4. Chemistry and Physics

Transcript: Thermal equilibrium is an important physical principle. When two substances have unequal temperatures, thermal equilibrium means that they will tend to evolve to a situation of equal temperature. At a microscopic level we know that temperature denotes the instantaneous random speed of atoms or molecules in the substance. Hot substances have faster instantaneous motions of their atoms or molecules. So when ice is dropped into a glass of water the ice melts, and the water cools down which means that the instantaneous motions of the molecules in the ice have increased while the instantaneous motions of the molecules in the water have decreased as it heads towards thermal equilibrium.

Transcript: Thermodynamics is the name for the scientific study of the way heat flows. When materials of unequal temperatures come into contact heat will flow from the hotter to the cooler material. Isaac Newton was the first person to show that the rate of flow of thermal energy from a hotter substance to a cooler substance was proportional to the temperature difference between them. We’re all familiar with this. When we drop an ice cube in a glass of warm water heat is transferred from the water to the ice. The ice melts. The water surrounding it cools down. When you poor water from a hot kettle into a tepid bath the bath warms up, so the heat is spread from the boiling water in the kettle to the cooler water in the bath. When you touch a hot car in the summer sun and your finger burns it is because the heat is flowing from the hot car into your fingertip. These are all familiar examples, but we should remember that when we talk about heat flowing it is not like a substance or a liquid. What we are really talking about is the transfer of energy from the microscopic motions of the atoms or molecules moving at different speeds in hotter and cooler substances.

Transcript: As the temperature of a gas is raised, the microscopic motions of the atoms and molecules increase. As the velocities of the atoms and molecules increase, they collide harder and harder. At a temperature of several thousand Kelvin, the atoms or molecules collide with sufficient energy to liberate electrons from the atoms. This process continues until eventually all electrons have been liberated, and the gas consists of a sea of nuclei with surrounding electrons, all in violent collisions. This is called a plasma. The process of liberating electrons by violent collisions is called ionization. The gas at the surface of the sun is partly ionized, and deeper into the sun, where the temperature is higher, the gas is fully ionized. Most of the gaseous material of the universe in fact, whether in the form of stars or the intergalactic medium, is highly ionized.

Transcript: Gases are typically at higher temperature and much lower density than liquids or solids. The process by which a liquid becomes a gas is called vaporization. In a gas the chains between atoms and molecules are broken. The atoms and molecules move freely in straight lines with occasional collisions. As an analogy for the differences between solids, liquids, and gases, consider people standing in a gymnasium. If the people are packed densely in a gymnasium with arms linked, that’s like the situation of a solid where the atoms are locked in chains and cannot move. If the people are moving freely but still packed in the gymnasium, moving one amongst the other, that’s like the case of a liquid where the atoms and molecules can move freely, but the density is still high. If the gymnasium instead only has a few dozen people moving in straight lines blindly, occasionally colliding with each other, that’s the analog situation for a gas.

Transcript: At a slightly higher temperature than most solids we find liquids. Liquids are still high density, only slightly larger density, than the solid form of the substance. The atoms in a liquid are still in close proximity, but they have broken loose from the rigid structure or lattice and so can move freely taking the shape of the container they are in. This is the basic form of a liquid. Most substances pass from solid to liquids to a gaseous form as they are heated up. Some are an exception however, such as carbon dioxide which goes directly from solid to gaseous form. This process is called sublimation.

Transcript: The lowest temperature materials in the universe are solids, such as are found for the rocky material of our solar system or other solar systems. Solids have a rigid structure with shared electrons and the atoms enclosed in close proximity. This is true whether the solid is made of individual atoms of one element such as a mass of carbon, or molecules such as glass, or a compound such as cement. There are two basic forms of solids: amorphous, where the atoms are not in any fixed structure, and crystalline where the atoms form a lattice. In many cases the lattice structure can be reflected at large scales, such as in the crystal of a salt. The crystal of a salt as viewed under a magnifying glass represents mappings of billions of atoms in a fixed lattice form scaling right down to the scale of individual atoms.

