Different points in the earth are at slightly different distance from the sun and hence experience different force due to gravitation. For a rigid body, we know that if various forces act at various points in it, the resultant motion is as if a net force acts on the `CM` (centre of mass) causing translation and a net torque at the `CM` causing rotation around an axis through the `CM`. for the earth-sun system (approximating the earth as a uniform density sphere).

Options

A. the torque is zero
B. the torque causes the earth to spin
C. the rigid body result is not applicable since the earth is not even approximately a rigid body.
D. the torque causes the earth to move around the sun.

Answer

A

Explanation

The earth orbits around the sun in an approximate circular orbit and the gravitational force F acts along the radius of circular path, towards the sun, `:.` angle between `vec(r )` and `vec(F)` is zero.
So `vec(tau) = vec(r ) xx vec(F) = r F sin 0^(@) = 0`
i.e., torque is zero.
The correct option is (a).