# How does gravity sling off objects if it is a force of attraction between two bodies with mass

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We often here of star systems loosing a planet or objects (satellites) getting accelerated away from a huge source of gravity (huge mass).

In: Physics

A collision with another object can eject objects from orbit. A bunch of extra force accelerating an obect in its current direction of travel, its orbit changes that it escapes the gravity well.

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If you have just two things, you can’t do this. You need to have at least three, and then the trick is you “take” speed from one of them to speed you up relative to the other.

Say you’ve got a planet going around the Sun and a spacecraft coming up to the planet. If you plan your approach just right, the very tiny gravity your spacecraft has will slow the planet down an incredibly tiny amount but in return your spacecraft gets sped up.

You’re probably thinking of a [Gravity slingshot](https://en.wikipedia.org/wiki/Gravity_assist). It only works because the planet (or whatever) is in motion. Thrusting while closer to a planet as it’s passing you has a bonus effect as opposed to thrusting in empty deep space. And the planet slows down a tiny fraction.

Likewise if you wanted to fly into a solar system and slow down to match speeds you’d thrust while Jupiter is coming towards you on it’s orbit path. It can work as a space brake too.

To get a gravity slingshot effect, you have to pass behind a heavy thing, like behind the moon as it’s orbiting the Earth. You’re already going fast enough to escape it, so you trace out what would be a hyperbolic orbit behind it. Except that as you pass behind it it pulls you in, and is also moving away from you, so it pulls you along with it into a slightly faster orbit, and you leave going faster.

It’s kind of the same interaction as if you physically bounced the object off the front of the moon as it’s orbiting. It bounces off faster than it was going, because the moon was going toward it. Like bouncing a rubber ball off the front of an approaching car.

If you had two mutually repulsive objects, like two objects with the same electric charge, they would “bounce” off each other this way, but without touching because their electric forces would mediate the interaction, and they would also follow a hyperbolic “orbit” that turns into straight lines at infinity. Because classical gravity is mathematically the same except that it’s attractive, you have to “bounce” off the back of it by orbiting behind it instead of bouncing off the front. So it’s more like the moon’s gravity is doing a hammer throw of the object.