Transcript: There are four basic states of matter in the universe. The temperatures of transition between them depend a little bit on the chemical substance or compound involved, but in general we can say that solids correspond to temperatures of up to one or two hundred Kelvin. The microscopic motions of the atoms and molecules are one or two kilometers per second, and these substances emit far infrared thermal radiation. Liquids are somewhat hotter, temperatures of three or four hundred Kelvin. Microscopic motions are two or three kilometers per second and liquids also emit far infrared thermal radiation. Gases are hotter still, temperatures of six hundred Kelvin up to a thousand Kelvin or so. Microscopic motions are three or four kilometers per second for the atoms or molecules, and thermal radiation is in the infrared region of the spectrum. The hottest materials are called plasmas. These range up from three thousand Kelvin up to many thousands of Kelvin. The microscopic motions are ten kilometers per second or larger, and the type of thermal emission from a plasma is visible radiation or ultraviolet radiation.

Transcript: One of the most important principles in physics is the law of conservation of energy. Energy can be neither created nor destroyed. In any closed physical system the total amount of energy is constant, although the energy may change forms multiple times. What do we mean by a closed system? We mean drawing an imaginary box around either an atom or an object or a planet such that all the processes occur within the box. Energy is conserved, but it can change forms. In an elliptical orbit, the kinetic energy changes constantly across the orbit, and the potential energy changes too. But the sum is constant. There are many examples of the transfer and change of one form of energy to another, but in every case, when the sums are done carefully, the total is conserved. In cases where it does not appear to be conserved the energy usually emerges in a subtle form. When a ball rolls and comes to a halt, or a pendulum swings and comes to a halt, it seems as if the energy of motion, the kinetic energy, has disappeared. But in each case the energy of motion has been replaced by thermal energy. For the rolling ball, the frictional energy increases the temperature of the surface due to the rubbing of atoms against each other, in the case of the pendulum, an increase in the microscopic motion of the air particles.

Transcript: It’s a fundamental principle of physics that energy can be easily and readily transformed from one form to another. Everyday life surrounds us with examples of this. For instance, in a battery the chemical energy in a stored chemical bond is converted into an electrical signal or electrical energy which can then be converted into the motion of something, say a fan, kinetic energy. In a hydroelectric dam the stored gravitational potential energy of water raised is converted into the kinetic energy of a turning turbine which is then converted into electrical energy for other subsequent uses. In a car the chemical energy of the stored bonds in petroleum is turned into the kinetic energy of motion and some into heat energy. In the heart the chemical energy from the stored food that gives us all our energy as living organisms is converted into an electrical signal in the heart and then into the kinetic energy of the motion of pumped blood. In an astrophysical example, think what happens when a meteorite impacts the Earth. As it is hurtling through space towards the Earth, the object has a large amount of kinetic energy and decreasing gravitational potential energy as it approaches the planet’s surface. As it impact the planets surface the gravitational potential energy does not change, but the chemical energy goes instantly to zero. What happens to this energy? It does not disappear. It is turned almost instantly into heat energy, thermal energy. We are surrounded by examples of the transformation of energy from one form to another.

Transcript: There are many situations in physics and astronomy where there is a mixture of gravitational potential energy and kinetic energy, or the energy of motion. An object in orbit around another object has both. In a circular orbit, there is one-half the amount of kinetic energy as the amount of gravitational potential energy, and both are constant through the orbit. In an elliptical orbit however, the amount of each type of energy changes continuously throughout the orbit. For example, in orbits in the solar system, or more particularly in the case of a highly elliptical comet orbit, the comet moves faster when it is closer to the sun. Thereby, its kinetic energy increases as its gravitational potential energy decreases, but the sum of the two remains constant. You can see these examples in more everyday situations right on the surface the Earth. When a ball rolls down an inclined plane, at the start, it has the maximum amount of gravitational potential energy and no kinetic energy, no motion. As it accelerates down the plane its gravitational potential energy decreases, and its kinetic energy increases. Or, consider a child on a swing. At each end of the swing the child is momentarily stationary and has no kinetic energy but is raised at the maximum height above the Earth’s surface and so has the maximum gravitational potential energy. As the child moves through the arc of the swing the kinetic energy increases, and the gravitational potential energy decreases until at the opposite end of the swing. Once again kinetic energy goes to zero, and gravitational potential energy is at maximum.

