Screws seen pretty intuitive untill you actually start thinking about how they work. How come this tiny ridge along their edge can pull pieces of wood together with pretty amazing force?
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Metals have the trait of being ever so slightly elastic. When you tighten a screw/bolt, but you’re actually doing is putting the shaft of the screw into tension and slightly deforming it, which pulls together the two ends. Ever tighten a bolt too tight and all the sudden it becomes easier to turn? That’s because you put so much tension on the bolt that it deformed and stretched out a bit.
Friction, for the most part.
When you screw into pieces of wood–or any material, really–you’re forcing the screw and the wood together very tightly. The threads provide a lot of surface area, all of which is in contact with the wood. That means you’ll need a lot of force to move it.
For materials like wood, it will also expand a bit after the screw has been put in. It’ll wedge itself into the threads, so in addition to providing friction that prevents rotational movement, you’re also preventing movement straight in or out of the screwhole, because you’d need to apply enough force to break the wood in between the threads. That’s why there’s often bits of wood that come out if you do pull apart a screwed-together object.
A screw is nothing but an inclined plane. If you unwound it, it would be a ramp or wedge.
Inclined planes convert a long distance forward into a short distance up (or down). The shallow the ramp, the longer you have to push, but the easier it is to get to the height that is the top of the ramp.
This allows the screw to exert force up and down that is greater than it would if it were straight up and down. You spend more time turning the screw (because the ramp makes the distance longer), but that converts to greater pressure against the material up and down from the ramp.
This effectively makes the screw into a much longer nail. It goes in more slowly and with less effort, but you get the same result. With a screw, you take longer to drive it in, but it is easier. With nails, you need a hammer, and it is easy to find that there isn’t even enough material for the nail to have full effect.
So, the screw exerts more force against the material around it than a straight nail of similar length and diameter, and once there has the friction grip of a much longer nail.
A screw is really a very long knife edge that’s firmly attached to something. If I told you that there was a 16″ (or 0.5m) long blade embedded into something, how hard do you think you could pull on it sideways?
If you screw a plank against a wall, it’s not the screws that holds the majority of the load like a “hook”, it’s actually the plank being pressed against the wall, increasing friction, that holds the most of the load.
HOWEVER, if you instead of a plank, screw OP’s mom against the wall, the load is not held at all.
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