Torque, Internal forces & external forces.

[pdpax]

Hi, I had a question about Torque, Internal forces & external forces.
Here are 2 examples helping me to state my question:

1) The Earth is rounding the sun with respect to the Sun.
The forces which are acting on the system are 2 gravitational forces that are action & re-action.
The Force vector & the Position vector will be in opposite direction; so their cross product is Zero & then the Torque will be zero.

2) In a system of particles, two objects are attracting each other with respect to a point (Q), which is located outside of the system.
With respect to (Q), We say these two forces cancel each other out & there will be no Torque on the system in view of (Q).

Why in case 2, we say these forces cancel each other; but in the first case, we don't consider the re-action of Sun's gravitational force?

Thanks for your helps...

 

[cepheid]

2) In a system of particles, two objects are attracting each other with respect to a point (Q), which is located outside of the system.
With respect to (Q), We say these two forces cancel each other out & there will be no Torque on the system in view of (Q).

I don't think I follow. These two forces can't "cancel each other out", because they act on two different objects, both of which actually DO accelerate under the influence of said forces. Furthermore, for each object, if there is a non-zero component of the force on that object that acts perpendicular to the line-of-sight between the object and point Q, then that force will provide a net torque on that object around the point Q.