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 torques 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

(a) The earth is revolving in circular orbit around sun due to gravitational force F which acts along the radius of circular path, towards the sun i.e., angle between `vecr` and `vecF` is zero.
As, Torque `vec(tau)=vecrxx vecF or tau=rFsin theta`
`:. tau=rF sin 0^(@)=0`