Energy is wasted due to friction. For many tasks, finding inexpensive ways of reducing friction is generally welcome. Rolling something, for example, requires less of our energy than dragging it, which is why ball bearings are useful. Creating various lubricants like oils and greases is big business! Water is also a useful lubricant for certain jobs.

Did you know that ice on a thin layer of water is one of the slipperier things you can make? Try it out!

Gravity, a force familiar to all of us, is a long-range force with an effective range that stretches toward infinity. Like gravity, the effective range of the electromagnetic force also extends to infinity. However, the electromagnetic force (as shown by the chart) is far stronger than gravity. The electromagnetic force is associated with electricity and magnetism.

Although the forces due to gravity and electromagnetism both extend to infinity these forces get weaker and weaker the further two bodies are separated. The two bodies being large masses in the case of gravity and two static charges (plus + or minus -) in the case of the electromagnetic force. In fact, the strengths of both gravitational and electromagnetic forces diminish with the inverse square (1/D²) of the distance (D) between the centers of the two bodies. We will clarify these relationships in the next few sections.

The word gravity is derived from the Latin word "gravitas," meaning "heavy" or "weight." Gravity is an incredibly weak attractive force. Of the four fundamental forces, gravity it by far the weakest. However, it is gravity and not the other forces that causes planets to orbit the Sun and apples to fall from a tree. Gravity is always a pulling force and is never a pushing force. What sets gravity apart from the other forces?

Gravity is a long-range attractive force, which means that it can pull objects together over incredibly large distances. Although the strong and weak nuclear forces are far stronger than gravity, they are both short-range forces and consequently act only over extremely short distances. This means that they serve no purpose in pulling large masses (like the Sun and the planets) together. Although gravity is weak, gravity is always an attractive force between two objects. The electromagnetic force, by comparison, is also a long-range force, however, it can be an attractive force or a repulsive force depending on the signs (+ or -) of the two electric charges. We will examine this later.

Sir Isaac Newton (1642-1727) was the first to derive a Universal Law of Gravitation. Using his observations of the cosmos along with Kepler's Three Laws of Planetary Motion, Newton constructed a law of gravitation that for most purposes has proved to be astonishingly accurate. Newton showed that heavenly (celestial) bodies were governed by the same laws as objects on Earth. It was Johannes Kepler (1571-1630) who showed that the planets of our solar system orbited the sun in elliptical rather than circular orbits as was previously believed. Using Tycho Brahe's (1546-1601) astronomical data, Kepler formulated what would eventually come to be known simply as Kepler's Laws. Tycho Brahe was Kepler's mentor.

Newton's Universal Law of Gravitation can presented in the following way:

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This equation shows how large or small the force of gravity F_g is between two masses m₁ and m₂. The larger the masses the larger is gravity. As you can see, the product of the two masses (m₁ x m₂) is divided by the square of the distance (D) between the respective centers of mass, m₁ and m₂. This result is multiplied by the gravitational constant G. However, because G is such a small number (G = 6.67 x 10^-11 N m² kg^-2) unless the masses are substantial, gravity will be relatively small. Because of the 1/D² term, Newton's Law of Gravitation is referred to as an inverse square law. In other word, mass m₁ attracts mass m₂ with a force that is directly proportional to the product of the two masses and inversely proportional to the square of the distance between the respective centers of mass.

In situations where gravitational fields are very strong, as in the case of collapsed stars and black holes, Newton's Law of Gravitation fails and Einstein's theory of General Relativity takes over the discussion.

What is the Cause of Gravity?__(Enter Albert Einstein)

Newton explained how gravity worked, but did not know the cause of gravity. It wasn't until Albert Einstein (1879-1955) and his Theory of General Relativity that the cause of gravity was realized. Newton's Law of Gravitation is independent of time. General Relativity is founded on the premise that our Universe is four-dimensional. Scientists refer to this as space-time, which contains the normal three dimensions (i.e., length, width and height) and time as one entity. Einstein imagined that gravity is created when matter distorts space-time. He visualized that space-time could be deformed (curved) by the matter it held. Some refer to this as the "warping" of space-time. We may visualize the warping of space-time using the following diagram called a gravity well:

_______Gravity Well

Imagine space-time as a bed sheet that you hold at one end and I hold at the other end and we pull tightly. Then we have someone gently toss a fairly heavy ball onto the taut sheet. What happens? Well, the heavy ball journeys to the center of the sheet and forms a depression in the once flat sheet. This is our gravity well. Let's imagine that this is the blue sphere in the figure above. Space-time has been curved or "warped" by the massiveness of the blue sphere. This could be the Sun, for example, of our solar system.

