In physics, a lever (from French lever, "to raise", c.f. a levant) is a rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force that can be applied to another object. This leverage is also termed mechanical advantage, and is one example of the principle of moments. A lever is one of the six simple machines. Archimedes once said, "Give me a lever long enough and a fulcrum on which to place it, and I shall move the world." First class levers are similar but not the same as second or third class levers, in which the fulcrum, resistance, and effort are in different locations.
Theory of operation
The principle of leverage can be derived using Newton's laws of motion, and modern statics. It is important to note that the amount of work done is given by force times distance. To use a lever to lift a certain unit of weight with a force of half a unit, the distance from the fulcrum to the spot where force is applied must be exactly twice that of the distance between the weight and the fulcrum. For example, to cut in half the force required to lift a weight resting 1 meter from the fulcrum, we would need to apply force 2 meters from the other side of the fulcrum. The amount of work done is always the same and independent of the dimensions of the lever (in an ideal lever). The lever only allows to trade force for distance.
The point where you apply the force is called the effort. The effect of applying this force is called the load. The load arm and the effort arm are the names given to the distances from the fulcrum to the load and effort, respectively. Using these definitions, the Law of the Lever is:
Load arm X load force = effort arm X effort force. If, for example, a 1 gram feather were balanced by a one kilogram rock, the feather would be 1000 times further from the fulcrum than the rock; if a 1 kilogram rock were balanced by another 1 kilogram rock, the fulcrum would be in the middle.
The three classes of levers
There are three classes of levers which represent variations in the location of the fulcrum and the input and output forces.
A first-class lever is a lever in which the fulcrum is located between the input effort and the output load. In operation, a force is applied (by pulling or pushing) to a section of the bar, which causes the lever to swing about the fulcrum, overcoming the resistance force on the opposite side. The fulcrum may be at the center point of the lever as in a seesaw or at any point between the input and output. This supports the effort arm.
- 1. Seesaw (also known as a teeter-totter)
- 2. Crowbar (curved end of it)
- 3. Hammer Claw , when pulling a nail with the hammer's claw
In a second class lever the input effort is located at the end of the bar and the fulcrum is located at the other end of the bar, opposite to the input, with the output load at a point between these two forces.
- 1. Nutcracker
- 2. Paddle
- 3. Wheelbarrow
- 4. Wrench
- 5. Bottle opener
Third class lever. For the lever in this diagram to work correctly, one must assume that the fulcrum is attached to the bar or acting in opposition to the other two forces.
For this class of levers, the input effort is higher than the output load, which is different from second-class levers and some first-class levers. However, the distance moved by the resistance (load) is greater than the distance moved by the effort. Since this motion occurs in the same length of time, the resistance necessarily moves faster than the effort. Thus, a third-class lever still has its uses in making certain tasks easier to do. In third class levers, effort is applied between the output load on one end and the fulcrum on the opposite end.
- 1. Baseball bat
- 2. Boat paddle
- 3. Broom
- 4. Fishing rod
- 5. Hockey stick
- 6. Shovel (the action of picking or lifting up sand or dirt)
- 7. Hammer
- Lever at Diracdelta science and engineering encyclopedia
- A Simple Lever by Stephen Wolfram, Wolfram Demonstrations Project.
- Levers: Simple Machines at EnchantedLearning.com