Tools and machines

Last updated: May 16, 2007.

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

May the force be with you

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!

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. Drawing pins are a bit like nails with built-in hammers. When you push on the large, flattened head, the force you apply is magnified into a much bigger force that pushes the pin into the wall.

There are five main types of simple machine: levers, wheels, 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:

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.

Wheels

The invention of the wheel, around 5500 years ago in the Middle East, revolutionized transportation and gradually brought huge changes to society—but what makes it so special? Obviously it's easier to push a cart loaded with heavy sacks of grain than to carry the sacks on your back—but why, exactly? When you push a cart, you apply a sideways force. You don't have to lift anything: you simply have to overcome its tendency to stay where it is (inertia) and set it in motion, which is much easier. Once it's moving, all you have to do is overcome the force of friction (the resistance between the smooth wheels and the rough ground beneath them). This is why a strong man can pull a lorry. At no point does he have to lift the weight of the lorry, which would be impossible; he just has to overcome its inertia and then keep it moving. So moving a lorry isn't anything like as hard as it looks.

Bigger wheels are usually better than smaller ones. Once a wheel starts moving, the rim (outer part) turns a greater distance than the axle (inner part), but there is more force at the centre than at the edge. If you turn the rim of a wheel, you multiply the force you apply at the axle. In other words, the wheel acts like a lever and multiples the force. That's why pipes have wheels called stopcocks on 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 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 pedalling has a much greater effect. Car wheels work the same way.

Photo: A wheel as a speed multiplier. The engine turns the axle at the centre. 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, the wheel at the front means you can push the load sideways instead of having to lift it upwards.

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 two wheels and the rope wraps around twice, the pulley works as though four ropes are supporting the load. So you can lift four times as much!

Ramps

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.

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.

How many more?

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!