Transcript: Almost all energy on Earth has its origin with the sun. Think about the chain of events that makes it possible for you to move. That’s a form of kinetic energy of your motion. Your motion is only possible because of the food you eat. You are turning chemical energy stored in the molecules of the food you eat into stored energy in your body and then into kinetic energy. But what about the food you eat? You’re at the top of the food chain, and that energy probably came from another form, such as meat. That’s another form of stored chemical energy. That part of the food chain, say a cow, was eating a grain, another form of chemical energy stored in the starches of a grain. Where did the grain get its energy? From photosynthesis. Photosynthesis is a process whereby sunlight is stored in chemical bonds. If we think of it this way, almost every energy source we have and all the energy in our everyday lives can be traced back directly to one place: the sun. The source of the sun’s energy is nuclear energy from the conversion of hydrogen into helium by the fusion process.

Transcript: Friction is a form of thermal energy. A hundred and fifty years ago U.S. cannon maker Count Rumford noticed that when he was boring out the barrel of a cannon a lot of heat was generated, but there wasn’t a fixed amount of heat contained in the metal of the cannon, because as he used his tools continuously more and more heat was generated. The blunter tools produced more heat than sharper tools. This turns into the concept of friction, a form of thermal energy where abrasion or rubbing or physical contact between two surfaces creates increased microscopic motions of the atoms or molecules in that surface, a form of heat energy. When you rub your hands together you are increasing the motions of the molecules in the skin of your hands and creating heat. Even two ice cubes when rubbed together will create heat.

Transcript: Heat sometimes seems like a fluid. We talk about heat flowing from one place to another, objects giving off heat, or absorbing heat. Heat is actually a measure of the random disordered motion of microscopic atoms and molecules in a substance. The larger the microscopic random motions, the more the heat. This is called thermal energy, or heat energy, and as in all other forms of energy it is measured in units of joules. The fact that heat can be quantified and is truly a form of energy was shown by James Prescott Joule. In 1840 he did an elegant experiment where he arranged for a falling weight to rotate a paddle in a column of water. The rotating paddle stirred the water up and was able to heat it measurably with a thermometer. Joule deduced that a one pound weight falling through the equivalent of 770 feet was able to heat a specific amount of water by one degree Fahrenheit, thus demonstrating that the gravitational potential energy of the falling weight was being turned into the kinetic energy of the paddle’s motion and then into increased microscopic motions of the molecules of water.

Transcript: Kinetic energy is the energy of motion. It depends on both mass and velocity and is defined mathematically as a half times the mass times the velocity squared. Thus, kinetic energy increases linearly with mass but as the square of the velocity. A small object moving very fast can have a large kinetic energy, and a slow object that is very massive can have a large kinetic energy. If you do the calculation, you’ll actually find out that a speeding bullet really does have almost as much energy as a running human.

Transcript: Gravitational energy is a form of potential energy. Any object raised in a gravitational field has the potential to do work, and this is called gravitational potential energy. For example, when water is raised above sea level and it falls through a hydroelectric plant, the gravitational potential energy of the raised water is converted into electrical energy. Anything that is dropped will of course gain energy of motion, and so you are witnessing gravitational potential energy being converted into kinetic energy. The motion of anything suffering gravitational potential energy is to move towards the center of the gravitating object, in most cases the center of the Earth.

