Lever

A lever is one of the six simple machines: a rigid bar that pivots around a fixed point called the fulcrum. Pushing down on one end lifts a load on the other end. By choosing where to put the fulcrum, you can trade force for distance: a long lever can lift a heavy weight with little effort. Levers are everywhere in everyday life: scissors, hammers, crowbars, seesaws, tweezers, bottle openers, even your own arms and legs.

• What it isRigid bar pivoted at a pointAround a fulcrum
• Three partsEffort, fulcrum, loadArranged in different orders
• Three classesFirst, second, thirdBased on positions
• Trade-offLess force, more distanceTotal work stays the same
• Discovered byAncient GreeksUsed since pre-history
• In your bodyBones + musclesMost joints are levers

The three parts of a lever

• Fulcrum: the pivot point.
• Effort: the force you apply.
• Load: the thing you are trying to lift or move.

By placing these in different positions along the bar, you create different kinds of lever.

The three classes of lever

Levers are grouped into three classes based on the relative positions of fulcrum, effort and load.

Class 1 lever

Fulcrum in the middle, with effort on one side and load on the other. Pushing down on the effort end lifts the load on the other side. Examples: seesaw, crowbar, scissors, pliers, the claw end of a hammer pulling out a nail. Mechanical advantage depends on where the fulcrum sits: nearer the load makes it easier to lift.

Class 2 lever

Load in the middle, with the fulcrum at one end and the effort at the other. The whole lever lifts when you push the effort end. Examples: wheelbarrow (wheel = fulcrum, dirt = load, handles = effort), nutcracker, bottle opener. Class 2 levers always multiply your force, but you have to move further than the load does.

Class 3 lever

Effort in the middle, with the fulcrum at one end and the load at the other. Examples: a fishing rod, tweezers, a broom, your forearm (elbow = fulcrum, biceps = effort, hand = load). Class 3 levers actually need MORE force than the load (mechanical advantage less than 1), but they give greater distance and speed at the load end. That is why a fishing rod can whip a hook out fast, even though it takes effort to do so.

Fact Most of the joints in your body are class 3 levers. Your elbow, for example: the elbow joint is the fulcrum, your biceps muscle (attached just past the joint) is the effort, and the weight in your hand is the load. Your muscles have to work hard, but you get fast, far-reaching motion. Your bodys design favours speed and range of motion over raw lifting strength.

Mechanical advantage of a lever

The mechanical advantage of a lever is the ratio of the effort arm (distance from fulcrum to effort) to the load arm (distance from fulcrum to load).

• If the effort arm is 2 metres and the load arm is 50 cm (0.5 m), the mechanical advantage is 4. You need only 25 kg of force to lift a 100 kg load.
• The catch: to lift the load by 10 cm, you have to push your end of the lever down by 40 cm.

Levers in everyday tools

• Scissors: two class 1 levers joined at the fulcrum. The blades are also wedges.
• Crowbar: classic class 1 lever, used to pry things open.
• Wheelbarrow: a class 2 lever for moving heavy loads with less effort.
• Bottle opener: class 2 lever; the lip of the cap is the load.
• Nutcracker: two class 2 levers joined at the fulcrum.
• Tweezers: class 3 levers for picking up small things with precision.
• Fishing rod: class 3 lever for casting and reeling.
• Pry bar: very long class 1 lever for great mechanical advantage.
• Stapler: a class 1 lever applying force to drive a staple.

Did you know? The famous Greek mathematician Archimedes said: "Give me a lever long enough and a place to stand on, and I will move the Earth." Of course, the lever needed would have to be longer than the Earth-to-Sun distance, and Archimedes would have to push his end through millions of kilometres to budge the Earth by even a tiny amount. But the principle is exactly right: there is no theoretical limit to how much force a lever can multiply.

Levers in the human body

Your body uses dozens of levers:

• Elbow: class 3. The fulcrum is the elbow joint, the effort is your biceps, the load is whatever is in your hand.
• Toes (calf raise): class 2. The fulcrum is the ball of your foot, the effort is your calf muscle, the load is your body weight.
• Neck (nodding): class 1. The fulcrum is the top of your spine, the effort is your neck muscles, the load is the front of your head.
• Jaw (closing): a complex class 3 lever powered by some of the strongest muscles in your body.

Try this Pretend a long ruler or stick is a class 1 lever. Balance a heavy book on one end and place a fulcrum (a pencil or eraser) close to the book. Press down on the other end and feel how much force is needed. Now move the fulcrum further from the book (closer to your hand) and try again: it takes much more force. Then move the fulcrum very close to the book and you can lift it with almost no effort. You have just felt the effect of changing lever-arm length.

Deeper dive: how levers built ancient construction

Long before cranes, hydraulics or motors, builders moved huge stones using nothing but levers, rollers and inclined planes. The Pyramids of Egypt, the temples of Greece, Stonehenge, the Roman aqueducts and the Gothic cathedrals of Europe were all built with simple-machine technology and a lot of human muscle.

A typical building site in ancient times might use:

• Long wooden levers to nudge and rotate stones into position. A 10-metre oak beam could let 4 to 6 men move a stone weighing several tonnes.
• Wedges and pry bars for inserting stones into tight gaps, splitting blocks from quarry walls, and adjusting fits.
• Capstans and treadwheel cranes to lift heavy loads. A treadwheel crane is essentially a giant wheel-and-axle: workers walk inside the wheel, turning the axle that lifts the load with rope and pulleys. Roman treadwheel cranes could lift several tonnes.

For centuries, the basic technology of construction barely changed. Even the great Gothic cathedrals of the 1200s were built with treadwheel cranes and wooden levers very similar to those used by the Romans 1,000 years earlier.

Only in the 1800s, with the invention of steam-powered cranes and hydraulic lifts, did the construction industry leave simple machines behind. Yet even today, modern construction sites have crowbars, jack handles, lever-action ratchets and other tools that an ancient Roman engineer would immediately recognise. The lever is one of the oldest and most enduring inventions of all.

For more, see pulley and the six simple machines.