by Barry Parker
When the siege tower, which was frequently several stories high, reached the wall, a huge battering ram with an iron (or bronze) “bit” was rammed again and again against the wall. It was powered by a large contingent of soldiers. Slowly it would chip away at the wall, and as this took place, fierce fighting would go on between the Assyrians and the defenders. Fire, of course, was a major weapon of the defenders, so the Assyrians had to cover their engine with a huge sheet of animal skins that was kept wet.
As time passed, walls were built thicker and thicker, and eventually stone walls were used. But the Assyrians merely built bigger and bigger siege engines with more effective metal bits. As stone increasingly came to be used for the construction of city walls, it became more and more difficult for the siege engines to batter them down. Nonetheless, they continued to have some success. One of the largest siege engine of ancient times was the Greek helepolis; over one hundred feet high and so stable it could not be tipped over, it far outstripped the scale of Assyrian siege weapons.
Over time, the Assyrian Empire began to weaken. It had collapsed by about 610 BCE.
GREEKS AND THE BEGINNING OF PHYSICS
While the Assyrian Empire began to fade, other nations began to flourish, including Babylonia, the Persian Empire, which lasted to 330 BCE, and Phoenicia, the seafaring state that lasted to about 539 BCE. But the ancient civilization that had the biggest influence on physics was Greece, which consisted of city-states that began coming into power about 800 BCE. Indeed, before the Greeks, there was little if anything that could be called physics, and there was little science in general. Furthermore, the first scientists were not referred to as such; they were referred to as philosophers. But there's no doubt that one of their major aims was to understand the world around them. They were particularly interested in motion and matter. Why did things fall? And what exactly was the role of air, water, fire, and the earth beneath their feet? What was time? And their curiosity extended to the sun, the moon, and the stars. How far away were they? How big were they? Why did they seem to move?
The first science was, no doubt, a form of physics. It was not what we think of as physics today, but it did include many of the same topics. It was drawn from astronomy, mechanics, optics, and areas of mathematics such as geometry. The early Greek philosophers set out to understand the mysteries of the earth and the known universe, and although they arrived at some ideas that may seem strange to us today, they did make important advances. One of the biggest advances was to move away from mythological explanations for the phenomena they observed. Instead, they developed logic and learned to look for reasonable and logical explanations.
One of the first of these philosophers was Thales, who lived from 624 to 546 BCE. He was the first to emphasize the importance of explanations based on reason, and he was particularly interested in why things happened. Because of his contributions he is sometimes referred to as the father of science. He is said to have predicted the eclipse of May 28, 585 BCE. There is some controversy about this, however, as most modern astronomers feel that such a prediction was not possible at that time. But there is no controversy about his most important contribution. At that time Greek sailors never left the sight of land because they had no idea how to navigate when no land was visible. Thales showed them how to use Polaris (the North Star) for navigation. He also studied the strange phenomena associated with magnetism and amber, and he took a serious interest in the phenomena of time and the basic nature of matter.
The two major philosophers who came after Thales, namely Socrates and Plato, were both giants of rational thought, but their interest was mostly in logic, philosophy, and mathematics. Socrates was considered one of the wisest people of his time, but science was not central to his thinking. Plato, a student of Socrates, was probably most famous for his founding of the Academy of Athens.
In 384 BCE, however, the ancient philosopher best known to us was born: Aristotle. He was highly influential in his own era, and he remains influential today. He was strongly interested in science, and he made several contributions, but because his influence has extended over such a long period of time, he is frequently regarded as someone who hindered the development of science. Nevertheless, his goals were admirable. As he stated in his writings, his main aim was to discover principles and causes of change, and not just describe them. Much of what he came up with, however, was erroneous. One of his major hypotheses was that there were four basic elements: earth, water, air, and fire. And he postulated that everything was made up in some way from these four elements. He also had a strong interest in the phenomena of motion, and he classified all motions as either “natural” or “violent.” A falling object had natural motion; a thrown object had violent motion. He also believed that everything beyond the earth—sun, moon, and stars—was made up of a fifth element he called “ether.”5
A number of other Greek scientists of the time also made important contributions. Eratosthenes (276–194 BCE) invented a system of latitude and longitude for the earth. He also calculated the circumference of the earth using the shadows of sticks at different positions. In particular he pointed out that if the earth was flat, there should not be shadows from vertical sticks at different positions at the same time (only one could be shadowless) when the sun was directly overhead. He used his new knowledge to calculate the circumference of the earth to be two hundred fifty thousand stadia (we're still not sure however what a stade is). He also calculated the distances of the sun and the moon, giving us first, but very approximate, estimates.
Another important early Greek scientist was Hipparchus, who was born in 175 BCE. He gave us more accurate measurements of the distances to the sun and to the moon, and he was the first to set up a catalog of most of the visible stars.