Transcript: Light is a form of electromagnetic energy, but it is just one example of a wide array of forms of electromagnetic energy. Light is an electromagnetic wave caused by the microscopic changing electric and magnetic fields. Light and all electromagnetic waves transmit energy from one place to another via these changing electric and magnetic fields. Even though electromagnetic energy is invisible, it carries energy from one place to another in the form of radiation.

Transcript: Very few books ever changed the world, but in 1543 Copernicus published such a book called On the Revolution of the Celestial Spheres. This book was the first presentation for many centuries of the idea of a heliocentric solar system, the sun at the center.

Transcript: Potential energy can be stored in electric and magnetic fields. A compass is one example of a moving object with kinetic energy created by a magnetic field. Magnets can store energy which can be released in other forms. Electrical charges can also store energy which can be released in other forms. You can feel the force of electrical charges if you have ever noticed static cling. If you rub a balloon on your head it will stick to the ceiling. In this case electrical charges are holding the balloon up against the force of gravity.

Transcript: Chemical energy is a form of potential energy stored in the bonds between atoms and molecules. Chemical energy is released when electrons change their configuration in atoms and molecules. When wood burns, oxygen in the atmosphere reacts with the carbon to release energy. When gasoline or natural gas burns, long carbon chains are rearranged, and energy is released. In addition to being the chemical energy source of dynamite and all of our fossil fuels, all of the world’s rockets depend on chemical energy. When we eat we are extracting chemical energy from the food.

Transcript: There are several broad types of energy. Energy is measured in units of calories in the English system or joules in the international system of metric units. One broad category of energy is kinetic energy, or the energy of motion. A second broad category of energy is electromagnetic energy, energy carried by waves of light and other forms of radiation. A third form of energy is heat energy which is the random microscopic motions of atoms and molecules. A final category of energy is potential energy. Energy stored in different forms could be within the chemical bonds that release fuel energy, could be energy stored in electromagnetic fields, could be nuclear energy stored within the nucleus of an atom. All of these different forms of energy can be quantified, and they can often be turned from one form into another.

Transcript: Scientists define energy as the ability to do work. You can also think of energy as something that can cause a change. This sounds vague. But scientists have defined energy in many careful ways, and it is a clearly quantifiable concept in physics. Next time you drive your car, consider the source of its energy. Millions of years ago sunlight was intercepted as it traveled through space and was used to grow plants on the ancient Earth. Those plants died, decayed, and were deposited into rocks where they turned into petroleum. Many millions of years later humans dug up the petroleum, purified it, and turned it into the fuel for your car. Your car extracts the chemical energy from that petroleum and turns it into the kinetic energy of motion. As you put your foot on the brake and bring your car to a halt, the kinetic energy is turned into the heat in the brake lining and in your engine, and it rises up into the air to eventually seep out into space and continue its journey onward.

Transcript: When you knock on a wall or a piece of metal it feels solid enough. But normal matter is actually fantastically empty, and this is a consequence of the atomic theory itself. If we need an analogy for the atom, the nucleus would be something the size of a tennis ball on the fifty yard line of a football stadium, and the electrons would form a swarm orbiting at the outer edges of the football stadium. The next atom along in a piece of solid matter would be several hundred yards away, another football stadium, with a tennis ball at its center. Ordinary matter, as shown from Rutherford’s famous experiments, is enormously empty, mostly empty space. The illusion of solidity is a function of the electrical force. Negatively charged electrons act like a shield around the positively charged nucleus and keep atoms well apart form each other. Most of the mass in every atom and in all of normal matter, resides in the tiny atomic nucleus.

Transcript: Atoms are tiny pieces of matter, but they are composed of even smaller particles. Orbiting the nucleus of every atom are electrons. Electrons have negative charge and a mass of only nine times ten to the minus thirty-one kilograms. There are 1,836 times lighter than a proton. The nucleus of an atom consists of a mixture of protons and neutrons. Protons are positively charged. Neutrons have no electrical charge. Both protons and neutrons have a mass of roughly 1.7 times ten to the minus twenty-seven kilograms. This is the basis of atomic structure. The neutrons and protons are themselves not fundamental particles. Each are composed of fractionally charged quarks. Electrons, we believe, are fundamental.

