FRICTION 65 



EXERCISE 62 

 FRICTION 



Apparatus and Materials. Book, flatirons, spools or dowels, wheel- 

 barrow or cart, machine oil, bicycle. 



a. Lay a book upon the table, and push it over the table. 

 What force must be overcome? Does the degree of smoothness 

 of the book cover and the table have anything to do with the 

 force required? Now put some flatirons or other heavy weights 

 on the book, and try the experiment again. What is the result? 

 Put under the book and its weights four spools, or two round 

 dowels (cf. Exercise 59), and try moving the book. Suggest 

 why " rolling" friction is less for a given weight than " sliding" 

 friction. 



b. Try spinning, as hard as you can, the wheel of a wheel- 

 barrow or cart that has not been oiled recently. Count the 

 time during which it continues to turn. Now lubricate ("oil") 

 the bearings with machine oil, and find for how long the wheel 

 continues to turn. Give the cause of any difference that you 

 notice. 



c. Why is oil put on "oil stones" that are used for the 

 sharpening of cutting tools? Have some one take off a bicycle 

 pedal for you, and examine the "ball bearings." Why are they 

 used? 



EXERCISE 63 

 APPLIED FORMS OF SIMPLE MACHINES 



Apparatus. For examination: bread mixer, egg beater, self- 

 sealing fruit jar, ordinary fruit jar, skates, sewing machine, bicycle, 

 typewriter, clothes wringer, washing machine, screw eye, "crosscut" 

 and "rip" saws. 



a. Examine a bread mixer. How many simple machines can 

 you find about it? 



