205. =Communication of Motion in Elastic Bodies.=--Momentum is transferred from one body to another very differently in elastic from what it is in non-elastic bodies. As you saw in § 201, when one non-elastic body strikes upon another the momentum is divided between them, and both move on together. Now if _a_ and _b_, Fig. 133, were elastic bodies, as ivory balls, and _b_ should be let fall against _a_, it would give all its momentum to _a_. Therefore _b_ would stop, and _a_ would move on to the same height from which _b_ came. The reason is, that the velocity lost by _b_ and received by _a_ is just double what it would be if the balls were non-elastic. For the same reason, if _a_ and _b_, being elastic, meet each other from equal heights on the arc, they will both rebound, and return to the same heights from which they came. But if non-elastic they simply destroy each other's momentum and stop. The effect produced in the former case is just twice as great as in the latter, as you may see by reckoning on the arc. For the same reason, too, if you have a row of elastic balls, as in Fig. 134, and let _a_ fall from the point _i_ upon _b_, it will stop there; and communicating all its momentum to _b_, this momentum will pass from _b_ to _c_, and so on through all the row of balls to _e_, the last one, which will fly off to the point _h_, at the same height with _i_, the point from which _a_ fell. If _b_ be held still, and _a_ be let fall upon it, _a_ will rebound to the height from which it fell, for then the compressed elastic spring (§ 39) of each ball, as _b_ is immovable, communicates all the motion to _a_. It is for this reason that an elastic ball, on being thrown against any thing fixed, rebounds. If what it is thrown against be perfectly elastic it rebounds with a force equal to that with which it is thrown. [Illustration: Fig. 135.] 206. =Reflection of Motion.=--If an elastic body be thrown perpendicularly upon a surface it rebounds in the same path in which it is thrown. But if it hit the surface obliquely it is thrown off or reflected in a different direction. Thus a ball thrown from _b_ upon _c_, Fig. 135 (p. 158), will return in the line drawn to _b_. But if it be thrown from _d_ it will be reflected in the line _c a_. Now the angle _d c b_, called the angle of incidence, is always equal to _b c a_, the angle of reflection. The same, you will find in other parts of this book, is true of sound and light and heat.