Transcript: A radioactive element or atom is something that spontaneously decays, often into a more stable form, by the emission of one or more particles. There are three forms of emission from radioactivity. One corresponds to the emission of an alpha particle, or a helium nucleus, two protons and two neutrons bound together. The second is called beta decay, the process by which a neutron decays into a proton with the emission of an electron and a small invisible particle. The third is gamma radiation which corresponds to the release of a high energy photon. In all of these processes it is an utterly random event where the probability of a particular atom decaying cannot be specified, but the probability that a particular fraction of atoms will decay in a particular time can be specified. The time taken for half of a collection of radioactive atoms to decay is called the half life.

Transcript: The chemical properties of an element are defined by its atomic number, or the number of protons. In normal, electrically neutral matter there are equal numbers of protons in the nucleus and orbiting electrons. Most elements have roughly equal numbers of protons and neutrons. However, it is possible to have different elements with different numbers of neutrons and the same number of protons. Their atomic number is the same, but their atomic weights are different. These are called isotopes. Deuterium, for example, so-called heavy hydrogen, consists of a nucleus with one proton and one neutron orbited by one electron. Many of the heavier isotopes are radioactive and will decay into different elements of lower atomic numbers.

Transcript: Each element is defined by a unique atomic number which is the number of protons and electrons in an atom of that element. The atomic weight is the sum of the number of protons and neutrons. Over ninety percent of the universe is made of the two simplest elements: hydrogen with one proton and an orbiting electron, and helium with two protons, two neutrons, and two orbiting electrons. The next elements are very rare in the universe: lithium and beryllium, atomic numbers of three and four. Boron is also rare, atomic number of 5. The next three elements carbon, nitrogen and oxygen, the basis of life itself, are atomic numbers 6, 7, and 8, and atomic weights 12, 14, and 16. These comprise the simplest elements in the universe and the first elements in the periodic table.

Transcript: The man who did a decisive set of experiments in the early 20th century to demonstrate the atomic structure of matter was Ernest Rutherford. Rutherford was born in New Zealand to a poor family and passed through his entire education dependent on scholarships. By the end of his life, however, he would have won a Nobel Prize, been head of the famed Cavendish Laboratory at the University of Cambridge, and he was made a lord by the British government. From his humble beginnings, Rutherford was relentless in his search for the fundamental nature of matter. He established a laboratory at the Cavendish where he did a beautiful series of experiments to understand the nature of normal matter. Rutherford was a bear of a man with a booming voice and an intense manner. He could be a tough boss, sometimes sweeping the lab at the end of the day to send people home not to be with their families or wives but to think more deeply about the experiments that they had just been conducting. Students however loved him, and they flocked to him large numbers. Rutherford thought a good theory had to be explained simply, and this was a great benefit to him in his science.

Transcript: In the early 1800s only 30 elements were known, insufficient to see any pattern in their behavior. Around this time the battery was invented which led to the isolation, by chemical means, of several dozen additional elements. The Swedish chemist Berzelius invented the modern notation of chemistry, H20 and CO2 for example, where a compound is represented by the relative number of atoms of different elements in it. In 1869 the Russian chemist Mendeleev took the existing set of elements and arranged them by weight and chemical properties into the periodic table. The modern form of the periodic table has 90 stable elements and several dozen elements that can be created fleetingly only in the laboratory. These are among the most heavy elements.

Transcript: The atomic theory is the basis of modern chemistry. Dalton published his work in 1810. There are a few key ideas. All elements are composed of atoms. Each element has fundamental and unique chemical properties and is composed of atoms, each of which has unique weight. Compounds are formed when atoms combine in specific proportions into a molecule. Each compound or molecule has unique chemical properties. Elements and compounds combine and separate in chemical reactions. Elements and their fundamental counterparts, atoms, cannot be further broken or subdivided, and one element cannot be changed into another by chemical means.

