Challenger Learning Center
Gravity: It's the Law!
- what is gravity...
- the zero-gravity myth...
- weightless in deep space...
- getting to Earth orbit...
- weightless in Earth orbit...
- the infamous "Vomit Comet"...
- parabolic flight demonstration
- fun with relativity...
- in conclusion...
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…
"Not only do I plead not guilty to all charges, your honor, but I also intend to expose the real villain in this case. Since the Earth’s gravity pulls all objects toward the center of its MASS, and since gravity is a product of MASS, clearly MASS is the guilty party and it is merely an unfortunate coincidence that the plaintiff happened to get in the way. "
what is gravity...
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 zero-gravity myth…
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?
weightless in deep 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.
getting to Earth orbit…
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.
weightless in Earth orbit…
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.
© 2012 Christa Corrigan McAuliffe Center for Education and Teaching Excellence
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