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4 main forces

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Look around you: everything you see shows that The Force is with us. Or, to be more precise, its four descendants, in the form of a quartet of fundamental forces spawned moments after the big bang.

Two of the four are familiar enough: gravity, which weighs us down on the Earth, and electromagnetism, which keeps compass needles pointing north and paper sticking to charged-up balloons. The two other forces are less familiar, but no less important to shaping our world: the strong nuclear force, which binds together the nuclei of atoms, and the weak force, which causes nuclei to disintegrate.

In their present guise, each of the four appears utterly unlike any other: they each rule different domains, and have radically different strengths - the strong force living up to its name by being more than a trillion trillion trillion times stronger than gravity, the weakest of the four. Yet physicists now believe these differences mask a deeper unity, one that existed for just a fleeting but crucial instant at the birth of the universe.

Current theories predict that in the searing heat of the big bang all four forces were unified into a single "superforce" of unimaginable strength. While the superforce itself may never be recreated, its existence has been hinted at in experiments that can reach energy levels that existed in the early universe. This is possible using colossal accelerators that smash together subatomic particles at speeds close to the speed of light - thus mimicking conditions close to the big bang.

In experiments conducted with accelerators like the Large Hadron Collider on the Swiss-French border, physicists have been able to wind back the clock to one hundred-billionth of a second after the big bang, and study the behaviour of some of the fundamental forces. The results show changes in the forces of just the kind expected if they all turn into a single, unified superforce at some ultra-high energy far beyond our reach.

The era of the superforce may have long since ended, but we are still left with its shattered remains, which are now the subject of intense research.
Gravity
The most familiar of the fundamental interactions, gravity will forever be associated with the story of Sir Isaac Newton and the falling apple. Whether watching the apple fall really did inspire him to formulate his law of gravitation (as he himself claimed), there is no doubt that he was the first to recognise the universal reach of this force. According to his law, every mass in the universe exerts an attractive force on every other, its strength increasing in proportion to the masses involved, and decreasing with the square of their distance apart.

As a mathematical rule, Newton's law is extraordinarily effective. It allowed Newton and his contemporaries to explain otherwise baffling laws of planetary motion and predict the return of comets, and is still used to calculate the paths of spacecraft. Yet for all its successes, it does nothing to cast light on the real nature of gravity. Newton himself refused to speculate, airily declaring: "I frame no hypotheses ... to us it is enough that gravity really does exist."

Privately, Newton believed it was a manifestation of God's all-pervading spirit, which was hardly more helpful. A more concrete view of gravity emerged with the publication, in 1915, of Einstein's theory of general relativity.

According to Einstein, space and time are not a static arena in which events take place, as envisaged by Newton. Rather, they are more like some form of cosmic fabric that can be curved and warped by the presence of mass.

Einstein's theory gives a precise account of how much distortion in the fabric of space-time is produced by a given amount of mass. And it shows that what we think of as the "force" of gravity is really a kind of illusion created by this distortion in space and time. When the concentration of mass in one place is relatively low - as it is on the Earth - Einstein's theory gives results in precise agreement with Newton's famous law. But as the concentration increases, the theory shows that the fabric of space-time can curl up completely, forming what we now call a black hole, from which not even light can escape.

General relativity is more than just a more sophisticated way of describing gravity, however. Unlike Newton's theory, it accurately describes the effects of intense gravitational fields around planets, stars and black holes, and has so far survived every experimental test of its predictions. Yet even Einstein's theory reaches its limits at the big bang, giving nonsensical values for the temperature, gravitational field and density of the universe.

This has led to the search for a successor to general relativity, which includes the crucial ingredient it lacks: the laws of the subatomic world in the form of quantum theory. Marrying these two theories is the greatest challenge facing theoretical physicists. But if they succeed, the result will be the so-called theory of everything, which will encompass all from the smallest subatomic particle to the entire universe.
Electromagnetism
Of the four forces, electromagnetism is the best-understood - and the most important in everyday life. Unlike the others, it can be controlled and directed, which has put it at the heart of our technological world. It is also the force at the heart of all matter, binding charged particles together and thus making possible the existence of atoms, molecules and life itself.

The practical value of electromagnetism has been recognised for millennia. Archaeologists in the Middle East have found 2,200-year-old electric batteries made from electrodes immersed in vinegar, while naturally magnetic lodestone, which makes an effective navigation compass, was mentioned by the Greek scholar Thales in the 6th century BC.

