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I am a high school student. I read in my book that moon and satellite are in free fall condition with Earth. I asked my teachers about this, but I'm not able to understand. Can anyone explain to me why this happens?

Edit- i want to add one more question what causes the planet to revolve in elliptical path why not circular

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Free fall means the only force acting on the object is gravity. For objects orbiting the Earth this is the case, so we say they are in free fall.

Indeed, you can think of objects that are orbiting the Earth as objects that are falling but never hit the ground.

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objects in orbit are falling, but they are moving forward fast enough so they never meet the Earth, they just keep falling around it. Picture throwing a ball, it is going forward, and it is falling, so it moves in a curve. If it were thrown in space where there is no air to slow it, and it were thrown at the right speed and the right height above Earth it would curve around Earth forever.

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  • Place a satellite high up and just leave it there. It falls and crashes.
  • Repeat, but this time give it a small push sideways. It still falls and crashes. But this time it didn't fall straight down. Because, it had some sideways speed that moved it sideways. It crashes slightly more to the side.
  • Repeat, but push much harder this time. It still falls and crashes, but it crashes maybe several hundred kilometers away from the point it is hovering above. This is because you gave it a large sideways speed; while falling it still moved sideways as well.
  • Now, finally, repeat, but push much, much, harder! As always, it still falls. But this time it misses Earth. It does fall towards Earth, but the sideways speed is so large, that before it lands, it has moved sideways away from Earth. It falls but misses.

When it falls but misses, it will fall past Earth. Now it is on the other side, moving away from Earth. Gravity still pulls in it, so it slows down until it stops and starts falling back towards Earth, this time from the other side. The same thing happens: it falls but misses. And everything repeats itself. This will repeat itself forever.

The path it takes in this way is an ellipsis. Give it a bit more sideways speed to start with, ant the ellipsis becomes slightly wider. With some specific sideways speed, the elliptic path is just wide enough to be just as wide as it is long - the path is now circular (which is just a "special-case" of an ellipsis). With an even larger sideways speed, the path will be wider than it is long, and we have an ellipsis again, just a "fat" one instead of a "thin" one.

This is how orbits work for any celestial object, including our own satellites, moons, Suns, starts and planets. If something orbits in a circular path, there is nothing special about it; it just happened to have the fitting initial sideways speed.

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Any object moving in a circle is falling towards circle's center at every moment. A centripetal force is always acting on the object towards the center of circle. Because of this force, object velocity is keep on changing. Centripetal force always try to pull object towards center.

In case of moon or satellite, speed is always constant but velocity keeps on changing. Because object is being pulled towards earth at every moment, it's said to be falling towards earth.

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An object in orbit has two vectors acting on it simultaneously. The first is the gravitational vector wanting to pull the object directly to the center of the earth. The second vector acts on the object at 90 degrees to the first and we call this "angular momentum". When the second vector is set to zero we call this "free fall". When the second vector has a velocity greater than zero the object will still be in free fall but it will be displaced down range a little bit before returning to earth. The faster it goes the farther down range. Finally it reaches a point where it never does return to earth. Now it is circling the earth in a state of free fall. The gravitational vector keeps pushing it towards earth but the earth keeps curving away. An even faster velocity will take the object completely out of orbit and escape velocity will be achieved. Set a course for Rigel three Zulu.

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    $\begingroup$ I think your answer is rather unclear. What do you mean by angular momentum is a vector that acts on the object? Your answer could be severely improved if you mentioned explicitly energy considerations instead of just talking about "velocities". $\endgroup$ – Alonso Perez Lona Jun 14 at 22:15
  • $\begingroup$ It is assumed that the energy has already been applied per Newton's second law. The object is now drifting freely as per Newton's first law. The only thing that is different is that a large gravitational mass point has been introduced into the consideration. Now Newtons linear momentum is bent around the mass point converting it to angular momentum. It is now the objects velocity that prevents it crashing to earth, staying in orbit, or escaping to space. $\endgroup$ – Metaman Jun 15 at 2:51

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