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3. Everything / Maths, Science & Technology / Physics

Gravity

Among the four basic interactions that make things stick together - electromagnetic, strong and weak nuclear and gravitational interactions - gravity is the least understood. Gravity is an interaction that happens between bodies with discernible mass.

It might seem ridiculous, but all interactions are thought of as being due to an exchange of particles. The exchange particle for a gravitational interaction would be called a 'graviton'. No one has yet detected one, despite the fact that gravity is a pretty straightforward force to calculate and its presence is manifestly obvious in mundane life. Then again, there's a lot about physics that doesn't really make much sense.

Since gravity plays a fundamental role in the function of our world and in our daily existence, it was the first of these basic interactions to be quantified by the biggest 'propellerhead' of his time, Newton, as part of his canonical laws of physics. The law of gravity dictates that discernible mass mutually attracts depending on the distance and the mass. The force1 (F) generated by this attraction is:

F = G m1m2 / R2

Where G = 6.67 × 10-11Nm2 / kg2, which is the gravitational, or Newton's constant, m1 is the mass of the first object, and m2 the mass of the second object, and R the distance between the two objects. Replacing m1 by the Earth's mass (6 × 1024kg) and R by the Earth's radius (6.4 × 106m) the equation becomes F = (m1× G / R2) × m2. The first factor, the one in brackets, can be calculated. It is 9.81m/s2 or g: the acceleration due to gravitational attraction of an unsupported object towards the surface of this planet. So F = m2× g, a formula horribly familiar to physics students everywhere.

This means that two bits of mass that are at a certain distance to each other will be drawn closer by gravitational attraction - the amount of force generated varies depending on the variables listed in the equations above. This also means that things lying obscenely far away from each other will not produce any detectable effect on the other, which is good news for people scared of black holes and wormholes.

A more modern view on this topic was presented by Albert Einstein. In a far more complex description, dealing with curved space, mass-energy tells space-time where to bend and vice versa. Obviously, the effects on everyday life are negligible. For the sake of completeness, it should be remarked that there are indeed observable relativistic effects, such as the trajectory of light being bent by the sun's mass. To summarise, Einstein's relativistic description of gravity is more accurate, far more complicated, of negligible effect on everyday life, and still incomplete.

Circumventing Gravity

There is not a classical, empirical, formal or elegant way to eliminate gravity as such. Luckily there are some tricks to defy the influence of gravity. They all have one feature in common; apply a force equal to or bigger than the gravitational force with opposite direction. For instance, a chair can be regarded as an anti-gravity device. It exerts a force equal to the gravitational force, but opposite in direction on the sitting person's bottom, preventing said bottom being accelerated groundwards and hitting the floor. Spoons can also act as 'anti-gravity' devices in similar fashion.

Spacecraft use the force generated by their booster exhaust to circumvent the gravitational force that ordinarily keeps them earthbound. Later they use the centrifugal force due to their orbit and stay 'up there', circling the Earth like other man-made satellites.

This trick couldn't be performed at sea level, though. One problem would be the resistance caused by friction with air molecules2, another would be the distance R between the two bodies being much smaller, hence the force of gravity stronger and the smaller object pulled towards the larger body.

Gravity and Quotidian Life

One doesn't have to be a genius to note that things fall towards the ground. This is the most obvious characteristic of gravity, as experienced here on the surface of the planet Earth. Mankind has been witnessing and even using gravitational interaction in many ways, be they obvious or obscure.

Tides are the result of the gravitational force of the moon affecting the world's oceans, which in turn affect humanity's existence in a number of ways, such as enabling surfing, causing the erosion of coastlines, and making possible the use of tides to produce electric power. Hydroelectric power plants convert gravitational force acting on water into electricity. The ballistic trajectories of arrows or other projectiles like rocks fired from catapults had been observed long before the concept of 'gravity' had even been thought of. Every time that anything falls towards the ground, be it droplets of rain, skydivers, cups, books, or slices of pie, one witnesses a direct manifestation of gravity.

The Ultimate Limits Imposed by Gravity

It seems that too much gravity isn't necessarily a good thing. When there is too much stuff in one place the gravity is so humongously strong that not even light can escape. If the necessary critical mass is reached, black holes can develop, and these make some people very scared, and even more non-scientific people utterly confused.


1 The unit of measurement of force is the newton.
2 This resistance would produce drag, which slows the orbiting body down to the point where gravity begins to act upon it once more so that the satellite will be drawn groundwards.

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Entry Data
Entry ID: A565832 (Edited)

Written and Researched by:
Dr Hell

Edited by:
NexusSeven


Date: 08   June   2001


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Referenced Guide Entries
Newton's Laws of Motion
Students
Critical Mass
Man-made Satellites
Physics and the Knowledge of Ignorance
Water
Relativity
Wormholes and Black Holes
Spoons
The Moon and the Tides
The Ballista & Trebuchet - Antique Catapults
Units of Measurement
Learning to Skydive


Related BBC Pages
Bitesize Physics: Gravity and Movement


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