K.U IN i\ .&.:> 



661 



along a level lawn, you must have noticed that the force 

 you applied is distributed in two directions. You push 

 the lawn mower forward, but you also push it downward. 

 This downward motion is applied in increasing the pres- 

 sure of the wheels against the ground, thus causing them 

 to turn around without sliding. The horizontal force 

 is used in moving the wheel forward, and also in turning 

 the wheel. If you turn a lawn mower upside down so that 

 the revolving knives do not hit the grass, you will notice 

 that it runs more easily. A large part of your energy, 

 then, goes in moving these knives, which are retarded by 

 pressure against the grass and 

 the lower part of the lawn 

 mower. If two boys work to- 

 gether in cutting a lawn, the 

 work can be made much lighter 

 by one boy fastening a long rope 

 in front of the mower and pull- 

 ing. Can you explain why? 



Wheels and runners. Why 

 do our wagons have wheels and 

 our sleds runners? Perhaps you know that when we 

 move one body over another, a certain amount of re- 

 sistance known as friction must be overcome. Have you 

 ever pulled with all your might to drag a heavily loaded 

 box over the floor and then placed one or more rollers, 

 such as can be made from a broom handle, under the box 

 and found how easily it could be pulled? In much the 

 same way a load on a wagon is moved over the bare road 

 more easily than the same load on a sled. The sled runner 

 slides over the road surface, while the wagon wheel rolls 

 over it; and sliding friction is greater than rolling friction, 



H.-WHIT. CIV. SCI. IN THE HOME 22 



The push () is divided into two 

 parts: force A downward, which 

 gives the necessary friction for the 

 wheels to turn, and force C, which 

 rotates the blades and moves the 

 mower forward. 



