The ionisation energy of hydrogen atom is 13.6 eV. Following Bohr’s theory the energy corresponding to a transition between 3rd and 4th orbits is

Options

A. 3.40 eV
B. 1.51 eV
C. 0.85 eV
D. 0.66 eV

Explanation

A tangential force is required to roll the disc. In this case, a frictional force is required for making the disc roll.
(a) Force of friction acts opposite to the direction of velocity of point B. The direction of linear velocity at point B is tangentially leftward. Hence, friction force will act tangentially rightward. The frictional torque before the start of perfect rolling is perpendicular to the plane of the disc in the outward direction.
(b) Frictional force acts opposite to the direction of the velocity at contact point B, perfect rolling will begin when the velocity at that point becomes equal to zero. This will make the frictional force acting on the disc zero.