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CBSE Class 11 Pre Board 2019 : Physics - Paper 1 (St Xavier's Sr. Sec. School, Delhi)

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Annual Examination in PHYSICS Std. 11 25-2-2019 Time : 3 hrs. Max. Marks : 70 INSTRUCTIONS: i) ii) iii) iv) v) vi) vii) Q. Nos. 1 to 5carry 1 mark each. Q. Nos. 6 to 12 carry 2 marks each. Q. Nos. 13 to 24 carry 3 marks each. Q. Nos. 25 to 27 carry 5 marks each. Use pencil for the diagrams and graphs. Answers should be to the point. Use log tables if necessary. Section A 1. A and B are two non zero vectors. Find the value of (A+B).(A-B) (1) 2. The escape velocity on the surface of the earth is 11.2 km/s. What will be the escape velocity on the surface of a planet of mass twice mass of the earth and radius is half the radius of the earth. (1) Sketch a graph to show the variation of acceleration due to gravity with the distance from the centre of the earth. (OR) A given object of fixed mass weighs more at pole than on the equator. Why? (1) 4. The focal length of a convex lens increases when the lens is immersed in water. What happens to the focal length of a mirror when it is placed in water? (1) 5. An air column in a closed organ pipe vibrates at its third overtone with a frequency of 1400Hz. What is its fundamental frequency? (OR) A segment of wire in sonometer experiment has length of 20cm vibrating at its fundamental mode with a frequency of 256 Hz. What is the speed of waves? (1) 3. Section B 6 7. 8. Std. 11 A force of F = 2i+2j+3k N acts on a particle of radius vector r= (8i+2j+3k) m. Find the magnitude of torque acts on the particle. (OR) The angular speed of a motor wheel is increased from 1200 rpm to 3120 rpm in 6 seconds. How many revolutions does it make during this time? (2) A bullet of mass 10g is fired on to a soft wooden plank of thickness 20cm. The bullet emerges with 50% its initial speed. Calculate the minimum thickness of the wooden plank required to just stop the bullet if the bullet is fired with the same speed. (2) Write the conditions for the total internal reflection to take place. Derive the relation for between refractive and index and critical angle. (OR) Draw a ray diagram to deviate a light ray by 90o using a right angled prism and explain its path. (2) -2- PHYSICS 9. 10. 11. 12. A police man blows a whistle with a frequency of 500Hz. A car approaches towards him with a velocity of 15 m/s. Calculate the change in frequency as heard by the driver of the car, as he passes the police man. Speed of sound in air is 300m/s. (OR) The distance between two points on a stretched string is 20cm. The frequency of the progressive wave is 400Hz and velocity is 100m/s. Find the phase difference between these two points. (2) Write the mathematical functions of kinetic energy and potential energy of a simple harmonically oscillating particle with respect time and sketch the graphs to show their variation. (2) Write Newton s formula for velocity of sound through air. Why there was a discrepancy in theoretical value with experimental value? Explain how this discrepancy was removed by Laplace. (2) Three identical particles each of mass m are placed on the vertices of an equilateral triangle of side a . Calculate the magnitude of the net force acting any one of the particle due to other two particles. (2) Section C 13. Draw a neat ray diagram to show the refraction through parallel surfaces of a glass slab and hence derive an expression for lateral shift produced in the path of ray of light on passing through a glass slab of thickness t when incident at an angle i on its surface. (3) 14. Explain the path of a light ray passing through an equilateral prism and hence drive the relation for refractive index when the light ray under goes minimum deviation. (3) 15. Show that a convex mirror always produces a virtual, erect and diminished image of an object placed in front of the mirror at any position. (3) 16. 17. Derive the expression for the maximum speed with which a vehicle can negotiate the banked circular road. In an experiment of determination of refractive index material an equilateral prism, the minimum deviation of 40o is obtained. Calculate the refractive index of material of the prism. Calculate the minimum deviation if the entire prism is placed in water of refractive index of 4 . 3 (3) 18. A cylindrical cork is floating in water. A height of h meters of the cylinder is submerged in water. The cork is given a small depression and then released. Show that cork will execute SHM and hence find its time period of oscillations. (OR) What is Doppler s effect? Source of sound is approaching towards a stationary listener with a constant velocity and hence derive the formula for the apparent frequency as heard by the listener. (3) 19. A body takes twice the time to slide down a rough plane inclined at 60 to an identical smooth inclined plane. What is the coefficient of friction of rough plane? (OR) Two blocks of masses m1 and m2 are connected by a mass less string which passes over a massless pulley. Assuming that m1>m2, find an expression for acceleration and tension in the string. Std. 11 (3) -3- PHYSICS (3) 20. 21. A light string is wound round a cylinder of mass M and radius R: carries a mass m tied at the free end. When the mass is released, establish the relations for (i) linear acceleration of descending mass. (ii) Angular acceleration of cylinder (iii) tension in the string. (OR) A fly wheel of mass 25 kg has a radius of 0.2m. It is making 240rpm. What is the torque necessary to bring it to rest in 20s? If the torque is due to force applied tangentially on the rim of the flywheel, what is the magnitude of the force? (3) State parallel and perpendicular axes theorem and find the moment of inertia of a uniform circular disc about an axis passing through the edge and parallel to the diameter. (3) 22. A bucket containing water is tied to one end of rope of length 2.5m and rotated about the other end in a vertical circle in such a way that water in it does not spill. What is minimum velocity of bucket at which this happen and how many rotations per minute is it making? (3) 23. a) How much below the surface, the weight of a body becomes 10 % of its value on the surface of earth. (Radius of Earth is 6400 Km) Calculate the period of revolution of Neptune around the sun, given that radius of its orbit is 30 times the radius of earth s orbit around the sun, both orbits assuming being to be circular. (3) What do you mean by escape velocity? Derive an expression for the escape velocity of a body from the surface of a planet. (OR) Write two factors affecting the acceleration due to gravity. Derive the expression for acceleration due to gravity at height h from the surface hence deduce the relation for very small heights as compared to the radius of the earth. (3) b) 24. Section D 25. Derive lens maker s formula. Using lens maker s formula, explain under what condition a concave lens will behave as converging lens? (3+2) (OR) a) An object is placed at a distance of 30cm in front of a convex lens of focal length 10cm. Calculate the position of the image and the new position of the image if a concave lens of focal length 10cm is placed coaxially at a distance of 10cm right of the first lens. b) Draw a neat ray diagram of a compound microscope when the image is formed at the least distance of distinct vision. Write its formula for its magnifying power. (3+1 + ) 26. What are standing waves? Explain with the help of sequential diagrams of formation of standing waves. What are nodes and antinodes? (OR) a) What are beats? Explain how beats are produced with the help of neat diagrams? What do you mean by beat frequency? b) Two strings A and B produce beats of frequency 5Hz. If the tension of the string B slightly increased the beat frequency decreases to 3 Hz. What is the original frequency of B ? Given frequency of A is 427Hz. (5) Std. 11 -4- PHYSICS 27. a) b) a) b) Two particles of masses m1 and m2 under head on elastic collision. The particle of mass m1 is moving with a velocity u and particle of mass of m2 is at rest before collision. Derive the expression for final velocities of the particles. Hence calculate the final velocities of the particles if m1 : m2 = 1 : 4. The kinetic energy of a particle is increased by 20%. What is the percentage change in its momentum? (OR) What do you mean by restoring force? Is it a conservative force or non conservative force and why? Derive the expression for potential energy stored in a stretched spring. Show the variation of potential energy during oscillation graphically. ********* (3+2) (2+3)

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