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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

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