An artist's rendering of some of the forces of the universe. The apple falling is of course from the story of Isaac Newton discovering the law of gravity as an apple fell from a tree he was sitting underneath. Windows to the Universe original image Related links: Gravity is one of the universal forces of nature. It is an attractive force between all matter, and is very weak as compared to the other forces of nature. The gravitational force between two objects is dependent on their masses, which is why we can only see gravity in action when at least one of the objects is very large (like the Earth). Isaac Newton was the first scientist to define gravity mathematically when he formulated his law of universal gravitation. The law of gravitation says that gravity is strongest between two very massive objects, and gets much weaker as these objects get further apart. One of the applications of this law is the concept of ‘escape velocity’, which is the velocity an object needs to achieve to escape the gravitational pull of another object (like the Earth). Escape velocity can be calculated from Newton’s gravitational law, and if we plug in the measurements we have for the planet Earth, we see that Earth’s escape velocity is about 11 km/s. This means that if you could throw a baseball at 11 km/s, it would never come down! The concept of escape velocity is especially interesting when you consider black holes. These objects are extremely dense and very small. When we calculate the escape velocity for these objects, we find that the number is actually the speed of light, so not even light can get out of a black hole! You might also be interested in: The interactions in the Universe are governed by four forces (strong, weak, electromagnetic and gravitational). Physicists are trying to find one theory that would describe all the forces in nature as...more

Editor's Note: We asked several scientists from various fields what they thought were the greatest mysteries today, and then we added a few that were on our minds, too. This article is one of 15 in LiveScience's "Greatest Mysteries" series running each weekday. In the deepest depths of space, gravity tugs on matter to form galaxies, stars, black holes and the like. In spite of its infinite reach, however, gravity is the wimpiest of all forces in the universe. This weakness also makes it the most mysterious, as scientists can't measure it in the laboratory as easily as they can detect its effects on planets and stars. The repulsion between two positively charged protons, for example, is 10^36 times stronger than gravity's pull between them—that's 1 followed by 36 zeros less macho. Physicists want to squeeze little old gravity into the standard model—the crown-jewel theory of modern physics that explains three other fundamental forces in physics—but none has succeeded. Like a runt at a pool party, gravity just doesn't fit in when using Einstein's theory of relativity, which explains gravity only on large scales "Gravity is completely different from the other forces described by the standard model, " said Mark Jackson, a theoretical physicist at Fermilab in Illinois. "When you do some calculations about small gravitational interactions, you get stupid answers. The math simply doesn't work." Gremlins of gravity The numbers may not jibe, but physicists have a hunch about gravity's unseen gremlins: Tiny, massless particles called gravitons that emanate gravitational fields. "We can detect massless particles such as photons just fine, but gravitons elude us because they interact so weakly with matter, " said Michael Turner, a cosmologist at the University of Chicago. "We simply don't know how to detect one." Turner, however, isn't despondent about humanity's quest for gravitons. He thinks we'll eventually ensnare a few of the pesky particles hiding in the shadows of more easily detected particles. "What it really comes down to is technology, " Turner said. Physicists aren't using mechanical wizardry to discover gravitons just yet, however. Efforts are currently focused on confirming the existence of the Higgs boson, which is the graviton's distant cousin particle responsible for giving matter mass. Finding the 'toilet' Sheldon Glashow, winner of the 1979 Nobel Prize in Physics, once called the Higgs the "toilet" of the standard model of particle physics. Turner explained that Glashow coined the term because the Higgs performs an essential function: Keeping the standard model functioning, at least in an intellectual way. "Really, the Higgs is more like a plumber with duct tape, holding the standard model together, " Turner said. "A lot of the inelegance of it's all wrapped up in the Higgs." And rightly so, he noted, because it's required to make the other forces involving mass—such as gravity—make sense. "At the same time, the Higgs can be frustrating because it doesn't shed much light on gravity, " Turner said, assuming that the particle is eventually discovered. Accelerating answers Discovering elusive particles such as the Higgs is something like traveling through time. By using enormous machines to whiz particles close to the speed of light, then smash them together, engineers can mimic the incredible energies present during the early universe. So early in the universe's existence, particles were too energetic to stick together and form more familiar protons, neutrons and the like. The Tevatron, Fermilab's 4-mile-circumference (6.3-kilometer) particle accelerator, may have already spotted the Higgs in accelerator data, according to physicists' Web logs. But Turner said the new Large Hadron Collider (LHC) circling 17 miles (27 kilometers) beneath France and Switzerland should clearly confirm it within a few years. "I think it will be a sigh of relief when the Higgs is discovered, " he said. Will particle accelerators, however, eventually pop out a graviton? Xavier Siemens, a gravitational theorist at the University of Wisconsin Milwaukee, said showing gravity acts like a wave needs to happen first. "Classically, we can measure waves, and waves are made up of particles, " said Siemens, who is also a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) that looks for wave-like evidence of gravity. By detecting gravitational waves, there would be grounds to suggest gravitons really exist—and begin seeking it out.