Now we make things really interesting by tossing another ball (the red sphere in the figure) onto the sheet, not directly at the blue sphere and not moving too fast. The red sphere, which is less massive in our demonstration, starts rolling towards the blue sphere following the curvature of space-time introduced by the more massive blue sphere. The red sphere continues to be pulled toward the blue sphere. If the conditions are right, the red sphere orbits the blue sphere, much like the planets orbit the Sun. We are witnessing gravity! According to Einstein's General Theory of Relativity, gravity is the consequence of an objects ability to curve space-time. The more massive the object, the greater the curvature. The greater the curvature, the greater is gravity.

New Theories of Gravity - (Graviton Theory)

The General Theory of Relativity although a marvelous achievement does not fully explain how gravity is affected by mass or by distance. According to more recent quantum theories about gravity it has been proposed that a quantum particle called a graviton (a particle with no mass and no charge) is responsible for gravity. It is theorized that everything possessing mass emits these graviton particles, which are believed to mediate (transfer) the gravitational force between masses. In other words, the graviton is thought to be a gravitational force-carrying particle. As graviton theory goes, the more massive the object, the more gravitons it has, which is said to explain how mass is proportional to gravity. Some physicists also envision an object being surrounded by a cloud of graviton particles with the "graviton cloud" becoming less dense as the distance from the object increases. In this sense, graviton theory also accounts for how gravitational attraction is affected by distance. The problem is that the graviton has yet to be discovered. More work is needed to solve this problem.

Unlocking the Secrets of Gravity

____ _________Brain Power Needed

Brain Power is needed to find answers to gravity and other "Secrets of the Universe." I invite you to take up the challenge!

Electromagnetic Force

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The electromagnetic force or EM force, for short, is responsible for the electric and magnetic properties of matter. It is the EM force, which binds negatively charged electrons to positively charged protons and is responsible for holding atoms and molecules together. The EM force is the "glue" so to speak, between atoms and molecules. Light along with other forms of electromagnetic radiation is due to the EM force. Like gravity, the EM force is a long-range force of infinite extent, getting weaker and weaker, for example, as we move two electric charges further apart from each other. We will have more to say about this as we go.

____ __The action-at-a-distance nature of the EM force.

The EM force obeys an inverse square law similar to the one we saw for gravity. The difference being that gravity is always an attractive force, whereas the EM force may be repulsive or attractive depending on the sign (+ positive or - negative) of the electric charges. For example, two positive (+) electric charges repel each other as do two negative (-) charges, while positive (+) and negative (-) electric charges attract each other.

______Repulsion___________Attraction
__ __The Behavior of Electric Charges

The EM force between two electric charges may be calculated using Coulomb's Law, which is named after the French scientist, Charles Augustine Coulomb (1736-1806). Coulomb's experiments using a torsion balance proved that the EM force (F) between two static charges Q₁ and Q₂ is proportional to the product of the charges ( Q₁ x Q₂ ) and inversely proportional to the square of the distance (d) between the charges. The EM force is also known as the Coulomb force. The Coulomb constant k is a proportionality constant.

__________________________________________________Charles Augustine Coulomb
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Coulomb's Law and the Coulomb constant k

Using a torsion balance similar in principle to the one found below, Coulomb measured the electrostatic force between two electrostatically charged pith balls. Oppositely charged pith balls are attracted to each other, while similarly charged pith balls are repelled from each other by the Coulomb force.

[ Pith is a material extracted from the soft spongy center of the stems of most flowering plants that pick ups and holds electrostatic charges rather well. More often it is styrofoam (a man-made material) that is used to make pith balls for such experiments. ]

____ __Torsion Balance used to Measure the Coulomb Force

In the figure, the blue pith balls are attached to a needle, which is suspended from a wire in the balanced position. The red pith ball is fixed to a glass rod. A negatively (-) charged blue pith ball is attracted to the positively (+) charged red pith ball. As the blue pith ball rotates toward the red pith ball, the wire twists causing torsion in the wire. The torsion, which is a twisting force, is measured using a scale at the top of the device. The distance between the red pith ball and the blue pith ball is measured using a second scale that circumscribes the jar. This scale is the yellow measuring tape in the figure. Using the measurements for torsion and the distance between the red and blue pith balls, Coulomb derived his now famous Coulomb equation for electrostatic force.

