Summary of Key Concepts

Mechanisms are devices that convert one type of motion to another. To use a tube of lipstick or a glue stick, you have to turn a knob. When you do so, the lipstick or glue comes straight out the other end. Inside of each device, there is a mechanism that takes the motion you supply, the input, and transforms it into the motion you want, the output. The two motions are different. The input motion travels around in a circle and is at one end of the glue or lipstick case. The output motion travels in a straight line, and is at the other end.

Many mechanisms are designed to produce more force at the output than is required at the input. This is true of a pair of nail clippers. A small amount of force at the input manages to produce a much larger force at the output - enough to cut through a resistant big toe nail! However, this big gain in force does not come for free. You have to move the handle through a much bigger distance, compared with the tiny movement of the jaws. This is a characteristic of all mechanisms: the larger the force, the less the distance traveled, and vice versa.

All mechanisms depend on a few basic principles, and the most important of these is the Principle of the Lever. It is possible to lift a heavy object, like a desk, by putting the end of a long board under it. Near the desk end, rest the board on a solid support that allows it to rotate. Then a small amount of force on the other end of the board will be sufficient to lift the desk. The board itself is a lever, and the pivot it rests on is called a fulcrum. The point where you apply the force is called the effort, which is just another word for input. The effect of applying this force, lifting the desk, is called the load, which is also called the output.

This lever works because the effort moves a lot more than the load, and therefore requires a lot less force. The amount of movement at each end is proportional to its distance from the fulcrum. The load arm and the effort arm are the special names given to the distances from the fulcrum to the load and effort, respectively. Using these definitions, the Law of the Lever is simply:

   Load arm X load force = effort arm X effort force

Force times distance is energy, so this law says that the energy input equals the energy output; in other words, energy is neither created nor destroyed. Using a little algebra, the Law of the Lever is equivalent to:

             load force    =    effort arm  
            effort force         load arm

In other words, the lifting force is bigger than the effort in the same proportion as the effort arm is longer than the load arm. This ratio is so important that it is given a special name: mechanical advantage. A pair of nail clippers has a large mechanical advantage, because the end of its handle is so far from the fulcrum.

Many books give the impression that the fulcrum of a lever is always somewhere in the middle, which is the case for a first-class lever, but there are two other arrangements that are equally possible. In these the fulcrum is at one end, and either the load or the effort is in the middle. A second-class lever, such as a garlic press or a wheelbarrow, has the load in the middle. In a third-class lever, like a pair of tweezers or a staple remover, the effort is in the middle.

By looking at the load and effort arms, it becomes clear that the mechanical advantage of a second-class lever is always more than one, but for a third-class lever it is always less than one. Third-class levers are used to increase the distance traveled by the load, relative to the effort, rather than the force. The most familiar example is the human forearm. Some mechanisms include more than one lever, and use one lever to operate another. These are called compound levers or linkages. Examples include the nail clippers, vise grips, adjustable desk lamp, pedal-operated wastebasket, and umbrella. Because they have multiple parts working together, they are all examples of systems.

Science books usually identify six simple machines: the lever, wheel-and-axle, pulley, inclined plane, wedge and screw. Two of these - the wheel-and- axle and the pulley - are really examples of the lever. A wheel-and-axle, used to drive a car, is a third-class lever, because the outside of the axle, which supplies the effort, is in between the fulcrum and the load. A pulley, used to lift a weight, is a first-class lever, in which the fulcrum is at the center of the pulley, in between the effort and the load. Two of the other three simple machines- the wedge and the screw - are examples of the third - the inclined plane. A wedge is a double-sided inclined plane, while a screw is an inclined plane wound around a cylinder or cone. The lever and the inclined plane are really the basis for all mechanisms.