Consider a pair of identical (typical) screws and held in opposite hands: the right pointed to your left, the left pointed to your right.

Then place them adjacent to each other (the one in your right hand "on top"), and "interleave" their helical threads together.

If I move the bolts around each other as one would twiddle ones thumbs (the top one comes toward you and the bottom one away from you), holding each screw firmly by the head so that it does not rotate, do the heads:

a) move inward
b) move outward
c) remain the same distance from each other?

You should determine the answer without resorting to the actual test

(In reply to Hold on a second!!! by ron)

While it is true, as explained in the previous comments, that the two screws neither approach nor recede from one another, it's not true that rotation would be necessary to do either.

Imagine that instead of two screws, one was a nail, held by the point so that the screw, held in the other hand, could engage the nail head. As the screw was moved around the nail head, without rotating, different sides of the screw thread would cycle completely around in their contact of the nail head.  Therefore, the screw, depending on whether it was twiddled around one way or the other, would approach or recede from the nail, as each rotation relative to the nail (but fixed--not rotating--with respect to the room at large) would carry the nail head one groove, so to speak, farther one way or the other along the screw.

Posted by Charlie on 2004-12-30 04:51:15