Understanding electromagnetism has taken the combined genius of some of the greatest scientists of the last 200 years. Michael Faraday showed how to turn electromagnetism into motion and vice-versa through his invention of the electric motor and the dynamo - the latter still being the basis of electric power generation.

The Scottish theorist James Clerk Maxwell created a mathematical theory describing Faraday's discoveries which revealed that electricity and magnetism were different facets of a single, unified phenomenon called electromagnetism. Maxwell went on to predict the existence of invisible waves of electromagnetism able to travel even through a vacuum at the speed of light - which itself is such a wave. This astounding prediction was confirmed in 1888 by Heinrich Hertz, and now forms the basis of all telecommunications.

At a more fundamental level, theorists have combined electromagnetism with quantum theory, the laws governing the subatomic world. The result, known as quantum electrodynamics (QED), describes what happens when photons of light interact with electrons of matter, though esoteric, its predictions have proved astonishingly accurate.
Weak nuclear force
Some crystals kept in a cupboard were the unlikely stimulus for the discovery of the weak nuclear force. In the spring of 1896, the French chemist Henri Becquerel was investigating compounds that glow in the dark after exposure to sunlight. A spell of overcast weather had prevented him from studying the effect, prompting him to gather up the crystals and put them in a cupboard along with a sealed photographic plate. After a few days, he examined the plate and was puzzled to find it had fogged, as if the crystals were still emitting rays despite not being exposed to sunlight.

Becquerel had accidentally discovered radioactivity. The crystals contained uranium, whose notoriously unstable atoms disintegrate in a process called beta decay, releasing fast-moving electrons. But these electrons are not those in orbit around the nuclei of atoms; they come from the uranium nuclei themselves, the product of a conjuring trick performed by the weak force, which breaks apart neutrons and turns them into other particles - including electrons.

The particle-changing abilities of the weak nuclear force seems esoteric, but they are essential to the nuclear reactions that keep the sun and stars burning. But for theorists, the force has the distinction of being the first to be unified with one of the other fundamental interactions: electromagnetism. At first sight, the two forces have little in common; the weak force is a billion times more feeble than electromagnetism, and is confined to the atomic nucleus. Yet during the 1960s, theorists in the US and UK found deep mathematical connections between the two forces, suggesting they were different facets of a single "electroweak" force. This underlying unity was finally confirmed in particle accelerator experiments in the early 1980s.
Strong nuclear force
The strong nuclear force resolves a paradox that emerges from two basic facts about our universe. First, every atom apart from hydrogen contains at least two positively charged protons in its nucleus; and second, like charges repel. How then can these atoms exist if their protons cannot bear to be in each other's company? The answer lies in the strong nuclear force, which is 10,000 times more powerful than the electromagnetic repulsion trying to tear the protons apart.

The source of this force lies within the protons and their fellow denizens of the atomic nucleus, the charge-free neutrons. Inside each lurks three constituent particles known as quarks (rhyming with "forks"), permanently trapped within their host by force-carrying particles aptly known as gluons.

First postulated in the early 1960s to explain odd regularities in the properties of subatomic particles, the existence of quarks was once regarded as little more than a mathematical mind game. However, by the early 1970s experiments with particle accelerators revealed the existence of nugget-like objects inside protons with precisely the properties predicted for quarks.

The strong nuclear force turns out to be just a side-effect of the interactions between quarks that keeps them caged within protons and neutrons. Even so, it has a profound effect on the cosmos. Without it, atoms more complex than hydrogen couldn't exist - and nor could we. Yet while it is much stronger than electromagnetism over very short distances, it rapidly becomes weaker with distance, making large atoms relatively unstable. Their instability is exploited by nuclear reactors, where fast-moving particles are used to split unstable atoms, releasing huge amounts of energy.

The strong force also powers the sun and stars, which crush atoms of hydrogen together so tightly that their nuclei overcome their natural repulsion and fuse together. The result is the release of a million times more energy, pound for pound, than any conventional fuel.
Topics

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Look around you: everything you see shows that The Force is with us. Or, to be more precise, its four descendants, in the form of a quartet of fundamental forces spawned moments after the big bang.

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The history of forces and friction

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As early as the Stone Age, hominids realized if they patiently rotated a dry wooden stick back and forth in the hollow of a limb, they could successfully kindle a fire. Ancient Egyptians, meanwhile, discovered lubricants such as oil, lime, and water allowed for easier transport of building materials and increased the speed of their chariots. And in the Bible, Noah sealed his arc with bitumen to make the vessel more hydrodynamic.

