cloudy night it cant loss heat by means of radiation due to clouds and cant attain temperature below dew point. So, dew are formed in clear nights but not in cloudy night. c) Air escaping from air hose at a gas station always feels cold. Why? Ans: We know the air escaping out of hot gas station is adiabatic process. Also,dQ= dU + dW In adiabatic process, dQ= 0. so, dU= -dW and PdV= - CvndT here, the volume of air gas escaping increases and work done is with the expense of internal energy. As internal energy decreases, temperature also decreases. 3. Answer in brief any one question. a) The sun is less bright is evening and morning as compared to noon although its distance from the observer is almost same. Ans: According to lambert cosine law, the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle θ between the observers line of sight and the surface normal. Since the value of θ is maximum at morning and evening the luminous intensity is less as the value of cos θ decreases as increase in θ. So, The sun is less bright is evening and morning as compared to noon although its distance from the observer is almost same. b) What is chromatic aberration? Why a mirror cannot give to chromatic aberration? Ans; The inability of lens to focus different colors of light at a point due to difference in the wavelength of rays is called chromatic aberration. Mirror cannot give chromatic aberration because the rays return in same medium and they arent refracted. 4. Answer in brief any one question. a) What do you mean by quantisation of charge? Every charged particle contains integral multiple of basic charge that is e= 1.6 * 10^-19 ie, q= ne This is called quantization of charge. b) Distinguish between Dielectric strength and Dielectric constant. Ans: The maximum electric field that a pure material can withstand under ideal conditions without breaking down to start conducting is called dielectric strength. Unit; V/m. Dielectric constant is property of an electrical insulating material (a dielectric) equal to the ratio of the capacitance of a capacitor filled with the given material to the capacitance of an identical capacitor in a vacuum without the dielectric material. It has no unit.
Posted on: Tue, 16 Dec 2014 14:30:00 +0000