Why do we lean back when carrying a load in front of us?
We lean back so that the line of gravity may be kept directly over our feet.
Why does a person standing up in a wagon fall when the wagon is suddenly moved?
Because the feet, being in contact with the wagon, move with it before the motion extends to the rest of the body, thus throwing the line of gravity behind the feet; hence, the person must fall, unless supported.
Why does a person standing up in a wagon fall when the wagon is suddenly stopped?
Because the feet, being in contact with the wagon, are suddenly stopped, while the body still moves forward, thus throwing the line of gravity ahead of the feet; hence, the person must fall, unless supported.
Why do animals with four feet stand firmer than those with only two feet?
Because the four feet afford a larger base to stand upon than the two feet do, and, therefore, support the centre of gravity better.
Why can a horse or a cow remain standing for many hours without seeming to be weary?
Because their four feet afford a good support to the centre of gravity.