Physics first appeared as a result of the studies and speculations of the above philosophers. It's important to note, however, that their contributions came almost entirely from “thought.” Experimental physics was not known at the time, and, indeed, the early philosophers did not perform any experiments in their attempts to prove their ideas. Nevertheless, even at that time they realized that there was a difference between what we call “pure physics” and “applied physics.” Pure physics is usually thought of as the accumulation of knowledge about the physical aspects of the world and universe, such as the basic principles of space, time, matter, motion, and so on, with no thought of how this knowledge should be applied. Applied physics, on the other hand, is the application of this knowledge to assist society in some way. At that time, the main application of physics was the design and manufacture of war machines. Early philosophers such as Socrates, Plato, and Aristotle argued that science should not necessarily have applied goals, particularly applications to war. Knowledge should be accumulated for its own sake.
In spite of the arguments against doing so, it wasn't long before the new discoveries of physics were being used to build new weapons of war. Many of the early advanced weapons built by the Greeks were based on an important physics concept called torsion. In physics, torsion is the twisting of an object due to an applied torque, where torque is a twisting force. And indeed, torsion soon became the basis of new terror weapons that were frequently referred to as machines or engines.
THE NEW WONDER MACHINES
The most common new wonder machines that came out of Greek physics (though not necessarily constructed by the Greeks) were the ballista, the onager, the trebuchet, and various other types of catapults. We talked about siege machines earlier that were used to break through walls; some of the above were also eventually used as siege machines. Let's look at each of them in turn. The ballista was invented by the Greeks and later modified and used extensively by the Romans. It was similar to a giant crossbow, but it used torsional energy that was stored in twisted skeins. Two wooden arms were used to twist the skeins; ropes were attached to one end of each of them, with the ropes extending back to a “pocket” that held the projectile. The ropes were pulled back by a winch. It had
a trigger on it, and when everything was ready to go, the trigger was pulled. Various types of projectiles were used, including stones, darts, shaped poles, and even body parts. It could throw them several hundred yards.6
The ballista.
A variation on the ballista came a little later in the form of the onager, which was used mainly by the Romans. It also used torsion, but was basically a type of catapult. It consisted of a large frame that was placed on the ground. A vertical frame of wood was rigidly attached to it. This vertical frame contained an axis that had a spoke or arm projecting out of it. This spoke was attached to ropes (or a spring) that could be twisted; the arm was pulled back, or armed, against the buildup of torsion in the ropes. Again, there was a pin to release it, and when the pin was struck with a hammer the projectile was launched toward its target. Large stones were usually used as the projectiles.
Soldiers arming an onager.
The third type of new weapon, the trebuchet, was actually the most powerful of the three. It was invented by the Romans and had three main characteristics:
It was not powered by torsion. Its power came from gravity acting on a counterweight.
It used what is called the “fulcrum principle,” where one arm was much longer than the other. The throwing arm was usually four to six times longer than the counterweight arm.
A sling with a pouch was attached to the end of the throwing arm to increase the speed of the projectile.
The device was loaded by placing a large and usually very heavy stone in the pouch. The throwing arm was then pulled down against the weight of the counterweight. It was tied down until ready. When it was triggered, it could throw rocks of three hundred pounds and more a distance of several hundred yards, but it was not nearly as accurate as the ballista or onager.7
The trebuchet and the onager were both a form of catapult. A catapult is a device that usually has an arm that is pulled back against a force and released. Several other types of catapults were also used, but the major ones were the two above. The physics of the above devices will be discussed in the next chapter.
ALEXANDER THE GREAT
One person who made extensive use of the new weapons was Alexander the Great. Born in Pella, the capital of Macedonia in 356 BCE, Alexander became the greatest military leader of his time, conquering most of the known world. Taught by Aristotle beginning at the age of sixteen, he developed a strong interest in science and physics. When he turned nineteen he began accompanying his father, Philip II, on some of his campaigns. Shortly thereafter, however, his father was assassinated, and because his father had multiple wives, and Alexander's mother was only one of them, his chances of inheriting the throne were not good. But he was determined to get it, and he took the necessary steps, killing several people in the process.8
When he became leader he quickly undertook a series of campaigns that lasted for almost ten years. In the end he had conquered Egypt, Mesopotamia, Persia, Central Asia, and even India. And by the time he was thirty he was considered one of the greatest military leaders the world had ever produced.
Aristotle had instilled in him a love of knowledge, and it remained even after he became king. As a result, he created one of the greatest learning centers the world had ever seen. After conquering Egypt, he founded Alexandria in 331 BCE, setting it up as a scientific research center. Although he only stayed in the city for a few days, he left with his viceroy and a general named Ptolemy an outline of the work he wanted done.9 At Alexandria what was called a “mouseion” was set up for the study of engineering, astronomy, navigation, physics, and the machines of war. The best scientists in the country and in surrounding countries were then invited to study there, including Eratosthenes and Hipparchus.