Electromagnetic Radiation

Light is a form of electromagnetic radiation or EM radiation, for short. EM radiation is caused by the disturbance of an electromagnetic (EM) field, which consists of alternating electric E and magnetic B fields, oriented perpendicular (at right angles) to each other as shown by the figure below. The electric and magnetic fields oscillate in time, propagating (spreading) through space as an EM wave.

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The EM wave moves along at (you guessed it) the speed of light, carrying the energy and momentum of the EM wave. The speed of light in a vacuum (empty space) is approximately 186,300 miles per second or roughly 300 million meters per second. Light and all other forms of electromagnetic radiation can travel through a vacuum. This is radically different from sound waves, which are mechanical waves, requiring a medium (solid, liquid or gas) for their travel and to transport their energy. This is why you cannot hear sounds in outer space. With zero atmosphere there is no medium for sound to travel through.

__________ _______Propagating EM Wave

The electric and magnetic fields both oscillate perpendicularly (at a 90^o angle) to each other and to the propagation direction of the EM wave. This type of wave is called a transverse wave. The oscillating electric field creates an oscillating magnetic field, which in turn creates an oscillating electric field and so the cycle goes. The energy of the EM wave is stored in the electric and magnetic fields. In fact, the energy is shifted back and forth between the electric and magnetic fields as the EM wave moves through space. There is no loss or gain in the total energy of the EM wave in accordance with the Law of Conservation of Energy, which states that energy, is neither created nor destroyed.

All forms of EM radiation oscillate in this periodic way with the waveform having peaks and valleys (troughs) as shown by the figure below. This waveform is called a sinusoidal wave or sine wave because it is a continuous wave that is described by sine functions. The height of the wave is called the wave amplitude. The distance from peak to peak or from valley to valley is known as the wavelength.

The Greek letter lambda__ is used to represent wavelength.

_______Sinusoidal Wave

The frequency of a wave, although not a part of the wave, refers to the number of complete waves that are made per unit time. This is usually given either as the number of wave cycles per second (cps) or as Hertz (Hz), where 1 Hertz equals 1 cps. For example, if ten complete waves pass a point in space in one second, then we say that the frequency is ten cycles per second (cps) or 10 Hz.

The Hertz is named in honor of the scientist, Heinrich Hertz (1857-1894), who was the first to demonstrate the existence of electromagnetic radiation. His experimental work in this area confirmed the theoretical predictions of James Clerk Maxwell (1831-1879) who proposed that light was a form of electromagnetic radiation. Maxwell's equations, as they are known, describe the behavior of electric and magnetic fields and how these fields interact with matter.

______Pioneers of Electromagnetic Radiation____

__Heinrich Hertz_________________________________________James Clerk Maxwell

Frequency is represented by the letter f and by the Greek letter nu (pronounced new). When we multiply the frequency of an electromagnetic wave by its wavelength, we obtain the speed of light c. This relationship is true for all electromagnetic radiation. The letter c is used for the speed of light because the word celera means "speed" in Latin.

_____ __x____=___c____(Speed of Light)

The speed of light is a constant. This means that as the wavelength of electromagnetic radiation decreases its frequency must increase. This is illustrated by the figure below, which compares wave (a) to wave (b). Wave (a) is of longer wavelength than wave (b), which means that the frequency (wave cycles per second) of wave (a) must be less than the frequency of wave (b). By comparing the number of peaks for the two waves it is easy to see that wave (b) is the higher frequency wave. Check it out!

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Light is certainly not the only example of EM radiation. Other forms include radio waves, microwaves, ultra-violet light and x-rays, which are all part of what is known as the electromagnetic spectrum. The EM spectrum may be examined both in terms of frequency and wavelength. This is shown in the following figure. The larger or "higher" the frequency the greater is the energy of the radiation. The larger or "longer" the wavelength the smaller is the energy of the radiation. This means that high frequency radiation is also short wavelength radiation and that low frequency radiation is long wavelength radiation.

__________________________Electromagnetic Spectrum

Visible light, for example, constitutes only a small part of the complete EM spectrum. The wavelength range for visible light begins with the lower energy red light, which has a wavelength of 700 nanometers (nm) and ends with the higher energy violet light, which has a wavelength of 400 nm. We call this visible light because it is the EM radiation of the EM spectrum that we can see. By the way, the nanometer, which is one billionth of one meter or 10^-12 meters, is used for its convenience in addressing the wavelengths of EM radiation.

EM waves that are higher in energy (meaning higher frequency, shorter wavelength) than visible light, include ultra-violet (UV) light, X-rays and gamma rays. Those that are lower in energy (lower frequency, longer wavelength) than visible light, include infrared light, microwaves and radio and TV waves, in order of decreasing energy.

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