Wedge

A wedge is a simple machine made of two inclined planes placed back to back to form a sharp triangle. When the thick end is pushed, the sharp end forces objects apart with great force. Knives, axes, chisels, doorstops, saws and the pointed prows of ships are all wedges. The sharper the wedge, the easier it splits things, but the more times you have to hit it. Wedges have been used for at least 10,000 years, ever since humans first chipped flint into sharp tools.

• What it isTwo slopes back to backForms a sharp triangle
• FunctionSplits, cuts or holdsMultiplies force at the edge
• Mechanical advantageLength / thicknessLong sharp wedges multiply more
• Knives useSharp wedgeCuts food easily
• Doorstops useFriction + wedgeForce from above holds it down
• Used sinceAround 2.5 million yearsEarliest stone tools

How a wedge works

Push on the wide end of a wedge and the angled surfaces force whatever is in front of it to spread apart sideways. A small force on the wide end becomes a much larger force perpendicular to the wedges sides. The narrower (sharper) the wedge, the more it multiplies the force.

Wedges are unusual among simple machines because they are active: you actively push them into something. Most other simple machines (levers, pulleys, ramps) are passive: they sit still and let you move things around them.

Mechanical advantage

The mechanical advantage of a wedge equals the ratio of its length to its thickness at the wide end:

• A long, thin wedge has a high mechanical advantage. Easy to push in, but takes many strokes.
• A short, thick wedge has a low mechanical advantage. Needs more force, but does the job in fewer strokes.

Carpenters and woodcutters choose wedges according to the job. A splitting wedge for logs is short and stout, designed to be hit hard but split fast. A wood chisel for fine work is long and thin, designed to ease into the wood with little force.

Fact Sharp flint flakes are some of the oldest known human tools, dating back over 2.5 million years. Early hominins struck stones together to chip off thin, sharp flakes that worked as wedge-shaped cutting tools. These simple wedges helped them cut meat from bones, scrape hides and shape wood, helping our ancestors survive in difficult environments.

Wedges everywhere

• Knives: a long thin wedge for cutting and slicing.
• Scissors: each blade is a wedge; together they make scissors a pair of wedges plus levers.
• Axes: a thick wedge on a long handle, perfect for splitting wood.
• Chisels: narrow wedges for shaping wood, metal or stone.
• Saws: a strip of metal with many tiny wedge-shaped teeth, each removing a little material as the saw moves.
• Pencils, needles, pins: pointed tips are wedges.
• Doorstops: pushed under a door, the wedge presses up against the door and down against the floor, holding the door open by friction.
• Nails: pointed tips are wedges that push wood fibres apart instead of cutting them.
• Plough blades: a curved wedge cutting through soil to form furrows for crops.
• Snow ploughs: the angled blade is a wedge that pushes snow to the side as the lorry drives forward.
• Aircraft and ship noses: wedge-shaped to part the air or water smoothly.

Why a sharper knife cuts better

The whole point of a knife is its wedge-shaped edge. A sharp knife has a very long, thin wedge profile, giving a high mechanical advantage. The same force on the handle becomes a much larger force at the edge, easily slicing through whatever you are cutting.

A blunt knife has a thicker, rounder edge: less mechanical advantage, more force needed, and more chance of slipping. Counter-intuitively, blunt knives are more dangerous than sharp ones, because you have to push so much harder that you are more likely to slip and cut yourself.

Did you know? The sharpest blades ever made are obsidian scalpels, made from volcanic glass. The cutting edge can be just a few molecules thick, sharper than the finest surgical steel. Some heart surgeons use obsidian scalpels because they cut so cleanly that wounds heal with less scarring. The same kind of blades were used by Mayan craftsmen over 1,000 years ago for surgery and ritual.

Other smart wedge designs

• Locking wedges: builders sometimes hammer a small wedge sideways into a wooden joint, jamming it permanently in place.
• Boat keels: wedge-shaped to part the water smoothly and reduce drag.
• Cattle plough: a curved wedge that lifts and turns over the soil as the bull pulls forward.
• Snow shoes: when you walk down a slope, the wedge-shaped front digs into the snow to grip.
• Door hinges: not literally wedges, but use wedge-like friction to hold a door at a specific angle.

Try this Compare cutting a piece of cheese with a sharp kitchen knife and a butter knife. The sharp knife slides through with hardly any pressure. The butter knife needs much more force, and even then often crushes the cheese rather than cutting it cleanly. The difference is entirely about the wedge angle. A sharp knife edge is about 15-20 degrees; a butter knife edge can be over 30 degrees. The difference in mechanical advantage is enormous.

Deeper dive: how stone tools made us human

The earliest known stone tools, found in Kenya, are about 2.6 million years old. They were made by an early human ancestor (perhaps Homo habilis or a related species) by striking one stone with another to chip off sharp flakes. The flakes are essentially wedges: simple cutting tools that helped open animal carcasses, scrape hides, and break bones for marrow.

Over the next 2 million years, stone tool making became steadily more sophisticated. Hand axes (about 1.8 million years old) were carefully shaped on both sides into beautiful symmetrical teardrop wedges. Mousterian tools (about 200,000 years old, made by Neanderthals) included specialised wedges for scraping, cutting and piercing. Modern humans began making delicate microblades (tiny stone wedges) about 50,000 years ago, attaching them to wooden shafts to make sophisticated spears, arrows and harpoons.

The ability to make and use wedges may have been one of the most important advances in human evolution. Wedge-shaped tools let our ancestors:

• Hunt larger animals, providing more protein for growing brains.
• Cut meat and tough plants into pieces that could be cooked and eaten.
• Shape wood, bone and antler into more sophisticated tools.
• Make warm clothing from animal hides.
• Build shelters, paddles, traps and other equipment.

Wedge-shaped tools also probably encouraged co-operation and skill sharing. Making a good hand axe takes practice and teaching. Communities that passed on tool-making skills did better than those that did not. Some scholars think tool-making may have helped drive the evolution of language itself.

Today, almost every kitchen, workshop, farm and construction site has dozens of wedges in use. The simplest shape in tool-making, still indispensable after 2.5 million years.

For more, see inclined plane and screw.