Conservation of Energy 



79 



When anything starts to move, it does so by pushing 

 on something else. When your arms start to move up, 

 they do so by pushing your body down a little. When 

 you swim, you push the water back and down with your 

 arms and legs, and this pushes your body forward and 

 up. When a bird flies up into the air, it pushes its body 

 up 1)y beating the air down with its wings. WTien an 

 airplane whirs along, its propeller fans the air backward 

 all the time. Street-car tracks are kept shiny by the 

 wheels, which slip a little as they tend to shove the 



FIG. 39. As he starts to toss the ball up, will he weigh more or less? 



