MOTION. 



b' 



/ 



of the motion of the earth, which is always revolving. "We do not, therefore, 

 really know any body to be in a state of absolute rest. 



Define uni- 126. A luoving body may have a Uniform 



rl^ble Mouir ^T a VARIABLE MoTION. UNIFORM MoTION is 



the motion of a body moving over equal 

 spaces in equal times. Variable Motion is the mo- 

 tion of a body moving over unequal spaces in equal 

 times. 



What is Ac- 127. When the spaces passed over in equal 

 Regarded Ho- ti^es iucrease, the body is said to possess Ac- 

 *'°°' CELERATED MoTiON ; wheu they diminish, the 



body is said to possess Retarded Motion. 



A stone Hilling through the air is an example of Accelerated Motion, since, 

 acted upon by the force of gravity, its rate of motion constantly increases ; 

 while the ascent of a stone projected from the hand, is an example of Re- 

 tarded Motion, its upward motion continually decreasing. 



What is Power 128. When a body commences to move from 

 MM? ^''"^'' a state of rest, we assign some force as the 

 cause of its motion ; and a force acting in such 

 a manner as to produce motion, is generally termed 

 " Power." On the contrary, a force acting in such a way 

 as to retard a moving body, destroy its motion, or drive 

 it in a contrary direction, is termed Resistance. The 

 chief forces which tend to retard or destroy the motion of 

 a body are Gravitation, Friction, and Resistance of 

 the Air. 



What is ve- 129- The speed, or rate, at which a body 

 locity? moves, is termed its Velocity. 



Moving bodies pass over their paths with different degrees of speed ; one 

 may pass through ten feet in a second of time, and another through a hun- 

 dred feet in the same period. We say, therefore, that they have diflerent 

 velocities. 



The velocity of a moving body is estimated by the time it occupies ia 

 moving over a given space, or by the space passed over in a given time. Tho 

 less the time and the greater the space moved over in that time, the greater 

 the velocity. 



Hov do we 130. To ascertain the Velocity of a mov- 

 veTocfty of^a ^^S ^ody, dividc the space passed over by the 

 moving body? ^^jj^g cousumed iu moving over it. 



