Machines When you tie a stone axe to a wooden handle, you have created a simple machine,

2.1.2 Machines When you tie a stone axe to a wooden handle, you have created a simple machine,

which is a mechanical device that changes the direction or magnitude of a force, and can use leverage or mechanical advantage to multiply the force. There are six classical simple machines: the wheel and axle, pulley, screw, inclined plane, wedge, and lever. The first three involve rotary motion around a central axis, and the other three provide mechanical advantages in linear motion.

Inventions involving wheels and other forms of continuous rotary motion began to appear around 3000 BCE. Road vehicles with wheels are undoubtedly among the greatest inventions in history, as they made possible the easy transporta- tion of people and heavy goods, as well as the waging of wars. Men and women could carry small items on their backs or heads, and larger items could be hung from poles carried between the shoulders of two people. A sledge with a flat bottom or runners that could be dragged across grass, mud, or snow was an improvement, especially when the friction could be lowered by lubrication with water. Another method, important in the construction of the Egyptian pyramids, was to move heavy stones over rolling logs. None of these methods could compare to the convenience of a wheel with a hollow hub rotating around a fixed axle, especially with the appli- cation of lubricating oil or grease in the hub. An obvious early use for carts and wagons was to carry heavy harvests from fields on the farm to living quarters, and to carry farm implements and manure in the opposite direction. Wall paintings in Egyptian tombs show families traveling in ox-carts with solid wheels. Even before 3000 BCE, wheeled vehicles and chariots were used in warfare. War chariots greatly increased the speed of travel and the quantity of weapons that could be carried, in comparison with the infantry, and were such prized possessions that they were buried with powerful kings in royal tombs.

Wheels enable people to make things as well as move them. Some of the most important early uses of wheels include the horizontal grinding stone to mill

53 wheat and corn into flour, and the crane with vertical pulley to lift stones for con-

2.1 TOOLS AND METHODS

struction. The potter’s wheel first appeared in Sumer around 3000 BCE. The centrifugal force of a spinning potter’s wheel throws wet clay outward into a per- fectly symmetrical shape that can be guided by a finger or tool. The wheel disk was probably first marked by tying a string to the center and rotating the loose end with a point to trace out a circle. A better design is to connect the wheel supporting the pottery to a co-rotating base wheel with a vertical post, so that a seated potter can rotate the base with his or her feet. The potter’s wheel diffused from the Middle East to the rest of the world, and became the first machine that was able to mass produce consumer goods.

Fibers from linen and wool must be twisted together to form yarns of greater strength and constant thickness, which can then be woven into fabric. The weaver’s spindle was developed around the same time as the potter’s wheel, and used the wheel to improve productivity. The spindle is a slender rod spun by hand to twist the fibers from a ball of fibers held by the distaff. A small weight called a whorl can

be attached to the spindle, making it easier to spin at a constant speed by hand. Since this work was done by the women in the ancient world, the distaff became a symbol of womanhood and home life. A heavier spinning wheel could be activated by a foot treadle, or by water power.

The carpenter’s screw uses a spiral groove along an axis to combine rotary motion with the mechanical advantage of a wedge, which makes it a stronger fastener than a nail. Archimedes introduced the screw to raise water from a lower level to a higher level with moderate force.

BIOGRAPHY: ARCHIMEDES Archimedes of Syracuse in Sicily (287–212 BC) was the most famous inventor and scien-

tist in the ancient world. He was related to King Hiero II, the ruler of Syracuse on the island of Sicily, and was a wealthy aristocrat with great influence. He was said to have traveled to Egypt and Alexandria to study. He worked on numerous inventions, including every day tools and weapons used against the Romans. The most famous anecdote about Archimedes was reported by Vitruvius about how he discovered the method to determine the density of an object with an irregular shape, such as a crown in the shape of a laurel wreath of King Hiero. Archimedes was to determine whether the goldsmith had added silver to solid gold, which is a heavier substance. While immersing himself in the bath,

he noticed that the level of the water in the tub rose, which can be used to measure the volume of a submerged object and thus, its density. In excitement of his discovery, he took to the streets naked, and shouted “Eureka!” or “I have found it!”

