Why Are There Three Kinds of Lever? Where the Pivot Sits Changes Everything
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A lever is a bar turning about a pivot, and the three arrangements of pivot, effort and load behave completely differently. One multiplies force, one multiplies speed, and which you get is decided by the layout alone.
The three arrangements
Every lever has three points that matter, being the pivot about which it turns, the place where effort is applied and the place where the load acts, and the three classes are simply the three possible orderings. In the first class the pivot sits between the effort and the load, which is the arrangement of a seesaw, a pair of scissors and a crowbar used to prise. In the second class the load sits between the pivot and the effort, which is a wheelbarrow, a nutcracker and a bottle opener. In the third class the effort is applied between the pivot and the load, which is a pair of tweezers, a fishing rod and a broom. Nothing else distinguishes them, and the differences in what they do all follow from these positions.
What each arrangement gives you
The consequences of the layout are fixed and predictable:
- •First class can multiply force or speed depending on where the pivot sits between the two
- •Second class always multiplies force, since the effort is always further from the pivot than the load
- •Third class always reduces force and multiplies speed and distance instead
- •The multiplication is the ratio of the two distances from the pivot
- •Nothing is gained overall, since a force multiplied is a distance divided
- •The direction of motion reverses in the first class and not in the other two
Why the body uses the losing one
Most levers in the human body are third class, which multiply speed at the cost of requiring more force, and that looks like a poor bargain until the constraints are considered. Muscles attach close to joints because a tendon attaching far out along a limb would require the muscle to be elsewhere, would make the limb bulky and would obstruct movement, so the anatomy is largely settled before the mechanics are considered. The arrangement means a muscle must generate considerably more force than the load it moves, with the biceps producing several times the weight held in the hand, which sounds wasteful and buys something valuable, since a small fast contraction near the joint produces a large fast movement at the end of the limb. For an animal that needs to move quickly and over distance, speed at the extremity is worth more than efficiency.
Finding the pivot in an odd case
Classifying a real object sometimes takes a moment and the awkward cases are instructive. A pair of scissors is two first class levers sharing a pivot, and cutting near the pivot gives more force than cutting near the tips, which is why thick material is cut at the base of the blades. A wheelbarrow has the wheel as the pivot, which is easy to miss because a pivot is usually imagined as a fixed point rather than something rolling. The human jaw is commonly classed as third class and is treated as second class by some analyses depending on which muscle and which teeth are considered, which shows that the classification depends on the specific action rather than on the object. A nutcracker is two second class levers. And a fixed pulley is a lever in disguise, with the axle as pivot and equal arms, which is why it changes direction and multiplies nothing.
Where the idea generalises
The same reasoning applies well beyond bars and pivots. Gears trade turning force against rotation speed in exactly the same ratio, and a bicycle changing gear is choosing a point on that trade-off according to the gradient. Pulley systems multiply force by the number of supporting rope sections, at the cost of pulling that many times the distance. Hydraulic systems multiply force by the ratio of piston areas, with the small piston moving far and the large piston moving little. Screws and inclined planes convert a long gentle motion into a short powerful one. In every case the product of force and distance is conserved apart from friction, which is the general principle behind all simple machines and the reason none of them produces something from nothing.
The takeaway
The three classes are the three orderings of pivot, effort and load along the bar, and nothing else distinguishes them. The second class always multiplies force and the third always multiplies speed and distance instead. Most levers in the body are third class because muscles must attach near joints, buying fast movement at the limb's end at the cost of requiring more force.