74 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 44. 



of the Science of Projectiles, or that department of 

 Natural Philosophy which considers the motion of bodies, 

 thrown or driven by an impelling force above the surface 

 of the earth. 



What iaa Pro- 1^^- ^ PROJECTILE Is a body thrown into 

 jectiie? ^]jQ g^^j. jjj ^j^y direction ; as a stone from the 

 hand, or a ball from a gun, or cannon. 



Wh f th d" If '"'6 project a body perpendicularly downward, or upward, 



rection of a it will move in a perpendicular line with a uniform accelerated 

 ^°^y tl'rown Qj. retarded motion, since the force of gravity and that of pro- 

 jection are in the same hne of direction. But if a body is 

 thrown in a direction oblique to the perpendicular, it is acted upon by two 

 forces,* the projectile force which tends to impel it forward in a straight line, 

 and the force of gravity, which tends to bring it 'o the earth. Instead, there- 

 fore, of following the direction of the projectile force, the path of the body 

 will be a curve, the resultant of the two forces. Such a curve ia called a 

 Pakabola. 



If a cannon-baU is fired Irom A to- 

 ward B, Fig. 44, in an upward direction, 

 instead of moving along the line A B, 

 it is, by the influence of the earth's at- 

 traction, continually drawn downward, 

 and its path is along a line which is in- 

 dicated by the parabolic curve A C ; 

 and although it has been moving on- 

 ward from the impulse it has received 

 from the force of the gunpowder, it oc- 

 cupies exactly the same time in falling 

 to the point C, as if the ball had been allowed to drop from the hand at A, 

 and faU to D. 



159. If a ball be projected from the mouth 

 of a cannon in a horizontal direction, it will 

 reach the earth in precisely the same time as 

 a ball dropped from the mouth of the gun. The force 

 of gravity is neither increased or diminished by the force 

 cf projection. 



T)ie same fact may be strikingly illustrated by placing a number of marbles 

 at unequal distances from the edge of a table and sweeping them off with a 

 ruler, or stick : those which are rolled along the farthest will be projected tbo 

 farthest; yet all wiU strike the floor at the same time. 



♦ The theoretical laws governing the motion of projectiles, as herewith given, are In 

 practice esaentially modified by the resistance of the air. 



What effect has 

 the projectile 

 force on the 

 action of grav- 

 ity? 



