Torque of the contact forces about the centre of mass

Question

A person leans in the opposite direction when he or she lifts a heavy load in one hand.

I read a reason for the same in a book.

According to the book, if a body is placed on a horizontal surface, the torque of the contact forces about the centre of mass should be zero to maintain equilibrium. This may happen only if the vertical line through the centre of mass cuts the base surface at a point within the contact area or the area bounded by the contact point. That's why the person leans.

But I am unable to understand that how does it explain the leaning of person on lifting a heavy load.

Answer 1

The person does not have to lean. When carrying a heavy load you should keep the load close to the vertical line through your centre of mass (CM). For stability, the combined CM (you plus load) must be between your feet. For heavy loads (eg a suitcase) you may need to lean slightly to achieve this, even when carrying close to your body.

If the object is bulky and does not have handles, this may prevent you from keeping the load close to your centre of mass. Then you are forced to lean backwards, even when using two hands, to place the combined centre of mass (your own plus that of the load) between your feet.

In all cases, it is the combined CM which must be between your feet.