Try to stand up

You'd think I was joking if I told you that you wouldn't be able to get up from a chair provided you sat on it in a certain way, even though you wouldn't be strapped down to it. Very well, let's have a go. Sit down on a chair in the same way the boy in Fig. 13 is sitting. Sit upright and don't shove your feet under the chair. Now try to get up without moving your feet or bending forward. You can't, however hard you try. You'll never stand up until you push your feet under the chair or lean forwards. Before I explain, let me tell you about the equilibrium of bodies in general, and of the human body in particular. A thing will not topple only when the perpendicular from its centre of gravity goes through its base. 







The leaning cylinder in Fig. 14 is bound to fall. If, on the other hand, the perpendicular from its centre of gravity fell through its base, it wouldn't topple over. The famous leaning towers of Pisa and Bologna, or the leaning campanile in Arkhangelsk (Fig. 15), don't fall, despite their tilt, for the same reason. The perpendiculars from their centres of gravity do not lie outside their bases. Another reason is that their foundations are sunk deep in the ground.


You won't fall only when the perpendicular from your centre of gravity lies within the area bound by the outer edge of your feet (Fig. 16). That is why it is so hard to stand on one leg and still harder to balance on a tight-rope. Our "base" is very small and the perpendicular from the centre of gravity may easily come to lie outside its limits.  Porters who carry loads on their heads are well-built a point, I presume, you have noticed. You may have also seen exquisite statues of women holding jars on their heads. It is because they carry a load on their heads that these people have to hold their heads and bodies upright. If they were to lean in any direction, this would shift the perpendicular from the centre of gravity higher than usual, because of the head-load, outside the base and unbalance them. 

Back now to the problem I set you at the beginning of the chapter. The sitting boy's centre of gravity is inside the body near the spine about 20 centimeters above the level of his navel. Drop a perpendicular from this point. It will pass through the chair behind the feet. You already know that for the man to stand up it should go through the area taken up by the feet. Consequently, when we get up we must either bend forward, to shift the centre of gravity, or shove our feet beneath the chair to place our "base" below.