Levers, and the three kinds
Where the fulcrum sits, and what each arrangement is good for.
A lever's whole behaviour, whether it multiplies force, multiplies speed, or does a bit of both depending on which end is used, comes down entirely to where its fulcrum sits relative to the load and the effort, and that single arrangement is what splits every lever in the world into one of exactly three kinds.
What is really going on
A lever is, underneath any specific shape, just a rigid bar turning about a fixed pivot, and everything interesting about how it behaves follows from the relative distances between three points on that bar, the pivot itself, the point where a load pushes back against it, and the point where effort is applied to move it. Placing the pivot between the load and the effort produces one kind of behaviour, placing the load between the pivot and the effort produces a second, and placing the effort between the pivot and the load produces a third, and these three arrangements are the entirety of what a lever can be, endlessly repeated across countless everyday tools that rarely look like they have anything in common.
This is, at heart, the same underlying idea as the four-bar linkage from earlier in this set reduced to its simplest possible case, a single rigid bar turning about a single pivot rather than four bars cooperating around four, and it is worth treating a lever as the smallest, most basic member of the linkage family rather than as an entirely separate topic, since the same bookkeeping of where each meaningful point sits relative to the pivot explains both.
The kitchen-drawer comparison
A pair of kitchen scissors has its pivot sitting between the handles and the blades, load and effort on opposite sides of a central fulcrum, which is the first kind of lever and can multiply force or multiply speed depending purely on which side is longer. A bottle opener hooked under a cap has its fulcrum at the far end, the cap itself sitting between that fulcrum and the hand pushing down on the opener's other end, which is the second kind, and it always multiplies force in the user's favour because the hand's effort always travels further than the cap does. A pair of tweezers works differently again, with the pivot at the closed end and the fingers squeezing somewhere in the middle, between the pivot and the tips doing the actual gripping, which is the third kind, and it always costs force rather than saving it, trading that cost for a fine, sensitive range of motion at the tips that a bulkier tool could never match.
Why the arrangement decides what a lever is good for
A first kind of lever, pivot in the middle, is the most flexible of the three, since sliding the fulcrum closer to one end or the other changes which side gains the advantage, a crowbar with its fulcrum near the load multiplying force enormously, the same crowbar used with the fulcrum moved toward the hand instead multiplying speed and reach at the cost of force. A second kind of lever, load in the middle, always multiplies force in the user's favour by its very arrangement, since the effort is necessarily applied further from the fulcrum than the load is, which is exactly why a wheelbarrow, with its wheel as the fulcrum and the load sitting between the wheel and the handles, lets one person move a load their own arms alone could never lift directly. A third kind of lever, effort in the middle, always costs force by the same logic in reverse, and exists specifically because some jobs need speed, reach or precision at the working end far more than they need raw force, a fishing rod's tip whipping through a wide arc from a small movement of the wrist being the clearest possible example.
The one number worth remembering
Moving a fulcrum on a first-kind lever from the exact middle to a point only a fifth of the way along its length can turn a modest mechanical advantage into one several times larger, since the advantage a lever offers scales directly with the ratio of the two arm lengths on either side of the pivot, which means a small shift in where the fulcrum actually sits can matter far more to a lever's usefulness than almost any other single design decision made about it. A crowbar's fulcrum, wedged deliberately close to the load it is prying against, is doing precisely this on purpose, sacrificing almost all of its own reach on the load side to buy a large multiple of extra force on the hand side.
What this changes in practice
Recognising which of the three kinds a given tool actually is explains, in one step, why it feels the way it does to use, whether it is trading force for speed or the reverse, and makes deliberately choosing or adjusting a fulcrum's position a genuinely useful design lever in its own right rather than an afterthought fixed wherever seemed convenient. A tool redesigned with its fulcrum deliberately relocated, rather than kept wherever the first prototype happened to place it, can trade an uncomfortable amount of required force for an easier, if slightly longer, motion, or the other way around, entirely without changing anything else about the tool.
What this does not explain
None of the three lever classes says anything about how efficiently a lever transmits the force it does carry, since friction at the pivot, flex in the bar itself, and how cleanly the load actually meets the tool all subtract from the ideal advantage the geometry alone promises. Knowing which kind of lever a tool is describes what it is trying to do, not how well a particular real example of it actually manages to do it, and the next article in this set picks up exactly that gap, the specific, unavoidable trade every lever makes between the force it delivers and the distance it asks the effort to travel to deliver it.