How close does an object have to be to earth to be pulled by gravity?

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.

Check out the following drawing. It was made by Isaac Newton, the first person to understand orbits. IMHO it is one of the greatest diagrams ever made.

What it shows is that if you could throw the rock hard enough, the Earth would curve away from underneath the rock so much that the rock actually never gets any closer to the ground. It goes all the way around in the circle and might hit you in the back of the head!

This is an orbit. It’s what the ISS and the moon are doing. We can’t actually do it here close to the surface of the Earth due to wind resistance, but on the surface of the moon, where there’s no atmosphere, you could indeed have a very low orbit.

This is the mechanism by which things “stay up” in space. It’s certainly true that gravity gets weaker as you go further out. The Earth’s gravity is much weaker at the moon than at the ISS (but it is almost the same strength at the ISS as at the surface of Earth – the ISS is only 400km up but the radius of the Earth is 6000 km, so the orbit is not really that high. In fact it’s called a “low Earth orbit”).

Because gravity is so much weaker at the moon, the moon orbits much more slowly than the ISS. It takes one month to go around. The ISS takes a few hours. An interesting consequence is that if you go out just the right amount in between, about six Earth radii, you reach a point where gravity is weakened enough that an orbit around the Earth takes 24 hours. There, you could have a “geosynchronous orbit”, a satellite that orbits so that it stays above the same spot on Earth’s equator as Earth spins.

Although gravity gets weaker as you go further out, there is no cut-off distance. In theory, gravity extends forever. However, if you went towards the sun, eventually the sun’s gravity would be stronger than the Earth’s, and then you wouldn’t fall back to Earth any more, even lacking the speed to orbit. That would happen if you went about .1% of the distance to the sun, or about 250,000 km, or 40 Earth radii. (This is actually less than the distance to the moon, but the moon doesn’t fall into the Sun because it’s orbiting the sun, just like the Earth itself is.)

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