A good example of a lever is a see-saw. Let's say that you're really light, and you want to lift a really heavy person on the opposite side. If you put the fulcrum in the middle, you won't have a chance. But if you slide the fulcrum closer to the heavy person, it will be easier to lift. Where's the trade-off? Well, to get this helping hand, your side of the see-saw is much longer (and higher off the ground), so you have to move it a much greater distance to get the lift.
Levers are used to:
First Class Levers:
The force must be applied over a long distance, in order to make the heavy load move
just a small amount.
By adjusting how far the fulcrum is from the load, you can control the mechanical advantage. The closer it is to the load, the more force is applied.
The second class lever is one where the fulcrum is at one end, and the applied force at the
other. The load that is to be moved is between them.
This lever is different in how it works ... it causes the load to move in the same direction as the force you apply.
Just as with a first class lever, how close the load is to the fulcrum determines by how much your force will be multiplied. If you want to move a very large load with a small applied force, you must put the load very close to the fulcrum.
The third class lever is the strangest ... and the one you use the most!The fulcrum is once again at one end of the lever, but this time the load is at the other end, and you apply a force in between.
This lever can not give any mechanical advantage. Regardless of where you apply the force, the
force you apply must always be greater than the force of a load.
If you were using this lever to lift an object at a distance, it would require less force to just stand above it and lift it up ... using the lever will require more force!
So why use a third class lever at all?
The answer lies in the fact that the load moves in the same direction as the force you apply, which is convenient. So is the application of force between the load and the fulcrum
What is mechanical advantage?
When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.
Jeffrey Janes, Kendra Penton & Kyle Hiscock
Info retreived from: http://www.wcsscience.com/lever/page2.html