You told us how an almost churchlike atmosphere is pervading your desolate house now. And justifiably so, for unusual divine powers are at work in there. Besso to Einstein, The basis of Einstein's general theory of relativity is the audacious idea that not only do the metrical relations of spacetime deviate from perfect Euclidean flatness, but that the metric itself is a dynamical object. In every other field theory the equations describe the behavior of a physical field, such as the electric or magnetic field, within a constant and immutable arena of space and time, but the field equations of general relativity describe the behavior of space and time themselves. The spacetime metric is the field. This fact is so familiar that we may be inclined to simply accept it without reflecting on how ambitious it is, and how miraculous it is that such a theory is even possible, not to mention (somewhat) comprehensible. Spacetime plays a dual role in this theory, because it constitutes both the dynamical object and the context within which the dynamics are defined. This self-referential aspect gives general relativity certain characteristics different from any other field theory. For example, in other theories we formulate a Cauchy initial value problem by specifying the condition of the field everywhere at a given instant, and then use the field equations to determine the future evolution of the field. In contrast, because of the inherent self-referential quality of the metrical field, we are not free to specify arbitrary initial conditions, but only conditions that already satisfy certain self-consistency requirements (a system of differential relations called the Bianchi identities) imposed by the field equations themselves. The self-referential quality of the metric field equations also manifests itself in their non-linearity. Under the laws of general relativity, every form of stress-energy gravitates, including gravitation itself. This is really unavoidable for a theory in which the metrical relations between entities determine the "positions" of those entities, and those positions in turn influence the metric. This non-linearity raises both practical and theoretical issues. From a practical standpoint, it ensures that exact analytical solutions will be very difficult to determine. More importantly, from a conceptual standpoint, non-linearity ensures that the field cannot in general be uniquely defined by the distribution of material objects, because variations in the field itself can serve as "objects". Furthermore, after eschewing the comfortable but naive principle of inertia as a suitable foundation for physics, Einstein concluded that "in the general theory of relativity, space and time cannot be defined in such a way that differences of the spatial coordinates can be directly measured by the unit measuring rod, or differences in the time coordinate by a standard clock...this requirement ... takes away from space and time the last remnant of physical objectivity". It seems that we're completely at sea, unable to even begin to formulate a definite solution, and lacking any definite system of reference for defining even the most rudimentary quantities. It's not obvious how a viable physical theory could emerge from such an austere level of abstraction. These difficulties no doubt explain why Einstein's route to the field equations in the years 1907 to 1915 was so convoluted, with so much confusion and backtracking. One of the principles that heuristically guided his search was what he called the principle of general covariance. This was understood to mean that the laws of physics ought to be expressible in the form of tensor equations, because such equations automatically hold with respect to any system of curvilinear coordinates (within a given diffeomorphism class, as discussed in Section 9.2). He abandoned this principle at one stage, believing that he and Grossmann had proven it could not be made consistent with the Poisson equation of Newtonian gravitation, but subsequently realized the invalidity of their arguments, and re-embraced general covariance as a fundamental principle. It strikes many people as ironic that Einstein found the principle of general covariance to be so compelling, because, strictly speaking, it's possible to express almost any physical law, including Newton's laws, in generally covariant form (i.e., as tensor equations). This was not clear when Einstein first developed general relativity, but it was pointed out in one of the very first published critiques of Einstein's 1916 paper, and immediately acknowledged by Einstein. It's worth remembering that the generally covariant formalism had been developed only in 1901 by Ricci and Levi-Civita, and the first real use of it in physics was Einstein's formulation of general relativity. This historical accident made it natural for people (including Einstein, at first) to imagine that general relativity is distinguished from other theories by its general covariance, whereas in fact general covariance was only a new mathematical formalism, and does not connote a distinguishing physical attribute. For this reason, some people have been tempted to conclude that the requirement of general covariance is actually vacuous. However, in reply to this criticism, Einstein clarified the real meaning (for him) of this principle, pointing out that its heuristic value arises when combined with the idea that the laws of physics should not only be expressible as tensor equations...