These are all early examples of budding engineers practicing "tribology" (from the Greek word tribos, "to rub"), the field devoted to the study of interacting surfaces in relative motion, including friction, corrosion, and lubrication.

It may sound like another obscure " ology " at first. But from a grandfather clock to a molecular protein to a roller coaster, every system is made up of connected parts-rivets, gears, cartilage, bone, wheels-that create friction and can break down without proper lubrication. The velocity created between those parts as they rub together can lead to removal of material at the surface; that is, to wear and tear.

While tribologists-specialists who work in the field-have traditionally focused on fatigue, waste, or failure in mechanical elements like these, recent developments in the field have pushed this little known branch of science into the spotlight. In fact, tribology's influence on global energy consumption, emissions, healthcare and sustainability has never been more immediate.

Tribology's applications now extend beyond the realm of mechanics: Biotribology, for example, might investigate replacing the body's natural ball-and-socket hip joint with a smooth metallic ball at the head of the femur. Geotribology considers the friction of tectonic plates and giant glaciers, while nanotribology examines phenomena at a microscopic scale-disk drive storage, diamond-reinforced materials, even molecular sieves. Most recently, green tribology works to address environmental problems such as how to lubricate the mechanisms of a wind turbine.

How can a field so pervasive in nature and everyday life remain virtually invisible in both private and public spheres? In part, because tribology itself is built on unseen forces all around us.

Tribology in Ancient Egypt.

Tribology Through the Centuries

While the term wasn't coined until the 1960s, tribology's first scholar lived over 500 years ago. Leonardo da Vinci wasn't just an artistic genius, but a keen observer and engineer who understood friction was a limiting factor in the efficiency of his revolutionary machines. The artist's sketches-initially dismissed as doodles until x-ray technology proved otherwise-distinguished between rolling and sliding contacts and identified surface roughness as a factor in material movement.

Nearly two centuries later, French physicist Guillaume Amontons published his findings on friction, laws that govern sliding surfaces and inform research today. In his 1687 work Principia, Sir Isaac Newton laid down the foundations of fluid viscosity, which led to the development of oil lubrication. The observations and experiments carried out by Leonardo, Amontons, and Newton form the basis for many modern tribological devices, from car brakes to shoe soles.

It's perhaps surprising that anyone would devote a lifetime to the study of friction. But tribology, "the most important subject no one has heard of," is actually a significant part of daily life: Each blink of an eye is lubricated with tears, each step we take requires grip, saliva lubricates the food that we swallow.

Surfaces as varied as yoga mats and iPhone touch screens are carefully calibrated for our touch, designed to be neither too slick nor too scabrous. Coins become worn as a result of continued contact with human fingers; the metal fillings in our teeth grind down; our kitchen knives dull with use and the rubber treads of our car tires wear out. Wear is such a universal phenomenon that rarely do two solid bodies slide over each other or even touch each other without measurable material transfer or material loss. But we often don't consider such problems until we encounter them.

"Tribology is an afterthought: a technology used to solve a problem whenever there's a new mechanical system," Chris DellaCorte, senior technologist at NASA's Glenn Research Center, tells Popular Mechanics.

DellaCorte works on complex friction wear and lubrication problems across aeronautics and space missions: bearings that can turn at cold temperatures, and lubricants that can endure the high vacuum of space, withstand the dramatic vibrations of a rocket launch, and survive heat generated by aircraft turbines and engines.

"I like to tell people that my job is to make sure that things that are supposed to move keep moving, and things that aren't supposed to move don't," DellaCorte explains. Advances in tribology, he adds, typically occur whenever society tries to do something new.

"My job is to make sure that things that are supposed to move keep moving, and things that aren't supposed to move don't."

Consider that mankind's earliest, and arguably most important inventions- fire, and the wheel-were actually attempts to work out tribological obstacles. The creation of fire required friction, while the wheel was a way to reduce friction. Millennia later, we continue to puzzle over these same basic principles.

In the 1990s, for example, when computer hard disk drives frequently crashed, tribologists developed fluorinated oils and diamond coatings to reduce friction. This meant disk drives could operate reliably, without wearing down during startup or shutdown. More recently, engineers discovered wind turbines withstood far greater gusts on actual mountaintops than inside testing facilities, which frequently led to their overload and breakdown. Better controls, special coatings and bearing alloys developed by tribologists allowed wind turbines to withstand high winds, which in turn conserved energy. In effect, innovations like these solve tribological problems and simultaneously reveal them.

