1. There gave been discussion earlier about using gravitational pull for space voyage. I mean, not only for changing directions but also to make acceleration and retardation for the ship. I understand that a body continues in its state of rest or motion unless an external force applied on it. A body falling into a gravitational field gains some momentum but require equal force to exert while escaping it. If that is the case, how something can gain or loose a momentum in space? Can anyone explain me in simple words?
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2.

3. If you pass close to another very massive object (like a planet) that is moving very fast relative to you, that object will "pull you along" as you pass near it.

4. Originally Posted by Scifor Refugee
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I thought gravity is not a function of rotation! I mean the field of gravity do not rotate hence pull a external object to accelerate. If so, how and how far one can gain speed? Because the pulling force will also will pull you back?

5. Originally Posted by sak
Originally Posted by Scifor Refugee
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I thought gravity is not a function of rotation! I mean the field of gravity do not rotate hence pull a external object to accelerate. If so, how and how far one can gain speed? Because the pulling force will also will pull you back?
He wasn't talking about moving in terms of rotation.

Let's do an example:

Assume that you place an space probe such that it is just ahead of Jupiter in its orbit and moving slower. (let's say at 5 km/sec compared to Jupiter's 13 km/sec. ). We'll also assume that the object is so placed, that it as it falls toward Jupiter it goes into a perfect parabolic orbit. (it swings around Jupiter and climbs back out going in the opposite direction from which it went in.

Since it started with a relative speed of 8 km/sec with respect to Jupiter when it started, when it climbs back to the same distance on its outbound trip it has the same speed relative to Jupiter, only this time it is moving away from Jupiter. So while before the encounter it had an orbital velocity wrt to the Sun of 13-8= 5km/sec, it now has an orbital velocity of 13+8 = 21 km. It hac gained 16 km/sec in orbital velocity with respect to the Sun.

In real life we can't obtain that perfect parabolic hair pin turn, but have to settle for a flatter hyperbolic orbit. This reduces the amount of boost we get, but you can still pick up a few km/sec.

6. Well, this description is much more explanative, Thanks

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