44 FORCE AND MOTION. 



cushion of a billiard-table, is an example of reflected 

 motion. 



97. Law of Reflected Motion. The angle in- 

 cluded between the direction of the moving body before it 

 strikes the reflecting surface and a perpendicular to that 

 surface drawn from the point of contact, is called the angle 



of incidence. The angle between the direction of the 

 moving body after striking and the perpendicular, is called 

 the angle of reflection. TJie angle of incidence is 

 equal to the angle of reflection, and lies in the 

 same plane. A ball shot from A will be reflected at B 

 back to O t making the angles ABD and CBD equal. 



EXERCISES. (Answers to le written.) 



1. Represent graphically the resultant of two forces, 100 and 150 

 pounds respectively, exerted by two men pulling a weight in the 

 same direction. Determine its value. 



2. In similar manner, represent the resultant of the same forces 

 when the men pull in opposite directions. Determine its value. 



3. Suppose an attempt be made to row a boat at the rate cf four 

 miles an hour directly across a stream flowing at the rate cf three 

 miles an hour. Determine the direction and velocity of the boat. 



4. A ball falls 64 feet from the mast of a moving ship to the 

 deck. During the time of the fall, the ship moved forward 24 ft. 

 Represent the actual path of the ball. Find its length. 



5. A sailor climbs a mast at the rate of 3 ft. a second ; the ship is 