Unimagined tribological challenges dot the horizon. Will electric cars generate spark discharges when we drive them across a wet road? Will satellite antennas unfurl properly in outer space? Will bionic limbs provide amputees with durable freedom of movement? Will the rovers we eventually send to Venus and Mars-chock full of moving mechanical assemblies-be able to withstand the extreme temperatures, corrosive acids and chemical dust storms of these interstellar environments?

"People assume that a can of WD40 can fix anything." But different systems require a wide variety of lubricants; elbow grease won't always do.

"Tribology's reach is far beyond comprehension," says Evan Zabawski, Senior Technical Advisor at and a member of the Society of Tribologists and Lubrication Engineers (STLE), a group of scientists and academics established in 1944. Zabawski leads training programs in lubrication fundamentals, including function and performance properties, as well as industry best practices. Most of his course attendees have little familiarity with the subject, he says, and they often arrive armed with misunderstandings and so-called "tribology myths."

"People assume that a can of WD40 can fix anything," Zabawski tells Popular Mechanics. "They rely on faulty wisdom passed down through the ages, on those Jiffy-Lube stickers that warn them to change their engine after 3000 miles." But different systems, in fact, require a wide variety of lubricants; elbow grease won't always do.

Biological, marine, and aeronautical systems, for example, rely on a wide range of "third body" lubricants-that is, the slippery emollient sandwiched between two hard surfaces. Magnetic levitation between instruments keeps surfaces from rubbing together in zero gravity interstellar environments. Sewage systems may use air lubricants, and ship propellers use water and rubber bearings instead of metal to maximize energy efficiency and minimize corrosion.

To solve this problem of the third body in resistance, tribologists often look to the natural world. To help Olympic swimmers minimize drag in the water, for instance, tribologists turned to one of the most efficient swimmers on earth: the shark.

"By studying sharks, we developed a low-drag swimsuit that reduced friction by nearly a quarter," says Zabawski. In fact, the shark skin-modeled suit helped athletes break 20 world records. Officials ultimately decided to ban its use, claiming an unfair advantage worse than steroids, yet the suit was a remarkable illustration of tribology's potential to improve quality of life. "While it appears we went too far, this also showed we have much to gain," says Zabawski.

Beyond the world of industrial engineering, tribology remains an almost sacral science: Most find the idea more theoretical than practical, especially given that the physics of surfaces is hard to describe or imagine. "There's a lack of awareness because the technology is so diffused," says DellaCorte.

Indeed, tribology is a highly interdisciplinary field, a branch of study made up of mechanical engineering, materials science, biology, mathematics, and others. To complicate matters, tribological problems-everything from adhesion ( when objects become stuck on surfaces) to humidity's effect on metallurgy-are often solved with practical, trial-by-error solutions, making them harder to identify.

As one of the earliest forms of resource conservation, tribology has, somewhat paradoxically, spurred incredible energy usage: Extending the service life of machines often delays the adoption of newer, more efficient designs. Yet, whether the aim is to prevent fatigue of our machines, protect equipment from corrosion, control temperature and contamination, transmit power, or provide a fluid seal, s cience friction, as it were, is increasingly relevant across industries.

Tribological applications allow human beings to interact with their solid planet, overcome the inconvenience of excessive wear, and mitigate the loss of time and precious resources. "We still have centuries of work ahead of us," Zabawski says, "but tribology continues to impact our lives in new and inventive ways."

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How the Obscure Science of Rubbing Built the Past and Will Shape the Future

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As early as the Stone Age, hominids realized if they patiently rotated a dry wooden stick back and forth in the hollow of a limb, they could successfully kindle a fire. Ancient Egyptians, meanwhile, discovered lubricants such as oil, lime, and water allowed for easier transport of building materials and increased the speed of their chariots.

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Einstein , Copernicus, Aristotle and Newtons views

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gravity, also called gravitation, in mechanics, the universal force of attraction acting between all matter. It is by far the weakest known force in nature and thus plays no role in determining the internal properties of everyday matter. On the other hand, through its long reach and universal action, it controls the trajectories of bodies in the solar system and elsewhere in the universe and the structures and evolution of stars, galaxies, and the whole cosmos. On Earth all bodies have a weight, or downward force of gravity, proportional to their mass, which Earth's mass exerts on them. Gravity is measured by the acceleration that it gives to freely falling objects. At Earth's surface the acceleration of gravity is about 9.8 metres (32 feet) per second per second. Thus, for every second an object is in free fall, its speed increases by about 9.8 metres per second. At the surface of the Moon the acceleration of a freely falling body is about 1.6 metres per second per second.

