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21. A short linear object of length b lies along the axis of a concave mirror of focal length f at a distance u from the pole of the mirror. The size of the image

       is approximately equal to

       a. b((u-f/f)^1/2          b. b(f/(u-f))^1/2                 c.b((u-f)/f)        d. b(f/(u-f))²

           

22. The work function of a substance is 4.0 eV. The longest wavelength of light that causes photoelectric emision from this, substance is approximately

       a. 540nm               b. 400 nm         c. 310 nm         d. 220 nm

 

23. Neglecting variation of mass with velocity, the wavelength associated  with an electron having the kinetic energy E, is proportional to

       a. E^1/2        b. E^-1/2  c. E      d. E^-1/2

 

24. After a time equal to five half lives, the amount of radioactive material remaining undecayed is

      a. 20%       b. 10%             c. 5%               d.3%

 

25. Two men of equal mass are hanging by their hands from the ends of a rope hung over a frictionless pulley. They begin to climb the rope. One person can climb with twice the speed of the other (w.r.t the rope). Who will get to the top first?

      a. The faster climber           b. The slower climber               c.Both will get to the top simultaneously d. It is indeterminate

 

26. A particle is dropped from a height h. Another particle which was initially at a distance d from the first, is simultaneously projected with a horizontal

      velocity u and the two particles just collide on the ground. Then

     a. d²=u²h/2g            b. d²=2u²h/g                 c. d=h              d. gd²=u²h

 

27. A very light planet is moving in a circular orbit round the sun with a force proportional to Rⁿ where R is the distance between the planet and the sun. Time

      period of revolution is found to be such that T² is proportional to R^7/2 Then the value of n is

      a.-1/2         b.-5/2   c.-7/2               d.-2

 

28. A metal ball of density 7800 kg/m3 is suspected to have a large number of inner cavities. It weighs 9.8 kg when weighed directly on a balance and 1.5 kg

      less when immersed in  water. The fraction of volume of the cavities in the metal ball is approximately

      a. 20%       b. 30%             c. 16%             d. 11%

 

29. The radius of gyration of a given rigid body with uniform mass distribution depends upon the

      a.  Mass and axis of rotation           b. Shape and axis of rotation                 c. Distance of c.o.m from the axis of rotation                        d. None of the above

 

30. A wire under tension vibrates with a frequency of  450Hz. What would  be the fundamental frequency if the wire were half as long, twice as thick and under 1/4^th tension

      a. 450 Hz               b. 225 Hz         c. 900 Hz         d. 112.5 Hz