5 ms·
What they mean is by "moving so fast" is, it's moving so fast horizontally. Imagine being at the top of a steep mountain and throwing a rock directly out from y
by thousandautumns 8y ago
What they mean is by "moving so fast" is, it's moving so fast horizontally. Imagine being at the top of a steep mountain and throwing a rock directly out from you. The rock will continue to move horizontally, due to the force behind your throw, as well as vertically, as it drops due to gravity. If you don't throw it hard enough, then the trajectory of the rock won't clear the mountain slope. If you throw it too hard, it will clear the base of the mountain by a wide margin. But if you throw it just right, the rock could hypothetically have a trajectory that is perfectly parallel to the mountain slope (in reality, due to things like air resistance, this isn't all that feasible).
Anything in orbit it literally the exact same. You get something moving fast enough horizontally that even though the Earth's gravity is still pulling on it, its trajectory towards earth is perfectly parallel (actually often not perfectly, but in principle) with the curvature of the planet. But by doing it high enough in the sky, you can escape the atmosphere of the earth, to where there is no wind resistance, meaning once you get up to a high horizontal speed, you can turn off the engines and coast perpetually without slowing down.
So everything in orbit, from the ISS to satellites, to the moon, are all in Earth's gravity well and are falling towards the planet, but they have a horizontal speed as well that keeps them from colliding with the surface. The Earth and the rest of the planets in the solar system do the same thing in terms of their orbit with the Sun.
It's also why sometimes satellites or space junk that have been in the sky for years will come crashing down to Earth. Sometimes the calculations for how fast you need to be going are off, or something will throw off it's horizontal momentum, and that will cause it's trajectory to dip just enough that it is no longer orbiting the earth perfectly, but instead is spiraling ever so slightly towards the surface, and will over the course of months or years or decades dip closer and closer until it enters the atmosphere, at which time air resistance becomes a factor again and it breaks up and really plummets.
- fmihaila 8y ago> But if you throw it just right, the rock could hypothetically have a trajectory that is perfectly parallel to the mountain slope (in reality, due to things like air resistance, this isn't all that feasible). Not quite. Ignoring air resistance, any throw you could possibly produce will have a parabolic trajectory, which means it could be parallel with the mountain slope only if the slope itself is parabolic.
- aw1621107 8y agoNitpicking here, but the trajectory would be approximately elliptical, not parabolic. A parabolic trajectory would require a uniform gravitational field.
- fmihaila 8y ago> A parabolic trajectory would require a uniform gravitational field. A uniform gravitational field is such a good approximation for the situation described that I didn't think of including that proviso. Ballistic trajectory calculations close to the Earth's surface assume constant g.
- Cogito 8y agoTo clarify for anyone reading, the difference is not constant vs variable magnitude of g, it's constant vs variable direction of g. Gravity is a vector pointing at the (weighted by inverse of distance squared) centre of mass, so once you are operating on a global scale you have to account for the direction of g changing as you move around. The force always pointing towards a single point is what causes an ellipse to form. If the force always pointed down it would be a parabola, and over short distances on the surface this is a really good approximation.
- aw1621107 8y agoOnly now when re-reading your comment do I realize that you said "any throw you could possibly produce", in which case you are correct. I was thinking that you were talking more about the "fire a stupendously powerful cannon to get an orbital (or closeish) trajectory" situation, in which case the velocities would be high enough to not use a uniform gravitational field. My apologies for the misunderstanding.
- thousandautumns 8y agoThis is correct, and I was making a simplification. A better example would be a curved hill.
- InGodsName 8y agoHow do they maintain horizontal velocity?
- matt_j 8y agoThe ISS? It's above the atmosphere so there is no air resistance to slow it down. The velocity is constant after the initial boost into orbit. Actually that's not quite correct, I believe it is not quite out of the atmosphere completely and as such there is a (very) small amount of air resistance and they have to fire engines to boost back into the appropriate orbit every once in a while.