The works of Isaac Newton and Albert Einstein dominate the development of gravitational theory. Newton's classical theory of gravitational force held sway from his Principia, published in 1687, until Einstein's work in the early 20th century. Newton's theory is sufficient even today for all but the most precise applications. Einstein's theory of general relativity predicts only minute quantitative differences from the Newtonian theory except in a few special cases. The major significance of Einstein's theory is its radical conceptual departure from classical theory and its implications for further growth in physical thought.

The launch of space vehicles and developments of research from them have led to great improvements in measurements of gravity around Earth, other planets, and the Moon and in experiments on the nature of gravitation.

Development of gravitational theory

Early concepts

Newton argued that the movements of celestial bodies and the free fall of objects on Earth are determined by the same force. The classical Greek philosophers, on the other hand, did not consider the celestial bodies to be affected by gravity, because the bodies were observed to follow perpetually repeating nondescending trajectories in the sky. Thus, Aristotle considered that each heavenly body followed a particular "natural" motion, unaffected by external causes or agents. Aristotle also believed that massive earthly objects possess a natural tendency to move toward Earth's centre. Those Aristotelian concepts prevailed for centuries along with two others: that a body moving at constant speed requires a continuous force acting on it and that force must be applied by contact rather than interaction at a distance. These ideas were generally held until the 16th and early 17th centuries, thereby impeding an understanding of the true principles of motion and precluding the development of ideas about universal gravitation. This impasse began to change with several scientific contributions to the problem of earthly and celestial motion, which in turn set the stage for Newton's later gravitational theory.

The 17th-century German astronomer Johannes Kepler accepted the argument of Nicolaus Copernicus (which goes back to Aristarchus of Samos) that the planets orbit the Sun, not Earth. Using the improved measurements of planetary movements made by the Danish astronomer Tycho Brahe during the 16th century, Kepler described the planetary orbits with simple geometric and arithmetic relations. Kepler's three quantitative laws of planetary motion are:

1. The planets describe elliptic orbits, of which the Sun occupies one focus (a focus is one of two points inside an ellipse; any ray coming from one of them bounces off a side of the ellipse and goes through the other focus).

2. The line joining a planet to the Sun sweeps out equal areas in equal times.

3. The square of the period of revolution of a planet is proportional to the cube of its average distance from the Sun.

During this same period the Italian astronomer and natural philosopher Galileo Galilei made progress in understanding "natural" motion and simple accelerated motion for earthly objects. He realized that bodies that are uninfluenced by forces continue indefinitely to move and that force is necessary to change motion, not to maintain constant motion. In studying how objects fall toward Earth, Galileo discovered that the motion is one of constant acceleration. He demonstrated that the distance a falling body travels from rest in this way varies as the square of the time. As noted above, the acceleration due to gravity at the surface of Earth is about 9.8 metres per second per second. Galileo was also the first to show by experiment that bodies fall with the same acceleration whatever their composition (the weak principle of equivalence).

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Gravity, in mechanics, the universal force of attraction acting between all matter. It is by far the weakest force known in nature and thus plays no role in determining the internal properties of everyday matter. Yet, it also controls the trajectories of bodies in the universe and the structure of the whole cosmos.

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Used when teaching class.

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Useful audiovisual learning

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Calculating Speed | Forces & Motion | Physics | FuseSchool

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Audio-Visual learning

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Useful for teacher for comparison.

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Revision handout at the end of unit

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For student revision

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Forces, motion and energy - Forces, motion and energy - National 4 Physics Revision - BBC Bitesize

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For National 4 Physics use Newton's Laws of Motion to describe how forces act on objects in motion; at rest; in freefall and in collisions.

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Useful memory aid or poster for room

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God for students to refer to

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handout

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Exam material for all sections of PW 2

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Copy of Physical World 2 Revision Questions

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For in class quizzing and recap questioning

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All resources for learning outcome PW 2.

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All resources for learning outcome PW 2.

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Good utility to help teachers expand learning. Covers everything

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Student recap and assessment