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Hydraulic digger

Tools and simple machines

by Chris Woodford. Last updated: December 12, 2014.

It's built to last you a lifetime—quite literally. But though the human body is the most amazing tool at your disposal, it often needs a helping hand. Tools made from metal, wood, and plastic work like extensions of your body, making you feel stronger and helping you work faster and more efficiently. In science, tools like this are called simple machines. And although you might think there's a big difference between a tiny little wrench and a huge great earthmover, exactly the same physics is at work in both. Let's take a closer look at tools and machines and how they work!

Photo: This hydraulic digger uses a collection of simple machines (wheels, axles, and levers) to magnify the force its driver can exert.

What is a machine?

To do anything at all—to lift a box, to push a car, to get out of bed, to jump in the air, to brush your teeth—you need to use a pushing or pulling action called a force. If you go around telling people you're strong, what you really mean is that your body can apply a lot of force. You may have watched incredibly strong people on TV pulling trucks or trains with their bare hands, but there's a limit to what even the most muscle-bound human body can do. Simple machines let us go beyond that limit. Simple machines can make us all strong!

Photo of a thumbtack (drawing pin) being pushed into a wall.

When you hear the word "machine", you probably think of something like a bulldozer or a steam locomotive. But in science, a machine is anything that makes a force bigger. So a hammer is a machine. A knife and fork are a pair of machines. And even an egg whisk is a machine. All these machines have one thing in common: when you apply a force to them, they increase its size and apply a greater force somewhere else. You can't cut meat with your hand alone, but if you push down on a knife, the long handle and the sharpened blade magnify the force you apply with your hand—and the meat slices effortlessly. When you pound a nail with a hammer, the handle increases the force you apply. And because the head of the hammer is bigger than the head of the nail, the force you apply is exerted over a smaller area with much greater pressure—and the nail easily enters the wood. Try pushing in a nail with your finger and you'll appreciate the advantage a hammer gives you.

Photo: Thumbtacks (sometimes called drawing pins) are a bit like nails with built-in hammers. When you push on the large, flattened head, the force you apply (to the large flattened end) is effectively magnified because it's concentrated into a much smaller area at the tiny pin head).

There are five main types of simple machine: levers, wheels and axles (which count as one), pulleys, ramps, and screws. Let's look at them more closely.

Levers

A lever is the simplest machine of all: it's just a long bar that helps you exert a bigger force when you turn it. When you sit on a see-saw, you've probably figured out that you need to sit further from the balance point (known as the pivot point or fulcrum) if the person at the opposite end is heavier than you. The further away from the fulcrum you sit, the more you can multiply the force of your weight. If you sit a long way from the fulcrum, you can even lift a much heavier person sitting at the other end—providing they sit very close to the fulcrum on their side. The force you apply with your weight is called the effort. Thanks to the fulcrum, it produces a bigger force to lift the load (the weight of the other person). The words "effort" and "load" can be very confusing, so we've avoided using them in this article. The important thing to remember about levers is that the force you produce is bigger than the force you apply:

Physics of a seesaw

With a long lever, you can exert a lot of leverage. When you use an axe or a wrench, the long handle helps to magnify the force you can apply. The longer the handle, the more leverage you get. So a long-handled wrench is always easier to use than a short-handled one. And if you can't budge a nut or bolt with a short wrench, try one with a longer handle.

Types of lever

Levers are all around us. Hammers, axes, tongs, knives, screwdrivers, wrenches, scissors—all these things contain levers. All of them give leverage, but not all of them work the same way. There are actually three different kinds of levers (sometimes known as classes).

Class-1 levers

In a class-1 lever, the force you apply is on the opposite side of the fulcrum to the force you produce. A see-saw is an example of a class-1 lever. So is a pair of scissors:

Physics of a class-1 lever (scissors)

Class-2 levers

A class-2 lever is arranged a slightly different way, with the fulcrum at one end. You apply force at the other end and the force you produce is in the middle. Nutcrackers, garlic presses, and wheelbarrows are all examples of class-2 levers:

Physics of a class-2 level (wheelbarrow)

Class-3 levers

A class-3 lever is different again. Like a class-2 lever, it has the fulcrum at one end. But the two forces switch around. Now you apply the force in the middle and the force you produce is at the opposite end. Class-3 levers are unlike other machines in that they reduce the force you apply, giving you much greater control. Tweezers and tongs are an example of class-3 levers:

Physics of a class-3 lever (tongs)

Pens are class-3 levers too: by pivoting them on our hands and holding them in the middle, we get much more control over the nib or ballpoint.

Wheels and axles

The invention of the wheel and axle (the rod around which a wheel turns), around 5500 years ago in the Middle East, revolutionized transportation and gradually brought huge changes to society, but what made it so special? It's easier to push a cart loaded with a heavy box than to push the same box along the ground because the cart's wheels and axles reduce friction and provide leverage. You can find out how in our main article on how wheels work.

