Ncert Exercises & Solutions

Different points in the earth are at slightly different distances from the sun and hence experience different forces 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 centre of mass causing translation and net torque at the centre of mass causing rotation around an axis through the CM. For the earth-sun system (approximating the earth as a uniform density sphere):

1.	the torque is zero.
2.	the torque causes the earth to spin.
3.	the rigid body result is not applicable since the earth is not even approximately a rigid body.
4.	the torque causes the earth to move around the sun.

Hint: The torque on a body depends on the angle between the position vector and the force.

Step 1: Find the angle between the position vector and the force.

As the earth is revolving around the sun in a circular motion due to gravitational attraction, the force of attraction will be of radial nature i.e. the angle between position vector r and force F is zero.

Step 2: Find the net torque.

So, torque =

|\vec{τ}| = |\vec{r} \times \vec{F}| = rFsin 0 ° = 0

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