"Train up a child in the way he should go; and when he is old, he will not depart from it."—Proverbs xxii.
814. Why does a top stand erect when it spins, but fall when it stops?
Because the top is under the influence of, and is balanced between opposing forces. The rapid rotation of the top gives to all its particles a tendency to fly from the centre. If the atoms of the wood were not held together by the attraction of cohesion, they would fly away in a circle outward from the top, just as drops of water fly off from a mop, while it is being twirled. If you take a spoonful of sand, salt, or dust, and drop it upon the top, it will be scattered in a circle, just as the atoms of the top would be, if they were free to separate, but not with the same force, because the atoms of the salt, &c., not being in an active state of rotation, would only be influenced by momentary contact with the rotating body. This tendency of the particles of a rotating body to fly outward from the centre, is called the centrifugal force.
Centrifugal.—From two Latin words meaning receding from the centre.
The other force influencing the top is the attraction of gravitation: the attraction which, were the top not spinning, would draw it towards the earth. The "spill" projecting from the bottom of the top stands in the line in which the top is drawn towards the earth and keeps it from obeying the law of gravitation. Therefore the rotatory motion given to the top, by the rapid unwinding of the string, and the tendency of its atoms to fly outward, balance the top upon the line in which it is drawn to the earth, and which is occupied by the spill, which prevents it falling to the ground.
815. Why does a top first reel around upon the spill, then become upright, and "sleep," and then reel again, and fall?
Fig. 25.—PEG-TOP "REELING."