Vorticey,
If you go back to my post on page 3 about Lu's incorrect use of the Goliath Theory Of Suddenly Flying Off Rocks, the answer to your quandry lies there. Your quandry is the same as has been raised by just about everyone whom learn't about this (well me anyway.)
Forget giving the force a name for a moment. I will go through the logic steps required to get to the answer. If you do not understand any step in that logic, then you will not understand the end game. Heedm, Nick and the other Gurus will be able to better define this, and please guys, fix flaws in my steps. Simplified, here goes:
1. An object will remain at a steady state of motion (either rest or at a constant speed) unless a force is applied to it.
2. Speed can be thought of as directional. An acceleration is a force.
3. To increase speed in a direction, you must apply an acceleration in THAT SAME direction. Think of a car which accelerates. If the direction of the acceleration was depicted as an arrow, the pointy end of the arrow would be pointing at the front of the car, I.E. in the direction of the speed change.
THE LOGIC THUS FAR: Speed is directional. Speed change requires an acceleration (which is also directional). THUS changing direction requires an acceleration, which is depicted as an arrow pointing toward the direction in which the acceleration force is acting.
5. When a car turns a corner, it changes direction, therefore it MUST have an acceleration (or force) applied to it (remember the speed is directional) in the direction of the corner.
6. Which way do we draw the arrow to depict this force? The arrow MUST be drawn toward the centre of the turn because that is the direction the force is acting, just like we did for the car's straight line acceleration.
THE LOGIC THUS FAR: when turning, you are changing direction, and an acceleration can be seen to be acting toward the centre of the turn.
7. David's sling is wrapped around the rock, and the rock is traveling at speed. but the rock is not travelling in a constant direction - it is traveling in a circle. Thus the rock is constantly changing direction. Thus a constant force MUST be being applied to make it constantly change direction.
8. We can depict the force acting on the rock by drawing an arrow. Just as we did with the car, the pointy end MUST BE pointing in the direction that the force is acting, I.E. toward the centre, I.E. toward David's hand.
THE LOGIC THUS FAR: There is a force that can be seen to be acting toward the centre of the circle.
Physcisists (who can spell rooly gudder than me) decided to give this force a name so that we could refer to it without saying "that force that acts on a mass that is changing it's direction"
They named it centripetal.
9. When the sling releases the rock the force trying to change it's direction is gone, therefore the rock will travel in a straight line again, along a tangent. If the rock were to fly out of the sling in any other direction, an acceleration must be applied (because we know that a change of direction can only be accompanyed by an acceleration).
If this does not help, just indicate which step (by it's number) that you are having difficulty with and I'll see if I can clarify.
Good luck.
Lu,
Please disregard all the above, it will not help you: it refers to a scientifically accepted and standardised theory from a high school text book.
[ 03 December 2001: Message edited by: helmet fire ]