As long as we're talking about one of the greatest scientists who ever lived, let's move on to Newton's other famous laws. His three laws of motion form an essential component of modern physics. And like many scientific laws, they're rather elegant in their simplicity. The first of the three laws states an object in motion stays in motion unless acted upon by an outside force. For a ball rolling across the floor, that outside force could be the friction between the ball and the floor, or it could be the toddler that kicks the ball in another direction. The second law establishes a connection between an object's mass () and its acceleration (a), in the form of the equation F = m a. represents force, measured in Newtons. It's also a vector, meaning it has a directional component. Owing to its acceleration, that ball rolling across the floor has a particular vector, a direction in which it's traveling, and it's accounted for in calculating its force. The third law is rather pithy and should be familiar to you: For every action there is an equal and opposite reaction. That is, for every force applied to an object or surface, that object pushes back with equal force.

In 1665, Isaac Newton recognized that all matter attracts all other matter, but he also recognized that the gravitational attraction of everyday objects for each other was far too small to be measured in his time. Newton tested his theory of gravitation with the large masses of astronomical objects like the moon, Earth, and sun. In 1797, Henry Cavendish succeeded in measuring the tiny gravitational force between two metal spheres. He fastened small spheres on the ends of a rod and hung it from a wire. Then he brought up two larger spheres, as shown in the schematic drawing, so that the gravitational forces twisted the wire slightly. The forces between a small and large sphere are only about a billionth of their weight. Nevertheless, from the amount of twist in the wire, and the physical properties of the wire and suspended spheres, Cavendish measured the tiny force, and it agreed with Newton's prediction. (See drawing at right) Dependence on mass and separation Photo of University of Washington experiment showing polished spheres Newton discovered that all matter in the universe attracts all other matter, with a force that decreases with the square of the separation. If you double the separation of two objects, the force they exert on each other is divided by four. The force is proportional to the mass of each object. Double the mass of one object, and the gravitational force doubles, too. We make an equation. So far we have that for the force of gravity F between two objects, 1 and 2, F is proportional to M1M2 R2 In the above relationship, M1 and M2 are masses, R is the separation between them. To make this relationship into an equation, we need a constant, fondly known as “Big ‘G’”. Here's the equation: Notice that if R gets big, the value of F gets small. Why “Big ‘G’” is important If we know "G" from lab measurements, we can find the mass of Earth by measuring the radius of the moon's orbit and the length of the month, or by measuring the acceleration of gravity on Earth's surface. Likewise, we can find the mass of the sun by measuring Earth's orbit and determining the length of the year. Science Marches Ahead? We expect measurements to get more and more accurate over time, as physicists improve experiments and employ new technologies. With "Big 'G'", however, for a while the accuracy was going down, and fast. Prior to 1987, "Big 'G'" was taken to be accurate to 0.013%. Subsequently, two research groups made measurements that were tenths of a percent from the then-accepted value, and in different directions! Consequently the accepted uncertainty was raised by more than a factor of ten. This unfortunate situation galvanized several other groups into action, including one at the University of Washington, whose measurements are accurate to 0.0015%, nearly 10 times more accurate than the 1987 value. Measuring Big 'G' Big news at an April 2000 scientific meeting was the announcement of a long-awaited higher precision measurement of the gravitational constant (affectionately known as “Big ‘G’”among physicists) by Jens Gundlach of the University of Washington. Although G has been of fundamental importance to physics and astronomy ever since it was introduced by Isaac Newton in the seventeenth century (the gravitational force between two objects equals G times the masses of the two objects and divided by their distance apart squared), it has been relatively hard to measure, owing to the weakness of gravity. Steve Merkowitzz (l) and Jens Gundlach (r) with the Cavendish apparatus developed at the University of Washington. (Credit: Mary Levin, University of Washington)

