PINWHEELS, KITES, AND SAILBOATS 



393 



Experiment. To show centrifugal force. 



Materials: Pail half full of water. A ball fastened to end of string. 



Method and Results: (a) Swing the pail of water rapidly overhead in a 

 large circle. Is there a time when there is no support under the water? 

 Does it fall out ? (&) Whirl the ball on the string. In what directions 

 may it go if you release it when the string is horizontal? When ver- 

 tical? Make diagrams. 



Conclusion: Centrifugal force is that force causing all bodies moving in 

 a curved path to pull away from that path and to go off in a 

 straight line. The force which pulls towards the center and keeps 

 the whirling body in the curved path is called centripetal force. Ex- 

 plain these terms by reference to the above experiments just performed. 



lifting jo 



Pin wheels, kites, 

 and sailboats. An- 

 other common toy is 

 the pinwheel. Most 

 of us know how 

 to cut the pinwheel, 

 and probably every- 

 body in the class 

 knows that the pin- 

 wheel revolves by 

 the force of the air 

 striking against it, 

 causing motion. The 

 difficult thing in kite 

 flying is to adjust the 

 cord at the point of 

 attachment so that 

 the kite balances and 

 w!ll go up readily. 

 Here again we use 

 the force of moving air, and by fastening the kite at 

 an angle to the direction of the wind, we cause it to rise 



Notice that the wind produces a greater lifting force 

 in A than in B, but that in B the greater part of the 

 force of the wind causes horizontal drifting. This 

 results from the angular adjustment of the kite 

 surface. 



