Physics Paper I

FEDERAL PUBLIC SERVICE COMMISSION COMPETITIVE EXAMINATION FOR RECRUITMENT TO POSTS IN B.P.S.17 UNDER THE FEDERAL GOVERNMENT, 2000. PHYSICS PAPER-I TIME ALLOWED: 3 HOURS MAXIMUM MARKS: 100 Note:- Attempt FIVE questions in all, including Question No.8 which is compulsory. All questions carry equal marks. 1. (a) Find a unit vector which is perpendicular to the vectors î +j and j + ?. (5) (b) If A = xz î – y2 j + 2x2y?ˆ and =x2yz2, find (15) 2. (a) Define angular momentum of a single particle and derive a relation between torque and angular momentum. (10) (b) Calculate the angular momentum, about the Earth’s center of a 50 kg person on the equator of the rotating earth. (10) 3. Describe with the help of necessary diagrams the details of Michelson-Morley experiment. What was the conclusion drawn from the results of the experiment. (20) 4. (a) Write down Maxwell’s distribution function for molecular speeds in a gas. Give the details of various parameters involved. (10) (b) A container filled with N molecules of Oxygen gas is maintained at 300 K. What fraction of the molecules has speeds in the range 599-601 m/see? The molar mass M of Oxygen is 0.032 kg/mole. (10) 5. Differentiate between Phase and Group velocities. Given that phase velocity V of waves in deep water is (g l /2p )1/2 ,find their group velocity v. (20) 6. What is interferometer? Describe the construction and working principle of an interferorneter with which you are familiar. (20) 7. Write short notes on the following; (a) Viscosity (b) Planetary Motion (c) Laws of Thermodynamics and their importance in daily life. (20) COMPULSORY QUESTION 8. Write only the correct answers in the answer book. Do not reproduce the questions. 1) If the speed of particle is doubled then it’s kinetic energy becomes a) doubled. b) unchanged c) four time. d) half of the initial value ii) A ping - pong ball of mass m moving with velocity v bounces off a concrete surface with the same speed in the opposite direction. The change in it’s linear momentum is

a) 2mw b) mv c) 1/2mv d) zero iii) The work done by a force of 20 N in displacing an object through a distance of 5 cm is a) 100 J b) 4 J c) 400 J d) 1 J iv) The weight of an astronaut in space far from any planet in a) Zero b) different than on earth c) same u on earth v) A net force needed to give a mass of 250 kg an acceleration of 5 cm/sec2 is a) 25N b) 12.5N c) 10N d) 5N vi) A light year is a unit of a) time b) distance c) velocity d) intensity of light vii. A spaceship is circling a planet at a distance of 10000 km from its center. Assume that the radius of the planet is 5000 km mid the acceleration clue to gravity on it’s surface is 10 rn/sec2. The speed of the spaceship is a) 5 km/sec b) 1 km/sec c) 10km/sec d) 25 km/sec viii) A uniform spring with force constant K is cut into halves. Whet is the spring constant for one of the halves? a) Half of the original one. b) Twice the original one c) 1/4 of original one d) Same original one ix) A watt is the unit of a) Energy b) Power c) Work d) capacity x) Two bodies are in thermal equilibrium with each other if they have same a) pressure b) density c) energy d) temperature xi) The number of grams in one mole of Helium gas are a) 1gm/mole b) 4gm/mole c) 6 gm/mole d) 2 gm/mole xii) A gas consists of a mixture of He and N2 molecules The speed of He molecules Intl. mixture is a) Slower than N2 molecules b) Greeter than N2 molecules c) equal to N2 molecule. xiii) If a gas is compressed constant temperature, then the mean free path of the molecules of the gas, a) decreases b) remain the same c) increases xiv. The melting point of the element Sulfur is 113 °C. It’s temperature at Fahrenheit scale is a) 45 °F b) 113 °F c) 63 °F c) 8l °F

xv) Diffraction of waves occurs because of a) medium change b) uncertainty principle c) geometry of the object obstructing the wave d) none of these xvi) Beats in sound are a result of a) two sound waves with different speeds b) in-homogenous medium c) superposition of two waves with different frequenceis d) the relative motion of one of the sound source. xvii) Light of certain colour is composed of train of waves, each 0.250 m long. The number of waves in 1 meter distance are a) 2500000 b) 5000000 c) 4000000 d) 250 xviii) Longitudinal waves do not exhibit a) refraction b) reflection C) diffraction d) plarization xix) If mass of a simple pendulum is doubled, it’s period a) remain unchanged b) doubled c) becomes half of the original d) none of these xx) Diagnostic ultrasound of frequency 4.50 MHz is used to examine tumors in soft tissue. If the speed of sound in tissue is 1500 m/sec. The wave length of this wave in tissue is a) 6750 m m b) 4.50 m m c) 1500 m m d) 333.3 m m *****************************************