Draw An Arrow To The Fulcrum Of Each Lever

Other classes of levers

In this chapter, you will learn about two more types of levers, which are also called classes of levers. In first-class levers, the fulcrum is somewhere between the effort and the load. In the other two classes, the fulcrum is at one of the ends.

Figure 1

Figure 2: These pictures show two corrugated cardboard sheets of about 20 cm long and 10 cm wide. The one piece has the corrugations over the width, and the other piece has the corrugations over the length.

Figure 3: Both pieces are folded in the middle to form springs. The edges are folded to form flanges.

The three classes of levers

Lift your finger in three different ways

Put your pencil on the desk in front of you.

Figure 4

Press the pencil down in the middle with right index finger, now try to lift your index finger by lifting the pencil at the sharp end with your left hand, as shown below. When you do this, the pencil acts as a lever.

Figure 5

1. The fulcrum of the lever is at the right end of the pencil, where it rests on the desk. Mark the input force with an arrow on the sketch above. Where is the load?

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2. In Figure 5 the input force is at one end of the lever, and the fulcrum at the other end. How is a first-class lever different from this?

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   Press the pencil down at the sharp end with your right index finger, and try to lift your finger by lifting the pencil in the middle with your left hand, as shown below.

   

   Figure 6

3. The fulcrum of the lever is at the left end of the pencil, where it rests on the desk. Mark the effort with an arrow on the sketch above. Where is the load?

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4. In the above case, the load is at one end of the lever, and the fulcrum at the other end. How is the situation on the previous page different from this one?

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   You used the pencil as a third-class lever in the above case. On the previous page, you used the pencil as a second-class lever .

   To use the pencil as a first-class lever, you need to add support somewhere between the two ends to act as a fulcrum.

   

   Figure 7

5. Do the experiments on the previous two pages again. When do you get the biggest mechanical advantage: when you use the pencil as a second-class lever or when you use it as a third-class lever?

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   Fulcrum is another word for pivot point.

   • Levers like this one, where the pivot point is between the input force and the load, are called first-class levers .

   

   • When the load is between the input force and the pivot point, it is called a second-class lever .

   

   • When the input force is between the load and the pivot point, it is called a third-class lever .

   

Practical examples of different classes of levers

This boy will swing the hammer to hit the nail into the wall.

Figure 11

In this situation, his forearm and the hammer together form a lever. The lever swings around the elbow, so the elbow forms the fulcrum.

1. Is his forearm and the hammer a first-class lever, a second-class lever or a third-class lever?

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2. Can you think of a sport where a person swings an object to hit something?

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3. Rest your right elbow on your desk, then pick up something with your right hand while keeping your elbow on the desk.

   Do it again, but this time hold your left hand lightly on your right arm, just above the elbow.

   Do you feel the muscle movement inside your arm?

   

   Figure 12

   The diagram below explains how your arm works.

   

   Figure 13

   When you pick something up in your hand, your arm works like a third-class lever and the input force is between your elbow and your hand. Your elbow acts as the fulcrum and the load is in your hand.

4. In each of the pictures below, draw a small triangle to indicate where the fulcrum is, and an arrow to indicate where the input force is. Make a letter L to show where the load is. Also state in each case what class of lever it is.

Figure 14

More practical examples of different classes of levers

Figure 15

When you use a wheelbarrow, the axle of the wheel is the fulcrum and your arms provide the input force. The load is between the fulcrum and the input force. This is how a second-class lever works. The nutcracker below is also a second-class lever.

An easy way to remember how a second-class lever works, is to think of a wheelbarrow or a nutcracker.

Figure 16: A nutcracker

Make a lever on a base plate

Use corrugated cardboard to make a lever on a base plate, as shown on this scale drawing. The scale of the drawing is 1:3.

Figure 17

For the lever, the corrugations must have the same direction as the length of the lever. Use a strip of cardboard 6 cm wide, and fold up the edges along the length to form flanges as shown on the right.

Figure 18

You can use this lever to move a small box filled with sand. You can do this in two ways: by using the lever as a second-class lever or by using the lever as a third-class lever.

1. Make free-hand sketches to illustrate the two ways in which your lever can be used.

2. Use your lever and sandbox to investigate when you get the biggest mechanical advantage, with a second-class lever or with a third-class lever. Write a brief report below.

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Next week

In the next chapter, you will investigate and learn how levers can be linked, and how they can be used for a variety of purposes.

Draw An Arrow To The Fulcrum Of Each Lever

Source: http://www.mstworkbooks.co.za/technology/gr7/gr7-technology-05.html

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