Q.Why does a porter bend forward while carrying a sack of rice on his back?

Step 1. Identify the two weights acting on the porter+sack system.

The system consists of the porter's own body weight $W_p$, acting through his own center of gravity, and the sack's weight $W_s$, acting through the sack's center of gravity — which, since the sack sits on his back, lies behind (and above) the natural position of his own upright center of gravity.

Step 2. Locate the base of support and the equilibrium condition.

For a person standing without toppling, they must be in both translational and rotational equilibrium: the net downward force must be balanced by the upward normal reaction from the ground through the feet, and — just as important — the combined center of gravity of everything the person is carrying (body + sack) must fall within the base of support defined by the feet. If the combined center of gravity strays behind the heels, the weight produces an unbalanced backward torque about the feet and the person topples over backward.

Step 3. See what happens if the porter stood upright with the sack on his back.

If the porter stood perfectly upright, the sack's weight, acting well behind his own spine, would add a center of gravity for the combined system that sits behind his feet's base of support (or at least much closer to its rear edge). This produces a large backward-toppling torque about the feet — plus, even before actual toppling, it forces the back and spinal muscles to supply a large continuous restoring torque, causing strain and quick fatigue.

Step 4. See how leaning forward restores balance.

By bending forward at the waist, the porter shifts his own body's center of gravity forward, ahead of its normal upright position. This forward shift of his own weight's line of action creates a forward-tilting torque about the feet that is opposite in sense to the sack's backward-tilting torque. By leaning forward by just the right amount, the porter arranges for the two torques (his own forward-shifted weight and the sack's backward-offset weight) to balance, so that the combined center of gravity of porter + sack once again falls within — ideally, close to the center of — the base of support given by his feet.

Step 5. Note the practical payoff — minimum effort, maximum stability.

With the combined center of gravity restored over the feet, the net torque needed from the leg and back muscles to stay upright is minimized — the skeleton itself, rather than continuous muscular exertion, bears most of the load. This is why bending forward isn't just a balance trick but also the posture that lets the porter carry a heavy sack for a sustained period without collapsing from muscular strain.

Center of gravity / base-of-support torque-balance reasoning

• Describing this only as 'reducing weight felt' rather than as a torque/center-of-gravity balance problem — the sack's weight on the back is unchanged; what changes is the LINE OF ACTION of the combined weight relative to the feet.
• Forgetting that it is the COMBINED center of gravity of porter + sack (not the porter's alone, and not the sack's alone) that must lie over the base of support.
• Assuming any forward lean helps — too little lean leaves the combined CG behind the feet (backward topple risk), too much moves it ahead of the feet (forward topple risk); the porter leans by just enough to center it.