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HOW ARE MACHINES USED IN EVERYDAY LIFE? (XV-1)
The problem. The more intelligent races have developed the use of simple machines and combinations of them in the performance of work. In modern life almost no work is done without the aid of some mechanical device. The simple machines that are
commonly used are the lever, the pulley, the inclined plane, the wheel and axle, the screw, and the wedge. Where are these found in the community and how are they used ?
What to use. The illustrations of the six simple machines shown in figure 30 and community applications of the same.
What to do. 1. Study the illustrations and applications carefully to understand the essential parts of each machine.
2. Prepare lists of the devices that you have seen which use the principles shown in the illustrations.
Questions. Are all the levers alike? What are the advantages and disadvantages of pulleys ? What advantage is gained with the inclined plane? Is there any disadvantage in the use of the wheel and axle? Is there any similarity between the screw and the inclined plane ? What advantage may be gained with the wedge? Which machine is most common in your community ?
Suggestions for report. Make a drawing of one illustration of each machine from
list. Reference work. Read sections 186 to 191.
Optional problems. Design and construct a model inclined plane, a wheel and axle, or any machine that you have studied.
PULLEYS AND PULLEY SYSTEMS (XV-2)
The problem. The pulley appears in everyday life in a variety of forms. It may be the small wheel for a window weight or shade, or the large wheel for a derrick or elevator. It may be one pulley or a group of several with a single rope running over all. In every case the pulley or the combination of pulleys is used in order to secure a distinct advantage that cannot be easily secured in any other way.
In order to use pulleys intelligently we ought to understand what the advantages are when used singly or in groups. How can this be done?
What to use. Ring stand, ring, spring balance, weight, single pulley, cord, meter stick, and burette clamp.
What to do. 1. Set up the apparatus as shown in figure 31. Arrange the load so that it is
Fig. 31 supported by one cord. Weigh the spring balance itself and add this weight to the readings indicated by the pointer. What is the smallest reading that can be made on this balance ?
2. Raise the balance slowly while holding the hook and note the reading. Lower the balance slowly and note the reading. Take the average of these values. Record all measurements on scrap paper for use in class discussion.
3. Next raise the load vertically 20 centimeters and note how far the force moves.
4. Arrange the apparatus so that two cords support the weight. It will not be necessary to add the weight of the balance to the scale reading, since the cord is fastened to the scale hook. Take a set of readings as suggested in 2 and 3. Complete the calculations given below under " Suggestions for report."
Questions. What advantage is gained in using a pulley with a single cord ? Compare the force and load, the distance of the load and force, the work done in each case. Answer the preceding questions for 4. Compare the work done in the two systems. Account for the fact that more work is put into the machine than is obtained from it.
Suggestions for report. After a class discussion of the work, correct all mistakes and then record the measurements. Make a set of calculations for the experiment suggested in figure 91 of the text. This exercise should be demonstrated by the instructor.
LOAD X FORCE X
= A; B
Reference work. Read sections 190 to 197.
Optional problems. Arrange a pulley system in which three cords support the load, and study it as suggested above. The approximate efficiencies of various machines are as follows:
Reciprocating steam engines
17 21 34 90
Can you make a bar diagram to show these facts graphically?