Or screwing workout equipment which you constantly pull with your own weight.
EDIT: Forgot to add, I’m not in the US, I’m talking about brick or concrete walls, not drywalls. Although probably the basic principle applies when it comes to explaining how force works.
In: Physics
You don’t screw it into the plaster. You screw it through the plaster and into the wooden post behind it. You find the wooden post behind it by using a magnet to detect the nails.
First, you don’t screw it into the wall, you screw it into the wooden studs within the wall. Screwing into drywall or plaster alone is insufficient for anything but the lightest loads.
Second, if you’re dealing with any sort of significant loads you don’t want to use regular screws you want to use lag screws, which are basically a cross between a big ass bolt and a screw. Normal screws just can’t handle the sheer forces or pullout forces a lag screw can.
If you’re screwing it onto the drywall or plaster instead of the wooden studs, then yes, it would eventually fall.
Screws, bolts, and nuts produce an incredible amount of clamping force. A screw is basically two very low incline ramps or shims being pushed against each other. It takes very little force turning the screw (torque) to create a huge force in the axis of the screw or clamping force. For example the force holding your wheels to your cars is over ten times the weight of the car PER WHEEL. Just from 5 little nuts torqued to ~100ft/lbs or someone pressing a foot long wrench with 100lbs of force.
Edit: Misread the point of the question, yes typically heavy things are not screwed into the drywall but into the frame of the house itself.
You guys have plasters and wood for walls? I got concrete.
Most of the force applied is pulling DOWN on the wall, not out from the wall, as long as the bolt/anchors are sufficiently deep into the wall. You can easily place a water tank or your body weight on top of a wall without worrying about whether the wall will be crushed, and that’s essentially what’s happening when you mount something properly. You’re creating a vertical force (mostly) at the mounting site. Note that the farther you place the center of mass of whatever object you’re attaching to the wall from the wall itself (like a really long platform), the more you’re going to need to drive the anchors/bolts deeper or add some additional support (like an angled support bracket) to transfer more of the horizontal load to the wall for vertical support.
Screws in bricks/concrete are basically friction fit: you put in the plastic sleeve which is already snug in a hole if properly drilled, then put the screw in. The screw is like an inclined plane as somebody else said it and has immense mechanical leverage. It expands the sleeve and crushes it into the rough surface of the hole with extreme force. A well chosen fastener will not be that much weaker than directly casting that screw into the concrete/brick. Nevertheless there are supplementary products than can be sprayed into thehole to make extra sure things stay in. The big ones used for mounting water heaters plus the epoxy hole filler get you a screw that [takes more than a ton to pull out](https://link.springer.com/article/10.1617/s11527-020-01536-2#Tab4).
A lot of people have added stuff but another major factor is most force is going to be towards the ground and so perpendicular to the fastener. Things that are designed to be attached to the wall generally have the most weight placed as close to the wall as possible so there is the least amount of pulling away from the wall.
In industrial settings, washing machine size transformers are often mounted to walls to provide more floor space below them. There’s usually a metal frame on the wall, and another on the opposite side connected to the interior by long bolts. The wall in between is usually cinder block or poured/reinforced concrete.
To add to all of the comments talking about downward not outward force – Screws are actually better at resisting that lateral force pulling out than the perpendicular force pulling at the ground. The threads provide a much larger surface area for friction to affect things.
Nails are better at perpendicular forces but as probably is apparent – can get pulled out relatively easily. Which is why you can pull a nail out of the wall with a crowbar/claw hammer pretty easily but you’re going to have a hard time pulling a screw out. This is why nails are used for things like framing a house instead of screws – there’s not much lateral forces to worry about but in some areas a LOT of weight being put on the nails where screws would just snap.
Screws are still generally plenty strong for most things you’d be hanging on a wall though. But as always check the specifications and instructions of for example, any TV Mount or other mounting hardware you are using so you use the correct fasteners, don’t hang too much weight, and have enough screws in studs.
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