In all cases of ratios like W/f in this expe experiments, work out the values by actual di necessary comparisons can be readily made. this case compare W/f with the number of cor EXPERIMENT NO. 114 COEFFICIENTS OF FRICTION. References: Stewart, Physics, Sect. 105-107; Physics, Sect. 74-77; Duff, College Phy Spinney, Text-Book of Physics, Sect. 71 It is the purpose of the experiment t coefficients of starting and sliding friction m certain materials, and to prove that starting than sliding friction. Tests are also made to ence of friction on nature of the materials in c the area of the rubbing surfaces. The bed may be a smooth metal plate or planed and sandpapered. A pulley is fasten the bed. Blocks faced with different material See that the surfaces are all wiped clean block on the bed and run a cord from the bloc to a scale-pan. Be sure that the cord runs para (a). Set a weight of one kilogram on th fully place weights in the scale-pan until the b move. No jar must be given to the block. M Repeat with loads of 2, 3, etc., up to 6 kilogra the same block resting on a face of smaller are block faced with another material, in each c names of the materials in contact. If W is th and load combined, and F is the weight of s tents which just starts the block moving, ther efficient of starting friction. Compute the coef trials. (b). Sliding friction. Repeat all the tests of (a) except that weights are now added to the scale-pan until a slight shove on the block is sufficient to produce a slow, uniform motion. Compute the coefficients of sliding friction for the same materials used in (a). (c). Remove the cord and place a single heavy weight on the block. It may be necessary to tie it in place. Tilt the bed slowly till the block just begins to slide, then measure the height and base of the triangle formed between the bed and the table. Make several trials with each combination of materials used in (a) and (b) but with only the one weight in each case. The coefficient of starting friction in this case is the ratio of the height to the base of the triangle, i. e. the tangent of the angle of tilt of the bed. Compare with results of (a). Do your tests show that starting or sliding friction is the greater? What is the effect of area change? Compare coefficients obtained with those given in tables of physical constants. Tabulate under following heads: Weight of Weight of Weight on Weight on FW F/W Block Scale-pan Block Scale-pan EXPERIMENT NO. 115 THE SIMPLE PENDULUM. References: Stewart, Physics, Sect. 55, 119; Kimball, College Physics, Sect. 101, 128; Duff, College Physics, Sect. 67, 101; Spinney, Text-Book of Physics, Sect. 49. The purpose of this experiment is to study the relation existing between the length and period of a simple pendulum and to compute the acceleration due to gravity. Suspend a small metal sphere from a rigid support by means of a fine wire or a thread. For the first trial make the pendulum 40 cm long. The length of this simple pendulum is the distance between the support and the center of the ball. Adjust the length carefully. The period of the pendulum is the time required for the pendulum to swing from one end of the path to the other end and back again, or the time elapsing between two successive passages through the central point in the SAME direction. To determine the period, we may measure the time required for a watch. If no stop-watch is to be had, an ordin second-hand or the laboratory clock may be u piece. It must not be thought that the use of sures accurate timing. The tendency is to pre late at the beginning and too soon at the end with an ordinary watch, a careful observer can r second, but his partner must count vibration the beginning and ending of a run by some a the tap of a pencil. Make three determinations of the time for brations and from the mean value calculate t pendulum. In the same manner find the period of the it has lengths of 80 cm, 100 cm, 120 cm, and 16 From the equation for the period of a s Work out the actual values of the quotients and compare the results for the different per product of two variable quantities is a constant inversely proportional to the other, but if the stant, one is directly proportional to the oth dulum has twice the period of another, how c the pendulums compare? The purpose of this experiment is to determine the gravinal acceleration by means of a simple pendulum, using the hod of coincidences. In Experiment No. 115, the same deination was made in a rough way; this method permits h greater accuracy. A large heavy pendulum, of which the time of vibration been accurately determined, is mounted near a small simple lulum. The first part of the experiment consists in detering the period of the simple pendulum by comparison with large, standard pendulum. This large pendulum may be of the laboratory clock, if its period has been accurately rmined. An electric circuit is so arranged that a buzzer is sounded never both pendulums are at rest and making contact with ably placed mercury cups. Suppose both pendulums are ating, and suppose that one pendulum vibrates faster than other. Then, after a time, there will be an instant when both dulums are passing through their positions of rest at the etime. The buzzer then sounds. The faster pendulum, ever, soon gets ahead of the other, and not until it has ed a half vibration will the buzzer again sound. There are ain advantages to be obtained by using a telegraph sounder telephone receiver in place of the buzzer. The time between successive coincidences is observed. We can compute the ber of vibrations the standard pendulum has made in this , and since the unknown pendulum has made a half vibramore (or less) than the standard, its period may be acculy calculated. Start both pendulums vibrating through small arcs-4 or 5 The simple pendulum is made to swing so that it passes ugh the mercury contact at every vibration. Light sidewise with a pencil accomplish this result. When the buzzer first ds, write down the time-hour, minute, and second. Owing he fact that the mercury contact is rather broad, several second and fourth coincidences, is the time pendulum to gain a complete vibration on the time, in seconds, t. Then t divided by the perio should give the number of vibrations made by this time. (If, owing to experimental error, a not obtained, take the nearest whole number tained.) The simple pendulum has made on vibration than this. To determine whether or not it has made than the standard, start both pendulums swing observe directly which one gains on the other the simple pendulum may be calculated by div quired for the standard pendulum to make the of vibrations by the number of vibrations ma pendulum during the same time, P = nT n+1 number of vibrations of the standard pendulum With a meter rod measure the distance fr the simple pendulum to the top of the ball. Me of the ball with a pair of calipers and determ Call the distance from the knife-edge support t ball, d. Then the length of the equivalent (neglecting the mass of the wire) is, |