What is a Simple Machine?

Well, believe it or not, we use simple machines every day! Simple machines can include common tools like bottle openers, axes, or even door handles. On the other hand they can be part of complex devices like automobiles and airplanes. The simple machines are divided up into six basic types which include:

These simple machines are a part of pretty much every device you can think of. In fact, people just don't realize how common the simple machines are. Every time you open a door, go upstairs, use scissors, and even move your limbs, you are utilizing simple machines. Without these machines we would all still be living in caves.

So What Makes a Simple Machine So Special?

Simple machines are useful because they can make a physical job easier by changing the magnitude or the direction of the force exerted to do work. Let's take a common example. Have you ever tried to unscrew a nut, bolt, or screw from something with you bare hands and discovered that it was just too tight to loosen even if you had a good grip? So what did you do? You got the proper tool, such as a screw driver or wrench, and unscrewed it! Why is it that it's so easy to unscrew with a tool when you can't with your bare hands? Well, in reality the wrench and screw driver are examples of a wheel and axle, where the screw or bolt is the axle and the handle is the wheel. The tool makes the job easier by changing the magnitude of the force you exert. All of the simple machines can be used for thousands of applications from lifting a 500-pound weight to propelling a boat. The reason why these machines are so special is because they make difficult tasks much easier.

Why are Tasks Easier With Simple Machines?

Simple machines aren't perfect. In order to make the job easier you have to supply a force over a greater distance. To demonstrate this, let's use the example of a lever (See picture). Let's say you wanted to lift a box off the ground. If you move the position of the lever support (called the Fulcrum) and push down on the lever, the box becomes easier to lift. But in order for it to become easier you have to push the lever down a great distance of 1.5 meters to lift the box up a short distance of .5 meters at the other end of the lever. The task becomes easier because it is a small force but over a large distance which is converted into a large force over a short distance. So a large force over a short distance must have the same amount of work as a small force over a large distance. This situation describes the definition for work, which is defined as the force applied times the distance that force is applied for. Thus the equation for work is:

In this equation the force (f) is usually measured in newtons, the distance (d) in meters, and when multiplied together they create the metric unit for work, called a Joule. In the English system the unit for work is called a lb-ft ("Pound-Feet"), and is known as "Torque".

How Much Easier Does a Simple Machine Make Things?

The work you put in is always equal to the work you get out. By changing the distance you put that work in for, you can alter the amount of force required to do the same amount of work. Let's use the lever above as an example again. In that example you supplied a force over a distance of 1.5 meters and lifted the box up .5 meters. Let us assume that you exerted a force of 50 newtons (about 11 pounds). How many newtons of force must have been exerted on the box? We can use the equation W = f x d. We can calculate the work put in by multiplying the distance times the force: W = 50N x 1.5m. This gives us 75 Joules of work. We know that the work put in must be equal to the work gotten out, so the work on the box must be equal to the 75 joules. So we can now calculate the force by setting up an equation and solving for f: 75J = f x .5m. Therefore the force must be 150 newtons. This force is three times greater than the 50 newtons originally put in. The ability of a machine to amplify force is called it's Mechanical Advantage. In this situation, the work is increased by a factor of three. Therefore the lever has a mechanical advantage of 3. Mathematically, the mechanical advantage (MA) is equal to the force exerted by the machine (resistance force - F_r) divided by the force you put in (effort force - F_e). Thus the equation for mechanical advantage is:

How Efficient are Simple Machines?

Because of friction and other small factors, no machine is perfect. In other words, some energy is always lost and you never get the exact amount of work out that you put in. How well a machine transfers the work put into it is called the Efficiency. The efficiency of a machine is a percent ratio between the work output (W_o) and work input (W_i). Therefore the equation for efficiency can be written as:

The efficiency of different machines varies greatly. The main factor determining efficiency is the amount of friction resisting the machine. If you could eliminate all friction, machines would have 100% efficiency. Unfortunately it is impossible to eliminate friction.