Perhaps the greatest feature of the new Mouseion at Alexandria was its library. It eventually became the largest library in the world, housing over seven hundred thousand manuscripts. It thrived for centuries, but much of it was eventually destroyed by fire.
ARCHIMEDES
One of the men who studied at Alexandria was Archimedes, who was born into 87 BCE in Syracuse, Sicily. He made a large number of contributions to physics, one of the most important of which was a principle now referred to as Archimedes’ principle. It states that a body immersed in a fluid experiences a buoyant force that is equal to the weight of the fluid it displaces. He also designed what is now called Archimedes’ screw. According to early accounts, the king of Syracuse commissioned Archimedes to design a large ship, but it was soon discovered that a considerable amount of water was leaking into the hull, and it was difficult to bail it out. Archimedes designed a machine with a revolving screw-shaped blade inside a cylinder that raised the water from the bottom of the hull as it was turned.10
Archimedes was also one of the first to explain the principle of the lever. And he is reported to have helped the people of Syracuse when they were attacked in 14 BCE. Presumably, he set up large curved mirrors that reflected the rays of the sun upon the attacking ships, causing them to catch fire. Most modern scientists doubt that this was possible at the time.
Physics is related to the early weapons of war just as it is to the more sophisticated later weapons. So far we have talked mostly about chariots, men on horses, bows and arrows, spears, and such things as the ballista, the onager, catapults, and trebuchets. Physics is involved in all of these things, but we haven't shown how it is involved. In this chapter we will do this, but first we will discuss the basic concepts of physics, beginning with the most basic ones, such as speed and acceleration, and proceeding through to more complicated ones, such as energy and momentum.
VELOCITY AND ACCELERATION
Everyone knows that if you shoot an arrow into the air it rises to a certain point before falling back to earth. It's also known that its speed as it leaves the bow depends on how hard the string pushes on it, and it's easy to see that its speed throughout its flight is not the same. After all, if you shoot it straight upward, it stops at some point before it starts to fall back to earth.
We have a slight problem in relation to motion on earth, however. Every object that moves has to pass through air, and this air has an effect on its speed as well as the path it takes. Dealing with the effect of air, however, is rather complicated, so for now we will ignore it.
The first thing we can say about an object in motion is that it has a certain speed relative to the surface of the earth. Speed is a useful concept, but even better (as far as physics is concerned) is velocity. Speed is defined as the distance something travels in a unit of time, say, a second, or even in hour. An arrow, for example, can have a speed of fifty feet per second. The problem with this is that it doesn't tell us anything about the direction that the arrow is traveling. If we specify both speed and direction, we have velocity. The velocity of the above arrow, for example, might be fifty feet per second in a northern direction.
If we look at this arrow a little closer, however, it's easy to see that it doesn't have a constant velocity. Its velocity is continually changing, and the biggest change will occur when it is shot directly upward. After all, it stops at its highest point. We refer to this change in velocity as acceleration. The arrow might leave the bow with a velocity of fifty feet per second, but a few seconds later it will be going only ten feet per second. Acceleration is clearly different than velocity, and it therefore needs a different unit. The unit in this case is feet per second squared (in the metric system it is meters per second squared). Velocity and acceleration are related by a simple formula: velocity (v) equals acceleration (a) × time (t), or more simply v = at.1
FORCE AND INERTIA
Closely related to velocity and acceleration is another important physics concept called force. For an arrow to gain speed—in other words, to accelerate—it must undergo a force, and as I mentioned earlier, it is the string of the bow that applies the force to the arrow. A force is simply defined as a push or a pull. And force is like velocity in that it has both magnitude and direction (we refer to such a quantity a
s a vector).2
We can relate force to acceleration, but before we do, let's introduce another important concept from physics. Everyone knows about weight, and how it seems to creep up on you when you eat too many chocolates. What we're interested in is closely related to weight, but it's not exactly the same. We refer to it as mass, and we abbreviate it as m. The mass of an object is its weight divided by the acceleration of gravity, which is usually abbreviated as g. I'll explain a little later why we need mass rather than weight.
The relationship between force and acceleration was given by the English physicist Isaac Newton. He included it in three laws of motion that he published in his Principia in 1687. He explained that an acceleration created by a force acting on a body is directly proportional to the magnitude of the force and inversely proportional to the mass of the body. We can write this an algebraic form as a = F/m. As it turns out, it is more convenient to use metric units in this formula (instead of the units you are probably more familiar with, namely feet, miles, and so on, which are units in what is called the British System). Within the metric system, however, we have two systems, referred to as cgs (centimeter, gram, second) and mks (meter, kilogram, second). In the mks system, acceleration is measured in meters per second squared, mass is measured in kilograms, and the unit of force is the Newton. In the cgs system, acceleration is measured in centimeters per second squared, mass is measured in grams, and the unit of force is the dyne, which is the force required to cause a mass of one gram to accelerate at a rate of one centimeter per second squared.