338 SOME SIMPLE MACHINES IN THE HOME 



But why not use wheels on ice and snow? If you ever 

 tried your roller skates on ice you know the difficulty 

 of getting started. This is because of the lack of sufficient 

 friction. Such smooth surfaces as glass, ice, and well- 

 worn snow have very little resistance to the movement of 

 hard objects over their surfaces. Sled runners have 



many more points 

 of contact with a 

 supporting surface 

 than have wheels ; 

 but because of the 

 very low friction 



Ball bearings. 



between ice and 



smooth steel, steel runners are better for snow and ice 

 than wheels. With skates there are fewer points of con- 

 tact and the friction is sometimes reduced still more for 

 another reason. When skating in moderate weather, the 

 pressure, due to your weight, melts the ice under the run- 

 ners and you really glide over water. This water freezes 

 instantly after the skates have left it. 



The law of machines. Machines do not make energy. 

 They simply transfer or exchange force or mechanical 

 energy from one place to another. We shall see as we go 

 on in our study of science that energy is simply the ability 

 to do work. Sometimes this energy is locked up, as in coal, 

 which gives heat when burned, or it may be evident as in 

 the power in the running stream, or in the waves, or in the 

 wind. By use of a machine we may transfer this energy 

 or power from one place to another, and by means of a 

 machine get work done more effectively. The chief value 

 of machines lies in their enabling us to overcome a large 

 resistant force by applying a small force. But we must 