It is said that during the Second Punic War, he produced many war machines that kept the Roman army at bay for 2 years. He made ballistic weapons shooting missiles and stones, he had a crane that can lift Roman ships out of the water, and he built burning mirrors that set ships on fire. However, all his ingenious military engineering was not enough to keep the Romans from breaking into the city walls of Syracuse. He was working on his mathematical problems at home, when a Roman soldier under General Marcellus came to his room and interrupted his work. Archimedes said “Do not disturb my work,” but the enraged soldier killed him.

54 CHAPTER 2 INVENTIONS FOR WORK

FIGURE 2.6 Lever.

The lever was used by Egyptians to move heavy objects, but Archimedes was the first person to write about its role to lift weights too heavy for the strength of unassisted men. A lever is shown in Figure 2.6 to have a fulcrum or pivot that divides

a rod into a longer force arm and a shorter weight arm. If your lever is 10 times longer on the force arm than the weight arm, a force of 1 kg on the long arm can lift a weight of 10 kg on the short arm. This leverage is not magic, as the product of force by dis- tance is fixed. When the force of 1 kg is pushed down by 10 cm, the weight of 10 kg rises by only 1 cm. Archimedes famously boasted, “Give me a lever and a place to stand on, and I will move the earth.” But this is only a figure of speech, as the mass of earth is 100,000 billion billion times greater than that of Archimedes. Where would

he find a sufficiently long and strong lever, and where would he stand? The screw, wedge, and inclined plane all provide mechanical advantages to amplify a small applied force onto a large resulting force. A ramp with a length of 10 and a height of

1 can be used to raise 10 kg of weight by the force of 1 kg. Strong forces are needed to squeeze out all the juice from an olive to produce oil and from grapes to produce wine, which are provided by the screws on the top of olive and wine presses.

The pulley can be used to change the direction of a force, so that you can lift up

a weight by pulling down on a rope with your body weight, shown in Figure 2.7. The upper wheel is fixed and the lower wheel is movable; when the rope is pulled down by

2 cm with the force of W, the lower wheel moves up by 1 cm to pull up with the force of 2W. A compound pulley with several blocks and tackles, set on top of a fixed wall or a movable crane, can also create more mechanical advantages. In the ancient Greek theater, when human affairs became hopelessly tangled, a god would descend from above, suspended by a pulley, to solve their problems. This plot device is called deus ex machina , or “god out of the machine.” Sailors can use compound pulleys to move heavy ships, which is perhaps the source of the legend that Archimedes made war machines that could pull Roman ships out of the harbor and dash them on the rocks.

More complex ancient machines involved simple machines as compo- nents, such as air and water pumps and treadmills. Egyptian wall paintings from 1500 BCE show scenes of metal working, where the fire was enhanced by

2.1 TOOLS AND METHODS

FIGURE 2.7 Pulley. Reprinted with permission # National Portrait Gallery, London.

air bellows operated by the feet of slaves. The bellow had an air intake con- trolled by a valve so that air could only enter but not leave, and an output nozzle also controlled by a valve so that air could leave but not enter. Hephaes- tus or Vulcan was the smith of the gods, and forged the thunderbolt for Zeus as well as the splendid armor for Achilles; book XVIII of the Iliad describes his equipment, which included bellows. Hero of Alexandria described a hydraulic organ for music, where air is propelled by the flow of water.

Rotary motion was needed for a mill to grind corn into flour, and for a hydraulic station to raise water to a higher place. These devices can be powered by human muscle, animal muscle, or wind and water. Our knowledge of ancient machinery comes from a number of distinguished sources. Marcus Vitruvius of Rome (80–15 BCE) wrote 10 books in De Architectura on architectural constructions, on machines used in structures such as hoists, cranes, and pulleys, and on war machines such as catapults and siege engines. Hero of Alexandria (10–70) wrote about the aeolipile—the earliest steam engine—and about a wind wheel that harnessed wind energy.

56 CHAPTER 2 INVENTIONS FOR WORK