You were caught red-handed dropping a big, fat water balloon on your boss’s head from the window of your third-floor office. Allow me to suggest a possible line of defense…
Gravity is an attractive force which all matter possesses. Every bit of matter attracts every other bit of matter. The strength of that attraction depends on two things - the mass of any two objects, and the distance between those objects. The relationship between gravity and mass is direct and simple. If an object contains 10 times more mass than another object, the former will exert 10 times more gravitational influence. The relationship between gravity and distance is a little different. If the distance between two objects were to increase by a factor of 10, the gravitational influence exerted by each object upon the other would decrease by a factor of 100. In the language of the physicist, the effect of gravitational force "varies inversely as the square of the distance". This principal is known as the inverse square law. It appears elsewhere throughout the natural world, describing patterns of magnetic attraction, as well as the light received from luminous bodies over great distances. The condition of weightlessness in space is one of the most commonly misunderstood concepts of popular science. This should come as no surprise; after all, faced with live video of orbiting astronauts floating about their vehicles, we the observers are left to conclude that there must be no gravity "up there". A complete lack of gravity in Earth orbit seems almost reasonable at first, until you ask yourself what is holding the vehicle in its orbit. If that’s not enough to dissuade you, try applying the inverse square law to the problem and you’ll find virtually no difference in gravitational effect from the Earth’s surface, out to a typical two or three hundred mile-high space shuttle orbit. So why do things float in space? Well, deep space and Earth orbit are two very different environments, requiring two very different answers. No doubt this accounts for a substantial share of the confusion. In either case we must deal with the effects of both gravitational attraction and acceleration. Weightlessness in deep space is due to the tremendous distances between massive objects. Stuff is so far apart out there that the gravitational attraction imposed on an interstellar spacecraft is very subtle, but certainly not escapable. So long as we avoid accelerationi by changing neither the speed nor direction of our motion during our observations, our vehicle and its contents will indeed exhibit that freefloating state of being we call weightlessness. But alas, we have escaped nothing - far from it. It is gravity, after all, that organizes solar systems into galaxies and galaxies into clusters. It is gravity that controls the expansion of the universe. Our spacecraft would be subject to these same influences. Although this subtle influence is not strong enough to create 'both feet planted firmly on the ground'-style gravity conditions, the very idea of actually "escaping" gravity is the cosmological equivalent to windmill jousting ii. The forces at work on a spacecraft in orbit around the Earth are the same forces as those encountered in deep space. In Earth orbit, however, we are solving a different problem. We are now very close to a very large mass. We know we haven’t got a comet’s chance on Mercury of escaping its gravity, but maybe, just maybe, we can outrun it. First we need to get into orbit, and as with so much in life, it all comes down to baseball. The curved path described by a baseball in flight can be thought of as a failed orbit. Hit a baseball straight ahead and it soon arcs back down to the ground. Hit the ball a little harder and it will travel a little farther before arcing down to the ground. Even with Babe Ruth swinging his lucky bat, that ball is only going to achieve a few hundred feet of distance before it eventually arcs back to the ground. In every attempt the ball’s forward velocity is soon defeated by the attractive force of Earth’s gravity, a force which is constant, and over which we can exert no control. We can, however, control the velocity imparted to the ball. The ball’s velocity is a variable. Back to the ball game. The Babe walks. The bases are loaded and you step up to the plate. A hush falls over the expectant throng. The tension of a gripping 3-2 count is ruptured as wood meets leather with such astonishing force that the ball never arcs back down to the ground (forget about air resistance for the purpose of this illustration). Check it out, sports fans – the ultimate grand slam. You have achieved Earth orbit! "No big deal", you declare to the usual mob of Jimmy Olsens gathered in the post-game locker room. You go on to explain how you "simply got the ball moving so fast that, as it curved downward to strike the ground, the ground curved out from under it". In other words, the curved path of the ball matched the curvature of the Earth, and the ball began "falling around" the Earth. Upon retiring from professional baseball you get a lucrative deal with an aerospace company explaining orbital mechanics to senior project managers. Enough baseball - back to the business of space travel. Earth orbit is achieved when the velocity of a spacecraft traveling in a closed circuit around the Earth exactly balances the gravitational attraction exerted by the Earth on the spacecraft. If the spacecraft neither accelerates nor decelerates by changing speed or direction, it will remain in a state of perpetual freefall. Its orbit will remain stable. Since the contents of the vehicle are also traveling at that special velocity, they float freely about its interior. Back on Earth this relationship between velocity and gravity is inescapably unbalanced; the same stuff that danced about the cabin so elegantly in orbit drops boorishly to the floor as it is accelerated toward the center of Earth's great mass. In the case of Earth orbit, then, weightlessness is a byproduct of a state of unaccelerated freefall. Newton's first law of motion nicely describes what's happening. It tells us that a body at rest wants to stay at rest - I like to call this the Monday morning rule. Before launch, a spacecraft and its passengers are at rest. Once launch commences, the passengers are reluctant to move from their present position, but the rocket has other ideas. Schmooshed against their seats in a futile act of defiance, these otherwise ambitious astronauts help illustrate a deeply rooted lack of self-motivation which all matter possesses (starting to sound like Monday morning yet?). This resistance to forward motion continues until the vehicle reaches the apex of its curved path, and starts back down. Once established on the freefalling, downward side of its flight path, the spacecraft and its contents are on their own. The attractive force of Earth's gravity, which happens to be an acceleration of 32 feet per second for each second of freefall, is equally imposed and obeyed by all. You may have heard tell of a modified KC-135 aircraft in which astronauts can experience brief periods of microgravity. Nicknamed the "vomit comet" by its anguished passengers, the aircraft creates a few seconds of weightlessness by flying the same curved path through the air as a free-falling baseball. Another name for this curved path that we've been talking about is parabola iii. A jet aircraft cannot fly fast enough to achieve Earth orbit. Friction with the atmosphere, or drag, and insufficient power are the most obvious impediments. Short periods of weightlessness are instead achieved by flying the aircraft through a parabolic curve, which is a mathematical way of describing the path of a free-falling object. An aircraft placed in such a controlled free-fall will exhibit the same properties of weightlessness as an object in orbit. Since the aircraft’s speed is insufficient to get the curve of its parabolic flight path to match the curvature of the Earth, a smaller parabolic path, one that falls within the safe operating limitations of the aircraft, must be flown. The pilot must eventually break off the maneuver or slam into the ground. In a large jet aircraft this technique will provide approximately 30 to 40 seconds of weightlessness at a time. The same technique can be applied in light aircraft as well, albeit with less impressive results. The breadth of the parabolic flight path depends upon the mass of the aircraft, so a small aircraft must fly a small parabolic curve. Since the duration of the weightless effect depends upon the breadth of the parabolic flight path, the weightless effect achieved by a light aircraft in parabolic flight will last only a few seconds.
