## Sunday, October 11, 2009

### IIT JEE Physics Final Revision Set 6

Two thin circular discs of mass 2 Kg and radius 10 cm each are joined by a rigid massless rod of length 20 cm. The axis of the rod is along the perpendicular to the planes of the disk through their centres. This object is kept on a truck in such a way that the axis of the object is horizontal and perpendicular to the direction of motion of the truck. Its friction with the floor of the truck is large enough so that the object can roll on the truck without slipping. Take x-axis as the direction of motion of the truck and z-axis as the vertically upwards direction. If the truck has an acceleration 9 m/s2, calculate:

1. the force of friction on each disk.
2. the magnitude and direction of the frictional torque acting on each disk about the centre of mass O of the object.
3. Express the torque in the vector form in terms of unit vectors i, j and k in x, y and z directions.

4. In the following column I lists some physical quantities and the column II gives approximate energy of them. Choose the appropriate value of energy from column II for each of the physical quantities in column I

Column I ------------- Column II
(i) Energy of thermal neutron---- (A) 0.025 eV
(ii) Energy of X-Ray ------------ (B) 0.5 eV
(iii) Binding energy per nucleon-- (C) 3 eV
(iv) Photoelectric threshold of a metal --- (D) 20 eV
------------------------------------------ (E) 10 keV
-------------------------------------------(F) 8 MeV

5. A band playing music at a frequency f is moving towards a wall at a speed Vb. A motorist is following the band with a speed Vm. If V is the speed of sound, obtain an expression for the beat frequency heard by the motorist.
Source JEE paper 1997