F. A. Coolidge

Before proceeding further with the study of machines we must see what influence friction has upon their action, and what means must be employed to make it as small as possible. Friction may be defined as the resistance a moving body meets as it passes over the surface of another body. It is largely due to the roughness of the surfaces in contact, but some of it is due to adhesion. In the study of the levers no mention was made of friction, as it played so little a part in their action; but in the other machines we must know how much of the force employed in moving a body is lost through the rubbing of the surface in contact. Machinery must be made with a view either to make use of friction, or to remove its influence so that as little of the force as possible shall be lost.

For the simple experiments we are to perform we need a spring balance with which to measure every force exerted. One graduated from 1/2 ounce to 64 ounces can be bought for 35 or 40 cents. As this is to be used for horizontal forces, a res; to hold it should be made from a block 2"x3"x4". A hole 1 1/4"indiam-

Elementary Mechanics II Friction 110

Fig. 4

Elementary Mechanics II Friction 111

Fig. 5.

eter should be bored through the block lengthwise and the block then sawed lengthwise. After we have smoothed down the sharp edges the rest for our balance will be 4" long, 2" wide, 1 i" high, and it will have a trough running lengthwise to hold the spring balance. An sectional view is seen in Fig. 6. Next we will take an inch board 4' long, 6" wide and plane it as smooth as possible on one face, leaving the other face rough. We will call this board A. A block B 3" x 4" x 1" with a strip 1/4" x J" fastened around the edge of one side, and fitted with a screw hook at one end. completes the apparatus needed. See Fig. II. Experiment VI. Place block B upon the rougher surface of board A and connect the hook of the block with the hook of the balance by means of a thread. Draw the block along the board several times noticing carefully the force used each time. Do not record the force necessary to start the block, but observe that is is always more than the force required to keep it in motion after it is once started. The average of six or eight trials will give us the force needed to pull the block along the board. Weigh the block carefully and divide the force used by the weight moved. The quotient F - W is called the coefficient of friction. Experiment VII.

Repeat experiment, I using the smooth face of the board. We shall find the force needed to move the block smaller than before. By making the surface in contact as smooth as possible, much of the friction is removed. Calculate as in experiment VI the coefficient of friction, which should be less than .4. Experiment VIII.

The block B has a base 3" x 4" or 12 square inches. One of its sides is 4" x 1 J" or 5 square inches. Pull the block B, resting on its side, along the board A and find, as before, the average of six or eight trials. We ought to get the same results as in experiment VII, and should see the truth of the next law of friction, viz., that the size of the surfaces in contact does not determine the amount of friction, although we might expect the friction would be more when the larger faces are touching each other.

Experiment. IX.

We will now make a series of trials in which block B shall carry different weights and exert an increasing pressure upon the board A. We can again use the weights made for the experiments in levers, or, if we have not made these, we must provide a number of small tin boxes and by filling them to different depths with sand we can obtain weights that will answer our purpose very well. A careful record of all our experiments, with a statement of the results obtained and inferences made, will make the value of these experiments much greater. Let us arrange the ex-periment in tabular form.

Weight moved

Force used

F W =

oeflicient of Friction.

8 oz.

2 oz.

2 8 =


16 oz.

32 oz.

48 oz.

64 oz.

The figures under force and coefficient are possible figures. In each case the average of several trials hould be made. In studying the figures obtained it is plainly seen that the force increases almost exactly in proportion to the weight, also that F - Wis the same for the five trials. These verify the laws that friction increases with the weight and that for two given substances the coefficient is the same. That other bodies will have a different coefficient may be seen by repeating experiment VII using a pane of glass instead of board A. Another way of reducing friction is the substitution of rolling motion for sliding motion. For this purpose three good, smooth lead pencils may be used under block B.

Experiment X.

Lay the pencils across board A with the block B resting on them. Compare the force needed to move B over these rollers with the force used in making it slide along the board. The force seems ridiculously small. Calculate the coefficient. Can it be a fraction so small ? Repeat with other weights and calculate as before.

From the experiments performed we see why so much pains is taken in making a railroad, and we now see how it is possible for a force of eight or ten pounds to move a ton weight. Now we know why the axles and bearings of wagons and machinery are made so smooth, and why oil, grease, and graphite are used so freely. The ball bearings of bicycles and other machines seem to be the height of man's achievments in preventing wasted work.

We must look for a moment at the other side of the question and see mischief would result if there were none. Walking would be impossible. Cars and wagons would not move, or the wheels would turn but no advancement be made. We oil the axles and sand the tracks; we oil the bearings of machinery to prevent and put something on the belt to create friction. In some cases, we need more than we have, and in others we try to remove what we do not need. In conclusion, we must acknowledge friction useful in its place, but worse then useless where we do not need it.