Transcript: Dalton came up with the idea of atoms as the ultimate particle. That is, an atom is a particle of each element which cannot be subdivided by chemical means. Different elements have atoms of different atomic weights, and Dalton discovered the rules by which they combine into compounds. He speculated that the microscopic particle corresponding to a compound is a molecule, a fixed proportion of two or more elements. For example, water can be understood in terms of one atom of oxygen and two atoms of hydrogen. The atoms of oxygen and hydrogen themselves cannot be further subdivided by chemical means.

Transcript: Dalton reached his theory of atoms by experimenting with gasses using entirely homemade equipment. For example, he deduced that air is not a uniform substance but is composed of a mixture of gases, primarily nitrogen and oxygen. He showed that a single gas corresponds to a chemical element, and he showed that gases combined in fixed proportions to form compounds. For example, hydrogen and oxygen combine explosively to form steam or water, and the proportions are always one part hydrogen to eight parts oxygen by weight.

Transcript: The person who gets credit for developing the first theory of atoms was a self educated man called John Dalton living in England in the mid 18th century. Born into a family of Quaker weavers, he was entirely self educated. He learned mathematics, foreign languages, and the basics of observational astronomy from a blind philosopher who lived nearby. At a young age he developed a theory of the aurora borealis, correctly speculating that it was connected with the Earth’s magnetism. He developed the first theory of atoms. Dalton was brilliant at synthesizing information, and the process of building a theory from a jigsaw puzzle of pieces, not all of which are present, is a fundamental scientific skill.

Transcript: Over two thousand years ago the Greeks speculated that the diversity of material in the natural world might conceal a basic simplicity. Democritus came up with the idea of atoms, reasoning that if you subdivided a particle over and over this process could not continue forever. These fundamental entities, invisibly small, had no primary characteristics. All the characteristics of color, smell, were secondary characteristics. Empedocles went further and speculated that the entire natural world was made up of four basic elements: earth, air, fire, and water. While these ideas seem a little naïve in retrospect, in other ways they are strikingly modern. However the Greek had no evidence as to the nature of atoms or the fundamental nature of matter. Such evidence would take another two thousand years to acquire.

Transcript: Humans will continue to explore space. In part they will do that for visionary reasons because it inspires us, and in part for pragmatic reasons. Global telecommunication depends on space travel, and we depend on global telecommunication. The fundamental basis of rocketry has not changed for seventy years. It is the chemical rocket. The Saturn 5 that took the men to the moon depended on burning kerosene and oxygen with the byproduct of water. Chemical rockets are not highly efficient, and research is underway to find more efficient energy sources which will in turn reduce the cost of space travel. Ionic systems are under development, and it may be possible, once in a low-Earth orbit, to use solar sails. It sounds like science fiction, but physicists are even speculating about the use of matter-antimatter as a propulsion device that would be the most efficient we could possibly come up with. Either way, it looks like our future is likely to be in space.

Transcript: The first use of space was for military purposes. Although we tend to forget it, there are still thousands of orbiting nuclear weapons. Commercial uses of space came afterwards. Space has been used for research; the International Space Station is advertised as a way to do research in space. Unfortunately, it is very expensive, and almost none of the experiments with drugs or biotech materials can be done in a cost effective way from orbit. Telecommunications is a different story. Most of the space program consists of the mundane business of launching satellites into space. There are many thousands in Earth orbit now. Tourism is perhaps the next frontier. At a cost of about 10,000 dollars per kilogram, even rich people could afford to go into orbit given a suitable launch vehicle. With new technologies for propulsion, it’s possible that the cost to launch into orbit could come down a few thousand dollars per kilogram. Other issues are raised by the future of space exploration. Up to now, only governments have carried out this activity. But what if there was privatization of the space program, and what about the commercial uses of space? The human legal systems of the world have barely begun to grapple with the issues of who owns space, or who might own an asteroid once minerals and metals begin to be mined from it.