When you drop an object from some height above the ground, it has an initial velocity of zero. Simple equations allow you to calculate the velocity an object falls after a given period of time and the velocity it reaches at a given displacement. The equations assume that air resistance is negligible. Examples demonstrate applications of the equations. Questions you may have include: What is the equation for the velocity for a given time? What is the equation for the velocity to reach a given displacement? What are some examples of these equations? Velocity with respect to time The general gravity equation for velocity with respect to time is: v = gt + vi Since the initial velocity vi = 0 for an object that is simply falling, the equation reduces to: v = gt where v is the vertical velocity of the object in meters/second (m/s) or feet/second (ft/s) g is the acceleration due to gravity (9.8 m/s2 or 32 ft/s2) t is the time in seconds (s) that the object has fallen Velocity of a falling object as a function of time or displacement Velocity with respect to displacement The general gravity equation for velocity with respect to displacement is: v = ±√(2gy + vi2) ± means plus or minus √(2gy + vi2) is the square root of the quantity (2gy + vi2) y is the vertical displacement in meters (m) or feet (ft) Since vi = 0, y is positive because it is below the starting point. Also, v is downward and positive. Only the + term of ± applies. Thus, the equation for the velocity of a falling object after it has traveled a certain displacement is: v = √(2gy) Examples The following examples illustrate applications of the equations. For a given time What will be the velocity of an object after it falls for 3 seconds? Solution Substitute in the equation: If you use g = 9.8 m/s2, v = (9.8 m/s2)*(3 s) = 29.4 m/s. If you use g = 32 ft/s2, v = (32 ft/s2)*(3 s) = 96 ft/s. For a given displacement What is the velocity of an object after it has fallen 100 feet? Since y is in feet, g = 32 ft/s2. Substitute in the equation: v = √[2*(32 ft/s2)*(100 ft)] v = √(6400 ft2/s2) v = 80 ft/s Summary There are simple equations for falling objects that allow you to calculate the velocity the object reaches for a given displacement or time. The equations are: Be a champion Websites - Physics Hypertextbook - Wikipedia - Calculator - Physics Classroom Books Top-rated books on Simple Gravity Science Top-rated books on Advanced Gravity Physics Share Click on a button to bookmark or share this page through Twitter, Facebook, email, or other services: Students and researchers

Yet stick a girl on live telly, and it's as if the cameras exert some weird gravitational pull on her blouse.A star wobbles on its axis when orbiting bodies like planets exert gravitational pull (force of attraction between bodies of matter).The suggestion is the moon's gravitational pull affects the amniotic fluid in the same way it affects the water in the sea.The shipwreck was first revealed when the gravitational pull of the sun, moon and planets led to extreme high and low tides in March.Tides are governed by the gravitational pull of the moon and, to a lesser extent, the sun.As a river basin soaks up water, the satellites record a stronger gravitational pull.Those Lebanese looks have quite the gravitational pull.Without taking into account gravitational pull or structural stress, tons of soil are transported to the roof of the mall, the weight greatly increased by rainwater, and guess what.A key operation called the Trans-Mars Injection (TMI) on December 1 will give Mangalyn enough speed to move out of Earth's gravitational pull and set it on a trajectory for Mars.India's Mars spacecraft has completed the first of a series of engine firings designed to free it from Earth's gravitational pull and propel it towards the Red Planet, scientists said Friday.It is believed 2011 QF99 is part of a larger-than-expected population of transient objects temporarily trapped by the gravitational pull of the Solar System's giant planets.The majority of the gas cloud has escaped from the black hole's gravitational pull but the tail continues to be stretched by the extreme gravity.

