>Hi Alex,
>
>Yes, you are correct, as is the example I gave. The force, or restraint, acting against the riders of the marry-go-round, that keeps the riders from flying off in a normal physics striaght line, is the screen around the marry-go-round's parameter. Although the screen isn't forcing the riders to the center of the merry-go-round, it represent the same force pushing the moon towards the center of the earth. However, because of the moon's speed, trend towards a straight line, and a force pushing the moon toward the center of the earth, the moon remain in orbit around the earth.
>
>By the way, what is the force that is pushing the moon towards the center of the earth, or holding the moon in orbit if you will? I know it is called gravity, but what is gravity? This is the question. <vbg>

Aside from the answers you got: There are currently four forces known to "act at a distance" (although scientists often try to avoid this term; instead, particles are thought to carry the force from one place to another).

There is the electric and magnetic forces (which are different forces, but closely related); gravitation; the strong force; and the weak force.

The strong force is what keeps an atomic nucleus together (despite the mutual repulsion between the protons).

The weak force is involved in the breakdown of some radioactive nuclei.