In this video segment from Cyberchase, Jackie, Matt, and Inez try to move a stack of slabs blocking a doorway. They decide to use a long board as a lever, but they soon realize they need a longer board in order to move the heavy slabs. The problem is that they are not sure exactly how long the lever should be. Since they do not have the time to try out levers of varying lengths, Inez decides to construct a scale model. While testing their model levers, they learn about the relationship between the length of a lever and its lifting capability.
If you have ever played on a see-saw or removed a nail from a wall using the claw on a hammer, you have seen a lever at work. A lever is a simple machine used to decrease the effort required to move objects. It consists of a board or bar positioned on a pivot point, called the fulcrum. A lever is used to lift a load using a force of effort applied at a second point along the board. In order for a lever to work, a force (either a push or a pull) must be applied to it. The amount of weight that can be lifted using a lever varies depending on the position of the fulcrum and the length of the board.
Not all levers are the same. There are three classes of levers which differ depending on the position of the fulcrum in relation to the load. In Class 1 levers, the fulcrum lies between the load and the effort. When Class 1 levers are used, a downward force causes the load to move up, while applying an upward force moves the load down. Common Class 1 levers include seesaws, crowbars, and balance scales. Some common tools are made up of multiple levers. For example, pliers and scissors are each made up of two Class 1 levers.
In Class 2 levers, such as a wheelbarrow, the load lies between the effort and the fulcrum. With a Class 2 lever, the direction of the effort is also the direction that that the load will move. So, in the case of the wheelbarrow, the wheel acts as the fulcrum and the effort is exerted at the handles, so when you pull up on the handles, the materials in the wheelbarrow move upward as well. A diving board, bottle opener, and the claw part of a hammer are examples of Class 2 levers.
In the case of Class 3 levers, the effort is placed between the load and the fulcrum. A hammer, when used to force a nail into something, is one example of a Class 3 lever. The wrist acts as the fulcrum while the load is the resistance of the material into which you are driving the nail. When a Class three lever is used, the load moves in the same direction as the force. Tweezers are an example of an object made up of two Class 3 levers. When you push the tweezers together at a point in the middle of the tweezers, the two tips are also pushed together, allowing you to grab onto a small object. Class 3 levers are also used when you want to increase the speed at which a load moves. A baseball bat and a golf club are examples of Class 3 levers. This allows the hitting end to move faster than your arm.
With all levers, if the effort is fixed, applying that effort at a location further away from the fulcrum will result in an increased amount of force. This is why a longer board is able to lift more than a shorter board. And we can say that the force is directly proportional to the length of the lever arm. More practically, this law can be applied when an adult and a child try to ride a see-saw. In order for the two sides to balance, the adult will have to sit closer to the fulcrum. How much closer? That will depend on the ratio of the adult's weight to that of the child.
To learn more about simple machines, check out Building Simple Machines: Plant Quencher.
To learn more about using models, check out Testing with Models.
MATT: The slabs aren’t moving! Try bouncing.
INEZ: Move all the way to the end like we did last time.
JACKIE: Excuse me, we’re already at the end.
MATT: Then I guess we need more end! Heh!
INEZ: More end - that’s it! We need a longer lever!
JACKIE: Definitely longer. Give me a hand.
MATT: Let’s try it!
INEZ: Close - but it’s still not working! Hmmm, we’ve just got to think of something!
MATT: Whoa! Guess you were thinking too hard!
MATT: Oh man!
JACKIE: C’mon guys...what do we know? The longer the lever, the more lifting power we have. So all we have to do is find a longer lever!
MATT: Yeah, but how long? Trying different lengths could take forever!
DIGIT: Matt! Inez! Jackie! Come in!
JACKIE: Didge! What’s up?
DIGIT: Hacker’s draining the magic from the Professor’s wand into his own body. We gotta stop him!
JACKIE: Okay, but we need Shari’s help! We’ll be there as soon as we get her!
DIGIT: Sheesh, I can’t wait that long. I need a Plan A.
BUZZ: You’ll need more that!
DIGIT: How about a Plan B?
INEZ: Matt, do you have more yo-yo’s?
MATT: Yeah...but why?
INEZ: I made this model to help me think. And I need more yo-yo’s. See? Lever...fulcrum...slabs...and...this is us!
INEZ: If we can figure out how things work with this model, maybe we can find a rule to use with our other lever! Since different lengths lift different weights, we can find the length we need.
JACKIE: Cool! Worth a try!
INEZ: I want to see how long the lever needs to be to lift two yo-yo’s, then three, so we can compare the lengths. That should tell us what we need to know.
MATT: Great idea! Here. It’s not lifting the two yo-yos. We need a longer lever.
JACKIE: Got it!
JACKIE: Yes! Long enough for a two yo-yo lift off.
INEZ: Hey, check this out! If you measure from the fulcrum to where we push down...the lever that lifted the two yo-yos is twice as long as the lever that lifted one yo-yo!
MATT: So maybe a lever three times as long will lift three yo-yos?
JACKIE: This one’s three times as long! Let’s try it. Cool!
INEZ: Twice the length lifts twice the weight!
MATT: And three times the length lifts three times the weight.
INEZ: And since we need to lift three slabs instead of one...we need a lever three times as long as the one we used the first time. Let’s find one!
JACKIE: Okay! Climb aboard, guys!
INEZ: Please work...please, please, please work!
MATT: It worked! Three times as long lifted three times the weight! Quick! We’ve gotta get Shari out and stop Hacker.
Academic standards correlations on Teachers' Domain use the Achievement Standards Network (ASN) database of state and national standards, provided to NSDL projects courtesy of JES & Co.
We assign reference terms to each statement within a standards document and to each media resource, and correlations are based upon matches of these terms for a given grade band. If a particular standards document of interest to you is not displayed yet, it most likely has not yet been processed by ASN or by Teachers' Domain. We will be adding social studies and arts correlations over the coming year, and also will be increasing the specificity of alignment.