EXERCISE 68 



HOW MUCH FORCE DOES THE WIND EXERT 

 AGAINST A TREE? 



Materials. A small tree one or two inches in diameter near the 

 ground ; ax ; rope ; heavy spring balance. Select a tree which is 

 not valuable, as it will be destroyed by the experiment. 



Directions for work. On a windy day cut the trunk nearly 

 through close to the ground, beginning on the side toward 

 the wind, so that the remaining bark and wood will act as a 

 hinge. Do not allow the tree to fall. Tie a rope to the trunk 

 as near the center of the crown as possible. Attach a spring 

 balance to the rope and support the tree against the force of the 

 wind by pulling on the- spring balance. It will be necessary for 

 someone to steady the tree at the same time so that it cannot 

 fall laterally. 



How many pounds force does the wind exert, tending to over- 

 throw the tree ? How was this force resisted by the uninjured 

 tree ? Attach the spring balance to the trunk one or two feet 

 above the ground and note the pull necessary to keep the tree 

 in an upright position. How great strain does the trunk sus- 

 tain at this point? 



Based on your experiment, estimate the pressure of the wind 

 against any large tree in the vicinity, supposing the wind to 

 have the same velocity as it had at the time of the experiment. 

 This estimate will necessarily be only a very rough approxi- 

 mation, but it should give you some idea of the stress to which 

 large trees are subject in time of storm. 



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