Levers

From High School Online Collaborative Writing

Levers are used to lift a weight or mass that would be too heavy to lift without a lot of force. The lever is one of the oldest Physics ideas that is still used today. Levers were used in Egypt to assist the Egyptians in erecting large obelisks. Levers reduce the amount of force that must be put into moving an object by increasing the distance, or achieve the opposite by reducing the distance and increasing the force.

Contents 1 Lever Classes 1.1 First-Class Levers 1.2 Second-Class Levers 1.3 Third-Class Levers 1.4 How To Remember 2 Torque 3 Levers Today 4 References 5 External Links

[edit] Lever Classes

There are three different types of levers that exist whose methods of input and output vary.

[edit] First-Class Levers

The First-Class Lever is the most easily recognizeable of the three. The most common, everyday example of a First-Class Lever is a see-saw. Scissors and pliers are also common examples of a Class One Lever. Anything that has the input and output on opposite sides of the fulcrum (located in the middle) is a Class One Lever. The goal of a Class One Lever is to raise something on the far end by applying force on the near end. For example, a little girl and little boy are on a see-saw. The little girl applies her weight to her side, causing the little boy on the far side to go up. In this case, the two people on the see-saw weigh approximately the same amount. It would be possible for the little girl to lift someone much heavier than her (insert fat joke here) using some physics and formulas we will get to later. Just remember, "When you have a fat friend, there are no see-saws. Only catapults." -Demetri Martin

[edit] Second-Class Levers

Class Two Levers are a bit different than Class One Levers. The fulcrum is on the far side and the effort that you exert is on the near side, meaning the load is in the middle. A bottle opener is a common example. You place the opener under the bottlecap (this is the fulcrum, or pivot point) and pull up, thus lifting off the load (the bottlecap). Note that you can push down on a Class Two Lever to achieve similar results. Staplers and nail clippers are common Class Two Levers that operate when pushed down upon.

[edit] Third-Class Levers

Class Three Levers are very similar to Class Two Levers, but the placement of the effort and load are switched. The fulcrum is still on the outside, but the load is on the other side, meaning that the effort is now in the middle. A baseball bat is a common example. The fulcrum would be your body, because your arms and the bat (they're connected in this example so we can treat them as one) pivot around it. The effort is exerted by your hands in an effort to move the load (the baseball) that is on the end of the lever (bat). A fishing rod, tweezers and tongs are all common examples of a class three lever.

[edit] How To Remember

A helpful way of remembering the classes of levers is with the mnemonic flex. f represents the fulcrum, l represents the load, and e represents the effort. The order of the letters in flex reminds you which part of the lever is between the other two. So the first letter, f, tells us that the fulcrum is between the load and effort in the first-class lever. The second letter, l, tells us that the load is between the effort and the fulcrum for the second-class lever. Finally, the third letter, e, tells us that the effort is between the load and fulcrum in the third-class lever.

[edit] Torque

Suppose you wanted to lift up a box that weighed 200 pounds (F₁ = 200 pounds). If you used a lever with 1 foot of distance from the fulcrum to the weight (D₁ = 1 foot) and the other distance was 10 feet to the fulcrum (D₂ = 10 feet), then you would only have to push down at your end with 20 pounds of force (F₂ = 20 pounds).

This is because of the rule of static equilibrium that F₁D₁ = F₂D₂. In order to to achieve equilibrium, the fulcrum must be located at the center of mass. In this case, we must add 20 pounds, because 20 pounds times 10 feet = 200 pounds times 1 foot.

Although you have what is called a mechanical advantage in being able to lift this heavy weight, you are actually doing the same amount of work, because you have to push the 20 pound force 10 times as far as the 200 pound box moves. For example, to lift the box 3 inches, you have to push the 20 pound force for 30 inches.

Since Work is Force times Distance, then you can see that: 20 pounds x 30 inches = 200 pounds x 3 inches.

[edit] Levers Today

Today, the role of large levers has been taken over by hydraulics and motors. Rather than finding the manpower to apply great force to one side of a lever in order to slowly lift, it is easier to use motors and pistons to. Modern day solutions have the capabilities to lift a load straight up, which means that all of the effort that is put in lifts the weight higher up. The effort put into levers moved the load diagonally (up and towards the fulcrum), meaning that more effort had to be put in in order to raise the load the same height. However, as shown above, many everyday aiding objects are levers. In fact, parts of our bodies act as levers constantly (ie, bicep curls). Although levers have been surpassed for large projects, it seems that for the common person levers are invaluable for getting through life.

[edit] References

• Torque Picture from "Levers" at Wikipedia.
• Explains the role of inertia in levers
• Gives good everyday examples of torque and levers
• Examples of rigid bodies in equilibrium
• [2]

[edit] External Links

• Cool java applet that demonstrates the principles of levers and torque.
• Website that shows the different kinds of levers in action.
• Shows how a trebuchet uses lever concepts.
• A website] that quizzes you on the various types of levers. See if I taught you anything at all