What do you mean by restoring force? And if the hammer is disturbed to an extent where the centre of mass is no longer under the pivot point, the mobile will collapse, won't it?
Can you explain to me why does positioning the centre of mass of the mobile below the pivot point, which I'm assuming is the edge of the table, makes the mobile stable and balanced?
The centre of mass of the hammer is near the hammer head, so the centre of mass of the whole mobile should be found there as well?
Does the centre of mass of the mobile have to be vertically aligned with the table and a part of the ruler which is resting on the table or does it have to be...
http://resources.scienceworld.ca/pdf/forces/Balance_Hammer.jpg
How do I explain this in terms of centre of mass?
And what's the definition of centre of mass in this context?