Transcript: The debate continues over the relative merits of manned space travel versus the use of robotic spacecraft. Manned space flight is dangerous. Astronauts have died in space, and the public is not always sure they want to pay that price. Manned space flight is expensive because the humans must be maintained in a hostile environment. If humans are ever sent to Mars the probable minimum price tag is three or four hundred billion dollars. By contrast, robotic space craft are cheap and easy to build. The miniaturization of electronics has made them more and more cost effective over time. They do not, however, capture the imagination the way sending people into space does. It is likely that for the foreseeable future, the space program will combine human space flight with robotic space craft.

Transcript: Space exploration is expensive, and space travel using humans is even more expensive. The high cost almost forces international cooperation. This started in the 1970s with the Apollo-Soyuz program. In the 1980s the United States developed the first reusable spacecraft, the space shuttle. The space shuttle was envisaged originally as something like a truck that could go into space forty or fifty times a year at a moderate cost. The reality has not been so happy. The space shuttle costs each flight fifty or a hundred times more than originally planned and has never gone up more than ten or twelve times a year. Two of the five space shuttles have been catastrophically lost with the astronauts on board. The Mir Space Station built by the Russians degraded so badly that it had to be abandoned. The International Space Station is now underway cooperatively between Europe, the United States, Russia, and Japan, but the cost is making all of those countries reconsider their involvement.

Transcript: Public interest in the space program and its cost probably peaked in the late 1960s with the Apollo program. In a series of six launches twelve astronauts set foot on the moon and brought back a few hundred kilograms of moon rocks. The program was cancelled in 1972. Thereafter, NASA turned to exploration of the solar system using unmanned probes. Both the Russians and the Americans sent probes to Venus and Mars, and by the end of the 1970s probes had been sent to explore all the planets of the solar system except Pluto. The result of all this activity was a burst of innovation in the field of planetary science.

Transcript: A low-Earth orbit corresponds to a distance of about 200 kilometers above the Earth’s surface. This is a tiny fraction of the Earth’s diameter but sufficient to be above the entire atmosphere. At this height and at a speed of 7.8 kilometers per second, the orbital time is about 90 minutes. Low-Earth orbit, for example, is the orbit of the Hubble Space Telescope. At the cost of considerably more energy an orbit can be created whose period is 24 hours. In this situation something can hover above one point of the rotating Earth. This is called a geosynchronous orbit, and it is obviously a favored orbit for things such as telecommunications satellites.

Transcript: The speed required to completely escape from the gravity of something is called the escape velocity. At the escape velocity the kinetic energy exactly equals the gravitational potential energy, and an object can escape to an infinite distance from the gravity of a large body. Earth’s escape velocity is eleven kilometers per second. The escape velocity is always the square root of two times the circular velocity, about 40 percent larger. Something that escapes the Earth’s gravitational field is still held by the gravity of the sun however. Earth’s circular velocity around the sun is 30 kilometers per second. Square-root two times larger is 42 kilometers per second. Therefore any object or rocket that can be accelerated to 42 kilometers per second can leave the solar system. Four man-made objects or satellites have done so, so far.

Transcript: The circular velocity is the velocity needed to create an orbit. Circular velocity for the Earth is 7.8 kilometers per second. This is about 17,500 miles per hour, so obviously a very large rocket and a lot of energy is needed to put something in orbit. Circular velocity scales up with the mass of the gravitating object and scales down with the distance from that object.

Transcript: Newton viewed both time and space as smooth, absolute, and Euclidian. Newton’s gravity law is an inverse square law, so the gravity of every object diminishes with the square of the distance. However it never reaches zero because one over the square of a large number is a very small number but not zero. Newton believed in an infinite universe, which means that in an infinite universe filled with objects, stars or galaxies, the gravity is infinite too. This was a problem for Newton’s theory of gravity and for his understanding of cosmology. He could not solve this problem at the time. Newton also did not understand what was the nature of the force of gravity that could act across the vacuum of space. As he said, “I frame no hypothesis.”