Incidentally, the engineers who design roller coasters and other "extreme" rides use the parabolic curve to induce episodes of zero-g, or "airtime" as enthusiasts call it, into their depraved machines. So far we've been playing safely inside the perimeters of Universal Gravitation Land, that classic tourist attraction of attraction founded by Sir Isaac Newton (1642-1727) and his somewhat less talented brother Roy. However, any further investigation of gravitation must begin with a visit to the premier academic theme park of the twenty-first century science, Relativity World (kids, don’t forget to have your pictures taken with the guy in the Albert Einstein suit). It would be downright silly to attempt a thorough recounting of these ideas in just a few paragraphs, so I will merely try to whet your appetite for a little self-directed study. Einstein's relativity does not replace Newton's universal gravitation, it merely modifies and partially supercedes aspects which proved inconsistent with developments in electromagnetic theory. There are those who would say I should have defined gravity as an apparent attractive force in the opening paragraph to better prepare the reader for these modifications. To avoid early confusion, I waited. You're welcome. When Einstein first published his theory, he knew he hadn’t worked out all the kinks yet. This is why there are two versions, the special theory of relativity, and the general theory of relativity. In his special theory of 1905, brainiac blows the whistle on the absolute invariability of intervals of both time and space. Distances in space, the passage of time, an object’s mass, and the energy involved, all vary according to the motion of the observer. Our clocks and tape measures serve us just fine, thank you very much, because we all live, work, and play within more or less the same reference frame. In other words, my motion relative to the speed of light is pretty much the same as your motion relative to the speed of light, so space, time, mass, and energy measure up the same. The special theory dealt only with observers whose motion relative to one another was the same. The general theory, published in 1915, goes on to include observers whose motions are accelerated relative to one another. It describes a universe in which space and time are interwoven threads of the same universal fabric, and in which gravitation alters the geometry of this space-time continuum. General relativity, above all, asserts that our universe does not conform to the familiar Euclidean iv form of geometry, which builds its constructs on a grid of perfectly parallel and perpendicular lines. Gravitation, according to general relativity, curves the grid which itself is space-time. When imagining the interaction of objects in our universe, says Einstein, don’t picture a flat piece of inflexible graph paper across which a large mass uses its powers of gravitational attraction to draw nigh a smaller mass. Instead, imagine a tautly stretched sheet of rubber graph paper in which ball-bearings of various sizes and weights create depressions - funnel-shaped concavities in space-time which alter the motion of passing objects. Imagine, also, how the grid lines on this rubber graph paper would be stretched out by the massive metal spheres. This distortion of the very grid itself is the non-Euclidean geometry Einstein refers to. Food for thought. To summarize… What goes up must come down
It all comes down to this common and uncomplicated axiom in the end. One needn’t master all the subtleties of Newton and Einstein to appreciate the role gravity plays in our everyday world. And by the way, anyone who tells you he has mastered all that is a big fat liar v. As to your own experimentations, a panel consisting of myself, the Assistant District Attorney, Miss Manners, and 4 out of 5 participating Physicists agreed that wide open spaces and soft objects with rounded edges are much preferable to a bag of balloons and a third-story window with a good view of the sidewalk. I mean, even with a defense as incontrovertible as MASS, why take your chances with the courts. Bruce Mattson is the Science and Technology Specialist at the McAuliffe/Challenger Center i Acceleration produces a reactive force, the effect of which is virtually indistinguishable from the attractive force of gravity. ii It is this notion that nothing can escape gravitational influence altogether that caused the term "zero-gravity" to fall into disuse. Micro-gravity is a much better descriptor given the pervasiveness of this natural force. iii The parabola, along with the circle, the ellipse, and the hyperbola, make up the four conic sections derived by slicing into a cone at specified angles. The circle and ellipse are "closed" curves, while the parabola and hyperbola are "open" curves. The ends of a parabolic curve eventually become very nearly parallel if the curve is continued; the ends of a hyperbolic curve, on the other hand, flare more widely apart when continued. iv Euclid was a Greek mathematician from around 300 BC whose work, Elements, forms the basis for textbooks on the subject of plane geometry even today. Trouble is, Euclid's brand of geometry can only handle three dimensions, all spatial. Relativity requires that there are in fact four dimensions - three of space and one of time. v A Big Fat Liar is anyone who tells you he would like to tell you more about relativity, but it would take too long so he'll just whet your appetite.
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