20 



FORCE AND ENERGY 



If we remove a body from the earth at any given place, that is, if we 

 take it "up in the air," or on a mountain top, it will also lose in weight. 

 The mass of the body will, of course, remain the same everywhere. 



21. Falling Bodies. We know that if we drop a stone 

 it falls "straight down.' 7 We have also seen bricklayers 

 using a string with a weight attached 

 a plumbline to be sure that they were 

 making a wall vertical. The position taken 

 by the plumbline, like the path of the 

 falling ball, shows that gravity pulls ver- 

 tically downward. 



But objects that are very light, like 

 feathers, seem to fall more slowly than 

 heavy objects. Why is this? Galileo, 

 dropping balls of different sizes and differ- 

 ent materials from the "leaning tower" of 

 Pisa (Fig. 15), insisted that all objects, 

 heavy and light, if let fall from the same 

 height, should reach the ground at the 

 same time. A feather and a bullet would 

 fall at the same rate were it not for the air, which resists 

 being pushed out of the way. In a tube free from air, that 

 is, in a vacuum (Fig. 16) they do fall at the same rate. 



On a very windy day even a heavy body may not fall straight down. 

 Thus an apple blown off the tree by a sudden gust will go in the direc- 

 tion of the wind (horizontally) and also downward. It cannot go in 

 either of these directions alone, so it goes down in a curved path. But 

 it will reach the ground by the longer, curved path in the same time 

 as if it fell vertically to the ground. This fact may be illustrated by 

 two marbles (Fig. 17) one of which is given a horizontal blow, while 

 the other is permitted to fall vertically from the same height. 



FIG. 16. 



Bodies Falling in a 

 Vacuum. 



