Pulled from Quora, here is one of the best, and most popular, answers to a question. Written by Mark Eichenlaub, a graduate student in physics.
How close does an object have to be to earth to be pulled by gravity?
This question doesn’t have a direct answer because, for lack of a less-direct way of saying it, that’s not the way it works. If there were no atmosphere, you could have the ISS be just above the surface of the Earth, high enough only to clear the mountains. On the other hand, you could have something as far out as the moon, and if it weren’t going fast enough and in the right direction, it would still fall back down. The ISS doesn’t stay up because of how high it is, but because of a combination of that and how fast it’s going.
One of the most difficult things to learn about physics is the concept of force. A force in a given direction does not make things go straight in that direction. Instead, it influences the motion to be a bit more in the direction of the force than it was before.
For example, if you roll a bowling ball straight down a lane, then run up beside it and kick it towards the gutter, you apply a force towards the gutter, but the ball doesn’t go straight into the gutter. Instead it keeps going down the lane, but picks up a little bit of diagonal motion as well.
Now we can talk about an very early thought experiment in physics. Imagine you’re standing at the edge of a cliff 100m tall. If you drop a rock off, it will fall straight down. If you throw the rock out horizontally, it will fall down, but it will keep moving out horizontally as it does so, and falls at an angle. (The angle isn’t constant – the shape is a curve called a parabola, but that’s relatively unimportant here.)
The the force is straight down, but that force doesn’t stop the rock from moving horizontally. If you throw the rock horizontally harder, it goes further, and falls at a shallower angle. The force on it is the same, but the original velocity was much bigger and so the deflection is less.
Now imagine throwing the rock so hard it travels one kilometer horizontally before it hits the ground. If you do that, something slightly new happens. The rock still falls, but it has to fall more than just 100m before it hits the ground. The reason is that the Earth is curved, and so as the rock traveled out that kilometer, the Earth was actually curving away underneath of it. In one kilometer, it turns out the Earth curves away by about 10 centimeters – a small difference, but a real one.
As you throw the rock even harder than that, the curving away of the Earth underneath becomes more significant. If you could throw the rock 10 kilometers, the Earth would now curve away by 10 meters, and for a 100 km throw the Earth curves away by an entire kilometer. Now the stone has to fall a very long way down compared to the 100m cliff it was dropped from. Continue reading