I've been reading many clever answers here about dark matter and dark energy that called my attention to this question. Since Einstein's theory relates matter and energy as different states of the same thing, is it valid to think about dark matter and dark energy in the same way? Are they two states of the same dark "thing"? Are they interchangeable? The short answer to your question is that we don't know if dark matter and dark energy are manifestations of the same dark "thing". We know they both must exist to explain certain phenomena, but we still know very little about their make up so we cannot assume they are linked. For now, we think of them as separate, and we believe the cosmos to be composed of roughly 0.03% heavy elements (anything other than hydrogen and helium), 0.3% neutrinos, 0.5% stars, 4% free hydrogen and helium, 25% dark matter, and 70% dark energy. Here is how we define them separately: Dark matter must exist to account for the gravity that holds galaxies together. If the only matter in the universe was matter we could directly detect, galaxies would not have had enough matter to have ever formed. The galaxies we observe today would fly apart because they wouldn't have enough matter to create a strong enough gravitational force to hold themselves together. Dark matter is also responsible for amplifying small fluctuations in the Cosmic Microwave Background back in the early universe to create the large scale structure we observe in the universe today. Dark energy, which also goes by the names of the cosmological constant or quintessence, must exist due to the rate of expansion we observe for our universe. Not only is the universe expanding, but this expansion is also accelerating so the unknown 'anti-gravity' force at work is termed 'dark energy'. Some researchers are searching for an explanation that encompasses both dark matter and dark energy. One example of such a theory uses a form of energy called a scalar field (it is a field because it has magnitude, energy and pressure, but it is scalar so it has no direction). Things would certainly be easier if we didn't need to have separate theories to explain dark matter and dark energy. However, other researchers look at dark matter and dark energy as two separate problems. For example, many string theories use supersymmetric particles to explain dark matter and make no connection to dark energy at all.

Matter is everything around you. Atoms and molecules are all composed of matter. Matter is anything that has mass and takes up space. If you are new to the idea of mass, it is the amount of stuff in an object. We talk about the difference between mass and weight in another section. Matter is sometimes related to light and electromagnetic radiation. Even though matter can be found all over the Universe, you will only find it in a few forms on Earth. We cover five states of matter on the site. Each of those states is sometimes called a phase. There are many other states of matter that exist in extreme environments. Scientists will probably discover more states as we continue to explore the Universe. You should know about solids, liquids, gases, plasmas, and one state called the Bose-Einstein condensate (BEC). Scientists have always known about solids, liquids, and gases. Plasma was a new idea when it was identified by William Crookes in 1879. The scientists who worked with the Bose-Einstein condensate received a Nobel Prize for their work in 1995. What makes a state of matter? It's about the physical state of the molecules and atoms. Think about solids. They are often hard and brittle. Liquids are fluidy, can move around a little, and fill up containers. Gases are always around you, but the molecules of a gas are much farther apart than the molecules in a liquid. If a gas has an odor, you’ll be able to smell it before you can see it. The BEC is all about atoms that are even closer and less energetic than atoms in a solid. Molecules can move from one physical state to another and not change their basic structure. Oxygen (O2) as a gas has the same chemical properties as liquid oxygen. The liquid state is colder and denser, but the molecules (the basic parts) are still the same. Water (H2O) is another example. A water molecule is made up of two hydrogen (H) atoms and one oxygen (O) atom. It has the same molecular structure whether it is a gas, liquid, or solid. Although its physical state may change, its chemical state remains the same. So you're asking, "What is a chemical change?" Let's start with a glass of pure water. If the formula of water were to change, that would be a chemical change. If you could add a second oxygen atom to a water molecule, you would have hydrogen peroxide (H2O2). The molecules would not be water anymore. The reality of creating hydrogen peroxide is more difficult. Chemical changes occur when the bonds between atoms in a molecule are created or destroyed. Changes in the physical state are related to changes in the environment such as temperature, pressure, and other physical forces. Generally, the basic chemical structure does not change when there is a physical change. Of course, in extreme environments such as the Sun, no molecule is safe from destruction. Or search the sites for a specific topic. Alien Matter in the Solar System (NASA Video) Encyclopedia.com: Wikipedia (Matter): Wikipedia (States of Matter):

Newton’s Law of Universal Gravitation is a fundamental physical law. We experience its effects everywhere on this planet, and it is the prime mover in the vast world of astronomy. It can also be expressed in a relatively simple mathematical formula on which SAT II Physics is almost certain to test you. Gravitational Force In 1687, Isaac Newton published his Law of Gravitation in Philosophiae Naturalis Principia Mathematica . Newton proposed that every body in the universe is attracted to every other body with a force that is directly proportional to the product of the bodies’ masses and inversely proportional to the square of the bodies’ separation...