How do Oort Cloud objects become short-period comets?

[O Great One]

Something I've thought about before but never asked anybody...The existence of short-period comets is a problem if one believes the solar system is 4 billion years old, because obviously after that length of time all of the short-period comets would have disintegrated. So, we need a way of replenishing them, hence the Oort cloud. So there's a dormant, potential comet floating out there in the Oort cloud. But what I am wondering is this...

If the object from the Oort cloud simply drifts into the solar system it seems that it will simply be pulled into the sun. In order to become a short-period comet orbiting the sun, it needs some kind of force directing it a point a distance away from the sun, so that when it approaches that point the sun's gravity will sling-shot the object and force it into an elliptical orbit around the sun.
Is my understanding of physics correct, that it needs something to force it towards a point a distance away from the sun?

But how does the object get that force, where does it come from?

 

[Nereid]

It's true that an object a long way away will fall into the Sun, IF:
1) there are no other forces - than the mutual gravitation - acting on it
2) it's not moving away for the Sun at faster than escape velocity to begin with
3) the component of its initial motion at right angles to the line to the Sun is exactly zero.

(There are other IF's too, but we'll ignore them for now).

If there is initial transverse motion (3), the object will fall in towards the Sun, whip round it, and shoot back out, in a parabolic (or hyperbolic) orbit. Not a short-period comet.

There are other forces which will act on a body such as a comet. Perhaps the most important is the gravitational pull of Jupiter (Saturn, ...), which may provide just the right amount of change to the comet's orbit to 'capture' it for the Sun. Only a fairly narrow range of initial conditions are right for this to happen, so most incoming comets don't become short period ones.

Just to note a few other things:
- the origin of most short-period comets is still somewhat of a mystery; from what we know of the lifetime of such comets, and the Edgeworth-Kuiper Belt (EKB), mutual collisions of EKB bodies cannot create enough short-period comets
- there's very little known about the Oort Cloud, in the sense that we don't know how many objects there are, what sizes they are, their compositions, etc
- this is an active area of reseach in astronomy, with quite a few interesting discoveries in the last few years; binary EKB objects, Pluto-Charon as merely the largest of a whole class of EKB objects, evidence that many (most?) aren't likely to be pristine relics of the early days of the solar system, ...

 

[FZ+]

If the object from the Oort cloud simply drifts into the solar system it seems that it will simply be pulled into the sun.

If an object is in the Oort cloud, it must already have a tangential velocity. Else, the oort cloud would spontaneously fall back in towards the sun.

 

[Nereid]

EKB - interesting new hypothesis on its origin

Following the recent finding that EKB seems to have a sharp outer edge - at least the normal belt, the 'excited disk' has objects that go out beyond 1,000 au - along comes an idea to account for this.
http://skyandtelescope.com/news/article_1115_1.asp