Fig. 27.—MARBLE ROLLING.
820. Why does a marble rebound when dropped upon the pavement?
Because the force of its fall to the earth compresses the atoms of which the marble is composed; and the atoms then exert the force of elasticity to restore themselves to their former condition; and by the exercise of this force the marble is repelled, or thrown upward from the pavement. Although a marble may be made of very hard stone, yet that stone may be elastic, and possess, though in a much less degree, the same kind of elasticity which causes the India-rubber ball to rebound from the earth.
821. Why does a marble, assuming it to be impelled with equal force, roll further on ice than on pavement, and further on pavement than on a pebble walk?
Because the friction is greater upon pavement than upon ice, and greater upon a pebble walk than upon pavement.
822. How many forces contribute to stay the progress of a rolling marble?
The friction of the air, the friction of the earth, and the attraction of gravitation, which tends to bring all bodies to a state of rest.