Nails and Screws without Friction In my physics class we're just starting the chapter on friction, and our book began the chapter by describing what a world without friction would be like.  Among other examples, it said "nails and screws would become useless."  I understand why nails wouldn't work (I imagine they would just slide out of the wall), but why should screws become "useless"?  Wouldn't it be much more difficult for a screw to come out of the wall, seeing as how it would need some rotational force?
 A: You may be familiar with parking structures that have a spiral "road" that goes from floor to floor. If you put a car in neutral on such a structure, and take off the brakes, it would roll down the slope. If you connected a train of cars together, they would still roll down the slope.
A conventional (regular pitch) screw is like that train of cars - but normally, all these cars have their brakes applied (friction) which makes it hard to remove the screw. Remove friction, and you take the brakes off. Any pull (gravity) on the screw will cause it to work loose. It will go a little bit more slowly (because it rotates as it pulls out, there is a higher apparent inertia) but it will fall right out of the hole.
As was pointed out in the comments, there are various mechanisms one can imagine - some of which exist in actual screws and nails - that would prevent them from working loose even if friction is low or non-existent. For certain building materials, like wood, this can be important as the hole can change size over time - reducing the "grip" of the fastener, and potentially allowing it to come loose.
An example of such a screw is the dual pitch screw used to fix bones (you really don't want those to come loose) - they screw in with one pitch, then have another pitch at the end. The result is "compression" of the bone, and it provides the ability to control that compression better than just having a countersunk head (effectively, you simulate a very fine pitch - namely the difference between the two pitches - for the last part where you tighten the screw) (image from http://www.vilex.com/vilex-surgical-products/internal-fixation/large-dual-thread-screws.html):

There are other screws like this one (from http://bcfasteners.com/wp-content/uploads/2013/01/GRK-Fasteners-RSS-Screw-400x400.png):

which has the ridges on the shaft that will dig into the wall of the hole, and as the material "bounces back" (for wood that will happen) it causes the screw to grip in a way that will not let go, regardless of friction. In essence, the "coarse pitch" ridges want to move the screw out at one speed; the fine pitch ones want to move it at another speed; and the only thing they can agree on is to do nothing.
A little color: it is said that the screw was invented before the screwdriver, and was driven with a hammer - because the thread makes the "nail" grip better. Only later did the screwdriver get invented, which allowed the screw to be removed easily. In the 80's I was told that a slang term for hammer is "Manchester screwdriver" - presumably a slight to the inhabitants of that city... (sometimes also called "Irish" or "Birmingham" screwdriver, incidentally - it all depends who you want to insult I suppose).
