First, Second, and Third Class Levers

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Juliet Sherman
5 years ago
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1 First, Second, and Third Class Levers Introduction: Levers, being simple machines, have only three simple parts. The load, the effort, and the fulcrum. Let s start with the load. The load is basically what it is you re trying to lift. The books in the last experiment where the load. Now for the effort. That s you. In the last experiment, you were putting the force on the lever to lift the load. You were the effort. The effort is any kind of force used to lift the load. Last for the fulcrum. It is the pivot that the lever turns on. The fulcrum, as we ll play with a bit more later, is the key to the effectiveness of the lever. There are three types of levers. Their names are first-class, second-class and third-class. I love it when it s that simple. Kind of like Dr. Seuss s Thing One and Thing Two. The only difference between the three different levers is where the effort, load and fulcrum are. Materials: Strong piece of wood 3-10 ft long A brick, think book, or smaller piece of wood (for the fulcrum) Books, gallons of water, or anything that's heavy and not fragile Scale Tape measure A parent or partner Procedure: Let's start by looking at three types of levers. First-class, second-class, and third-class levers. We will first use all three and get familiar with how they work, then we will use the third-class lever to do some measurements and calculations. First-Class Lever: A first-class lever is a lever in which the fulcrum is located in between the effort and the load. This is the lever that you think of whenever you think of levers. The lever you made in Experiment 1 is a first-class lever. Examples of first-class levers are the see-saw, a hammer (when it s used to pull nails), scissors (take a look, it s really a double lever!), and pliers (same as the scissors, a double lever) Supercharged Science Page 1

2 Be careful with this. Don t use something that s so heavy someone will get hurt. Also, be sure not to use something so heavy that you break the wooden lever. Last but not least, be sure to keep your head and face away from the lever. I ve seen folks push down on the lever and then let go. The lever comes up fast and can pop you pretty hard. 1. Put your fulcrum on the ground. 2. Put your lever on the fulcrum. Try to get your fulcrum close to the middle of the lever. 3. Put some weight on one end of the lever. 4. Now push down on the other side of the lever. Try to remember how hard (how much force) you needed to use to lift the heavy object. 5. Move the fulcrum under the lever so that it is closer to the heavy object. 6. Push down on the other side of the lever again. Can you tell the difference in the amount of force? 7. Move the fulcrum closer still to the heavy object. Feel a difference now? 8. Feel free to experiment with this. Move the fulcrum farther away and closer to the object. What conclusions can you draw? What you may have found, was that the closer the fulcrum is to the heavy object, the less force you needed to push with to get the object to move. Later we will look at this in greater detail, but first let me tell you about the other types of levers. Second-Class Lever: The second-class lever is a little strange. In a second-class lever, the load is between the fulcrum and the effort. A good example of this, is a wheel-barrow. The wheel is the fulcrum, the load sits in the wheel-barrow bucket and the effort is you. Some more examples would be a door (the hinge is the fulcrum), a stapler, and a nut-cracker. Again, be careful with this. Don t use something that s so heavy someone will get hurt. Also, be sure not to use something so heavy that you break the wooden lever. Last but not least, be sure to keep your head and face away from the lever. I ve seen folks push down on the lever and then let go. The lever comes up fast and can pop you pretty hard. 1. Put your fulcrum, the book or the brick, whatever you re using on a nice flat spot. 2. Put the end of your lever on the fulcrum Supercharged Science Page 2

3 3. Put the books or gallon jugs or whatever you re using for a load, in the middle of the lever. 4. Now, put yourself (the effort) on the opposite end of the lever from the fulcrum. 5. Lift 6. Experiment with the load. Move it towards the fulcrum and lift. Then move it toward the effort and lift. Where is it harder(takes more force) to lift the load, near the fulcrum or far? Where does the load lift the greatest distance, near the fulcrum or far? Third-Class Lever: This fellow is the oddest of all. The third-class lever has the effort between the load and the fulcrum. Imagine Experiment 1 but this time the fulcrum is at one end of the board, the books are on the other end and you re in the middle. Kind of a strange way to lift books huh? A few examples of this are tweezers, fishing rods (your elbow or wrist is the fulcrum), your jaw (the teeth crush the load which would be your hamburger), and your arm (the muscle connects between your elbow (fulcrum) and your load( the rest of your arm or whatever you re lifting)). Your skeletal and muscular system are, in fact, a series of levers! 1. Put your fulcrum on the ground in a nice flat place. 2. Put your lever on the fulcrum so that the fulcrum is at the very end of the lever. 3. Put your load on the lever at the end farthest from the fulcrum. 4. Now, put yourself (the effort) in the middle of the lever. 5. Lift. You may need someone to hold down the lever on the fulcrum 6. Experiment with the effort (you). Move towards the fulcrum and lift the load Then move toward the load and lift. Where is it harder(takes more force) to lift the load, near the fulcrum or far? Where does the load lift the greatest distance, near the fulcrum or far? We ve had a lot of fun levering this and levering that but now we have to get to the point of all this simple machine stuff. Work equals force times distance, right? Well, what have you been doing all this time with these levers? You ve been moving something (the load) a distance against a force (gravity). You ve been doing work. You ve been exerting energy. See how it all ties in nicely? 2014 Supercharged Science Page 3

4 Now we're going to need the scale, tape measure, partner, and a longer piece of wood (at least 8 ). Be very careful during this part of the experiment, as it involves standing on the wood, we don't want any injuries! Start by weighing both you and your partner, and record the values in the table below. Begin with the fulcrum in the center of the board (first-class lever). Have the heavier person stand on one edge of board. Then, the lighter person carefully stand on the other edge. The heavier person should stay planted on the ground. Now, using your tape measure, move the fulcrum closer to the heavier person in 6 increments and record the distance in the table below. After each movement, try to lift the heavier side by simply standing on the other end. Weight of Heavier Person (lbs) Weight of Lighter Person (lbs) Fulcrum Distance form Heavier Person (inches) Did the Heavier Person lift up?? (Yes or No) Once you've successfully lifted the heavier person by simply standing on the other end, do some fine tuning until you can both balance on the board Supercharged Science Page 4

5 What's the math behind this balancing act? Well, the equation for how much force is required to lift the heavier person is as follows, where W is the weight of the heavier person (lbs), X is the length from the heavier person to the fulcrum (inches), L is the distance form the lighter person to the fulcrum (lbs), and F is the weight of the lighter person (lbs) F = (W * X) / L Try your own values in the equation. You're answers should be very close! Problems: 1. Imagine you weigh 120 lbs and need to lift a 200 lb water jug. All you have handy is a first-class lever (like a see saw) made of a 12 foot beam. How far should you place the fulcrum from yourself, if you plan to use all your weight to lift the water? 2. Now what if the board was 24 feet long (assuming it wouldn't bend). But, the water jug now weights 1000 lbs! Can you lift it simply by standing on the board? If so, how far from you should the fulcrum be placed? 2014 Supercharged Science Page 5

6 Answers: feet 2. Yes you can! Place it 21.4 feet away form you 2014 Supercharged Science Page 6

Levers. Simple Machines: Lever 1

Levers. Simple Machines: Lever 1 Levers In the last lesson, we spent a lot of time on this strange concept called work. Work happens when something moves a distance against a force. Swell...who cares?! Well, believe it or not, this is