Question

The work done by Sun on Earth at any finite interval of time is 

(a) positive, negative or zero 

(b) Strictly positive

(c) Strictly negative 

(d) It is always zero

Answer 1

(d) It is always zero 

2 & 3. No work is done on the earth revolving around it in perfectly circular orbit. The earth revolves around the sun due to gravitational force of attraction between the sun and the earth. This force around the sun. This centripetal force is always perpendicular to the linear displacement. 

Work done = W = F.d cos θ Since θ = 90°

= F.δ (0)  cos 90° = 0

W = 0