Gravity or gravitational forces are forces of attraction. We're not talking about finding someone really cute and adorable. It's like the Earth pulling on you and keeping you on the ground. That pull is gravity at work. Every object in the universe that has mass exerts a gravitational pull, or force, on every other mass. The size of the pull depends on the masses of the objects. You exert a gravitational force on the people around you, but that force isn't very strong, since people aren't very massive. When you look at really large masses, like the Earth and Moon, the gravitational pull becomes very impressive. The gravitational force between the Earth and the molecules of gas in the atmosphere is strong enough to hold the atmosphere close to our surface. Smaller planets, that have less mass, may not be able to hold an atmosphere. Obviously, gravity is very important on Earth. The Sun's gravitational pull keeps our planet orbiting the Sun. The motion of the Moon is affected by the gravity of the Sun AND the Earth. The Moon's gravity pulls on the Earth and makes the tides rise and fall every day. As the Moon passes over the ocean, there is a swell in the sea level. As the Earth rotates, the Moon passes over new parts of the Earth, causing the swell to move also. The tides are independent of the phase of the moon. The moon has the same amount of pull whether there is a full or new moon. It would still be in the same basic place. We have to bring up an important idea now. The Earth always produces the same acceleration on every object. If you drop an acorn or a piano, they will gain velocity at the same rate. Although the gravitational force the Earth exerts on the objects is different, their masses are just as different, so the effect we observe (acceleration) is the same for each. The Earth's gravitational force accelerates objects when they fall. It constantly pulls, and the objects constantly speed up. People always say, "What about feathers? They fall so slowly." Obviously, there is air all around us. When a feather falls, it falls slowly because the air is in its way. There is a lot of air resistance and that resistance makes the feather move slower. The forces at work are the same. If you dropped a feather in a container with no air (a vacuum), it would drop as fast as a baseball. But what keeps the Moon from falling down, if all of this gravity is so strong? Well, the answer is that the moon IS falling; all the time, but doesn't get any closer to us! Remember that if there wasn't a force acting, the Moon would be traveling in a straight line. Because there IS a force of attraction toward the Earth, the moon "falls" from a straight line into a curve (orbit) around the Earth and ends up revolving around us. The Earth's gravity holds it in orbit, so it can't just go off in a straight line. Think about holding a ball on a string and spinning it in a circle. If you were to cut that string (no more gravity), the ball would fly off in a straight line in the direction it was going when you cut the string. That direction, by the way, is not directly away from your hand, but tangent to the circle. Tangent is a geometry term used to describe a direction that are related to the slope of a curve. Math stuff. The pull of the string inward (toward your hand) is like the Earth's gravitational pull (inward toward the center of the Earth). Or search the sites for a specific topic. How Gravity Affects Molecules (NASA-eClips Video) Encyclopedia.com (Specific Gravity):

Definitions of Fact, Theory, and Law in Scientific Work Science uses specialized terms that have different meanings than everyday usage. These definitions correspond to the way scientists typically use these terms in the context of their work. Note, especially, that the meaning of “theory” in science is different than the meaning of “theory” in everyday conversation. Fact: In science, an observation that has been repeatedly confirmed and for all practical purposes is accepted as “true.” Truth in science, however, is never final and what is accepted as a fact today may be modified or even discarded tomorrow. Hypothesis: A tentative statement about the natural world leading to deductions that can be tested. If the deductions are verified, the hypothesis is provisionally corroborated. If the deductions are incorrect, the original hypothesis is proved false and must be abandoned or modified. Hypotheses can be used to build more complex inferences and explanations. Law: A descriptive generalization about how some aspect of the natural world behaves under stated circumstances. Theory: In science, a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.

Gravity is a natural occurrence in which physical objects are attracted toward one another. This attraction is proportional to the objects’ masses. Since the mass of each planet is different, the gravitational pull on an object will be different on each planet as well. Hence, an individual’s weight would vary depending on what planet they are on. Below is a list of the gravitational pulls of each planet from strongest to weakest. 1. Jupiter has an incredible gravitational pull of 24.79 m/s2. This is nearly 2.53 times the gravity of what we experience here on planet Earth. If you weighed 100 lbs on Earth you would weigh 236.4 lbs on Jupiter. 2. Neptune has a gravitational pull of 11.15 m/s2 compared to Earth’s pull of 9.81 m/s2. An individual weighing 100 lbs on Earth would weigh 112.5 lbs on Neptune. 3. The planet Saturn has a gravitational pull of 10.44 m/s2. A human weighing 100 lbs on Earth would weigh an additional 6.4 lbs on Saturn. 4. Our home planet of Earth has a gravitational pull of 9.81 m/s2. If the gravitational force was cut in half, objects would fall at half the speed that they currently do. 5. Venus has a slightly lower gravity than Earth with a pull of 8.87 m/s2. If you weighed 100 lbs on Earth you would weigh in at 90.7 lbs on Venus. 6. Uranus’ gravitational pull of 8.69 m/s2 is very close to that of Venus. Similarly, if you weighed 100 lbs on Earth you would weigh 88.9 lbs on Uranus. 7. Mars has a very low gravitational pull of only 3.71 m/s2. If you weigh 100 lbs on Earth you would only come in around 38 lbs on the red planet. 8. At 3.7 m/s2, Mercury’s gravitational pull almost exactly the same as that of Mars. Someone weighing 100 lbs on Earth would only weigh 38 lbs on Mercury. Gravity on Mercury is therefore 2.65 times less than what we experience here on Earth.

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.