FALLING BODIES. 57 



Recapitulation. In this section we have considered 

 Gravitation ; Facts concerning it ; its Law ; 

 Gravity; Weight; Law of Weight; Centre 

 of Gravity; Equilibrium and Stability of 

 Bodies. 



ECTfON III. 



FALLI NG BODIES. 



118. A Constant Force. The tendency of force 

 is generally to produce motion. Acting on a given mass 

 for a given time, a given force will produce a certain 

 velocity. If the same force acts on the same mass for 

 twice the time it will produce a double velocity. A force 

 which thus continues to act uniformly upon a 

 body, even after the body has begun to move, is 

 called a constant force. The velocity thus produced 

 is called a uniformly accelerated velocity. If a constant 

 force gives a body a velocity of 10 feet in one second, it 

 will give a velocity of 20 feet in two seconds, of 30 feet in 

 three seconds, and so on. The force of gravity is a con- 

 stant force and the velocity it imparts to the falling body 

 is a uniformly accelerated velocity. 



119. Velocities of Falling Bodies. If a 



feather and a cent be dropped from the same height, the 

 cent will reach the ground first. This is not because the 

 cent is heavier, but because the feather meets with more 

 resistance from the air. If this resistance can be removed 

 or equalized, they will fall equal distances in equal times, 



