A 200 W sodium street lamp emits yellow light of wavelength 0.6 um. Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is

• 1.5 x 10²⁰

• 6 x 10¹⁸

• 62 x 10²⁰

• 3 x10¹⁹

A ball is thrown vertically downwards from a height of 20m with an initial velocity v_o, It collides with the ground, loses 50% of its energy in collision and rebounds to the same height. The initial velocity v_o is,

(Take g = 10 ms^-2)

• 14 ms^-1

• 20 ms^-1

• 28 ms^-1

• 10 ms^-1

A block of mass 10 kg, moving in the x-direction with a constant speed of 10 ms^-1 , is subjected to a retarding force F= 0.1x J/m during its travel from x = 20 m to 30 m. Its final KE will be

• 475 J

• 450 J

• 275 J

• 250 J 

A body of mass (4m) is lying in x-y plane at rest. It suddenly explodes into three pieces. Two pieces each of mass m move perpendicular to each other with equal speed (v). The total kinetic energy generated due to explosion is,

• mv²

• 

• 2mv²

• 4 mv²

B.

As per the question, the third part of mass 2m will move because the total momentum of the system after explosion must remain zero.

Let the velocity of the third part be v'.
According to the law of conservation of momentum,

Total kinetic energy generated by the explosion,

A body of mass m taken from the earth's surface to the height equal to twice the radius (R) of the earth. The change in potential energy of the body will be

• mg2R

• 2/3 mgR

• 3 mgR

• 1/3 mgR