How a wheel multiplies speed or force.

Big wheels are used to multiply force in other ways too. Pipes, for example, have wheels called stopcocks (or stop valves) fitted to them. When you turn the outer rim of a stopcock, the inner axle turns with much greater force—so the pipe is easier to close. Steering wheels work this way too. A truck or a bus often has a bigger steering wheel than a car, because it takes more force to turn its wheels. The bigger wheel gives the driver more leverage.

Wheels can multiply distance and speed as well as force. Bicycles have large wheels so they go faster. When you pedal, you power the inside of the wheel. But the wheel's outer rim turns around faster and covers more ground, so your pedaling has a much greater effect. Car wheels work the same way.

Photo: A wheel can work as either a force multiplier or a speed multiplier (but not both at the same time). If you turn the outside (rim) of a wheel, the axle at the center turns with less speed but more force, so the wheel works as a force multiplier. If you turn the axle instead (as a car does), the wheel becomes a speed multiplier. The axle turns only a short distance (blue arrow), but the leverage of the wheel means the outer rim turns much further (red arrow) in the same time. That's how a wheel helps you go faster.

Wheelbarrows combine wheels and levers to brilliant effect. A wheelbarrow makes it really easy to transport a load from one place to another—for two reasons. First, its long frame acts like a lever, so the load is much easier to lift. Second, it's easier to push the load using a wheelbarrow because the only friction is between the wheel and the axle. If you pushed the load across the rough surface of the ground without using a wheelbarrow, the friction would be much greater.

Gears

Gears are pairs of wheels with teeth that can multiply force. They take quite a bit of explaining so we won't go into details here. You can read all about them in our gears article.

Pulleys

Put two or more wheels together and loop a rope around them several times and you create a powerful lifting machine called a pulley. Each time the rope wraps around the wheels, you create more lifting power or mechanical advantage. If there are four wheels and the rope wraps around them, the pulley works as though four ropes are supporting the load. So you can lift four times as much, although the catch is that you have to pull the rope four times further. Read more in our pulleys article.

Ramps

How the head of an axe works like a ramp helping you cut wood more easily

If you've ever helped pull a boat out of the sea, you'll know it's easier to do it if there's a ramp on the shore. Instead of lifting the boat vertically, straight up, you can get it out of the sea with much less force if you go up the ramp. You use less force, but you need to pull the boat a longer distance—so you use the same amount of energy in each case. Hillwalkers sometimes use the idea of a ramp to get to the top of a steep climb. By zig-zagging from side to side across their climb, they effectively create their own ramp. The hill becomes less steep, but they have to walk quite a bit further to get to the top.

Ramps are sometimes known as inclined planes or wedges. The head of an axe is a wedge working in a different way. An axe forces wood apart in two ways. The handle works like a lever, magnifying the force you apply. The wedge-shaped blade concentrates the force over a smaller area, increasing the pressure on the wood and splitting it apart. The blade of a knife works the same way.

Artwork: The head of an axe works like a ramp. When it powers into wood, the wood splits apart along the diagonal. That means you can cut the wood by applying a smaller force over a larger distance. If you wanted to pull a log apart with your bare hands, you'd need to apply a much bigger force (though over a much shorter distance).

Picture of a woodscrew standing on its head pointing upward

Screws

A screw bites into wood when you turn it around. You often read science books that say a screw is "like a ramp wrapped around in a circle", which is pretty confusing and hard to understand. But imagine you're an ant and you want to climb from the bottom of a screw to the top. If you climb vertically up the outside, you go a relatively short distance but it takes an awful lot of climbing force. If you walk up the screw thread, winding around and around, you're really walking up a kind of spiral staircase—a ramp wrapped around in a circle. Yes, you walk much further, but it's a whole lot easier. There's another good thing about a screw too: because the head is bigger than the shaft beneath it, a screw works like a wheel (or lever): each time you turn the head, the sharpened point beneath bites into the wood with greater force. The tapering (cone-shaped) design makes it easier to drive in the screw.

Photo: The spiral thread on a screw means it takes longer to drive it into wood but—theoretically at least—you need less effort. The grooves also help the screw to remain in place.

Machines are all around us!

That's pretty much all there is to the science of simple machines. Once you understand how machines work, you start seeing them everywhere. Even your body is packed with machines. Your skeleton, for example, is a collection of levers! Take a look around your home and see how many more "simple machines" you can spot. You'll be amazed how many there are!

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Text copyright © Chris Woodford 2007. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007) Tools and simple machines. Retrieved from http://www.explainthatstuff.com/toolsmachines.html. [Accessed (Insert date here)]

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