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# Concept Day

### SLIDE MACHINES-1-1

In this Investigation, we begin our discussion of simple machines with the pulley. However, while we discuss pulleys here and then the simple machine levers in Investigations 2 and 3, we wish to at least introduce the other four of the traditional six simple machines as well. These include the wheel and axle, wedge, inclined plane, and screw. We will review these three machines again in Investigation 2.

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### SLIDE MACHINES-1-2

• This slide gives a quick review of the concept of work from the fifth grade Work and Simple Machines CELL. Work is the product of force (F) in Newtons times distance (d) in meters. That is Work = F x d.

Note: It is important to keep the concept of work in mind during this Investigation. This is because simple machines do not reduce the amount of work done but rather makes the same amount of work seem easier.

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### SLIDE MACHINES-1-3

• This slide simply lists the six simple machines; the lever, wheel and axle, wedge, inclined plane, screw, and pulley. We will devote the next two Investigations to the lever and this Investigation to the pulley.
• Simple machines often help us do work by provide mechanical advantage (MA). This is discussed further on the following slide.

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### SLIDE MACHINES-1-4

• Mechanical advantage. Simple machines do not reduce the total amount of work, but rather increases mechanical advantage (MA). Increased mechanical advantage makes work seem easier to us, but does not change the amount of work done.
• A good way to illustrate this is to think about an inclined plane like the one pictured at the bottom of this slide. In this example, a 750N (about 75kg or 165 pound) box must be lifted to a height of 1 meter. Instead of lifting it straight up, which might be very difficult or impossible, it is much easier to push it up the inclined plane. However, using the inclined plane results in us having to move the box 5 times further! Either way, the box must still be lifted the same distance of 1 meter. Therefore, the work done is the same regardless of how we do it.
• When we talk about simple machines during this CELL, we will often speak in terms of the mechanical advantage they provide.

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### SLIDE MACHINES-1-5

• This slide shows examples of tools that utilize the concept of the lever. One of the most common images of a lever is when a long object, such as a plank or metal rod is used to lift a heavy object such as a rock (center illustration). We will discuss load and effort arms of levers in Investigations 2 and 3. Every lever has a fulcrum. As shown in the illustration; the fulcrum is at the point where the bar makes contact with the small rock. As we go through the other examples of levers on this slide, try to identify where the fulcrum is located.
• The hammer at the lower right of this slide demonstrates a similar principle as the rock and bar illustration. In this case, the head of the hammer serves as a fulcrum. The effort arm is the long wooden handle and the load arm is the “claw” of the hammer.
• The pliers are constructed of two levers sharing the same fulcrum. The effort arm is the long, blue handle and the load arm is the much shorter, griping “jaw”. Tremendous force can be achieved with a “pair” of pliers.
• The bottle opener at the lower left has its fulcrum at the end of the lever. When the effort arm is lifted by its handle, the shorter load arm (distance from the fulcrum to the load: the edge of the bottle cap) exerts force on the cap.
• Finally, the nutcracker depicts yet another rendition of a lever simple machine. In this case, like the pliers, there are two levers sharing the same fulcrum. However, in this case, like the bottle opener, the fulcrum is at the end of the lever.

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### SLIDE MACHINES-1-6

• This slide shows examples of tools that utilize the concept of the wheel and axle. The important concept in terms of mechanical advantage is that wheels have a larger diameter than axles.
• In the case where effort or force is used to turn the wheel; the doorknob, sprinkler valve, screwdriver, and steering wheel in the examples on this slide, notice that one complete turn of the wheel is always coupled to one complete turn of the axle. However, the distance that any point on the edge of the wheel (its circumference) travels can be much further than a point on the edge of the axle (its circumference). This is what provides the mechanical advantage when effort is applied to the wheel.
• The mechanical advantage (MA) of a wheel and axle is simply the wheel diameter (Wd) divided by the axle diameter (Ad):

• A simple way to demonstrate the mechanical advantage of a wheel and axle machine is to try to screw in a screw using the shaft of a screwdriver rather than its handle!

Note: It is interesting to note that the tire (wheel) and axle on a car is quite different than the examples discussed above. In this case, the effort provided by the engine is applied to the axle, not the wheel. A simple calculation would demonstrate that this gives a negative mechanical advantage. This, therefore, requires a tremendous amount of force to be applied by the engine.

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### SLIDE MACHINES-1-7

• This slide shows examples of tools that utilize the concept of the wedge. The important dimensions in terms of mechanical advantage with a wedge are its side length and its thickness. The side length is measured from the point of the wedge to the point where the tapering to the point begins. In the ship example on this slide, the width would be from the point where the full width of the ship begins to taper to the bow.
• The longer a wedge’s side length is compared to its thickness increases the mechanical advantage of a wedge.

Thus, a long, sharp wedge will have more mechanical advantage than a short stubby wedge. A good way to appreciate the relationship between side length and width of a wedge is to imagine what happens as side length is shorted to a point where there is almost no point on the wedge at all… the bow of the ship, for example, would become flat, not pointed. All mechanical would be lost at this point.

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### SLIDE MACHINES-1-8

• This slide shows examples of simple machines that utilize the concept of the inclined plane. At this time, we are merely showing this slide to include in the six types of basic simple machines. We will discuss inclined plans as simple machines in somewhat more detail in Investigation 2.

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### SLIDE MACHINES-1-9

• This slide shows examples of simple machines and tools that utilize the concept of the screw. At this time, we are merely showing this slide to include in the six types of basic simple machines. We will discuss screws as simple machines in somewhat more detail in Investigation 2.

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### SLIDE MACHINES-1-10

• This slide shows several examples of pulleys. Pulleys can offer no mechanical advantage, as in the case with the weight machine on the left. On the other hand, they can offer a very large mechanical advantage, as shown on the crane head in the middle.
• Perhaps nowhere has the application of pulley technology been so elaborately developed as in the rigging systems of the large sailing ships of history.

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### SLIDE MACHINES-1-11

• This is simply an introductory slide to a discussion of the pulley.

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### SLIDE MACHINES-1-12

• Anatomy of a simple pulley. This slide highlights the essential details of a fixed single pulley.
• The load force is essentially the weight of the object to be lifted. It is measured in newtons (N) that, on Earth, is very close to the mass of the object, but not quite. We may use the following simple formula to calculate load force:

• where g is equal to the force of gravity = 9.8 m/s2. Thus, 1kg = 9.8N and .75kg (that is 750 grams) = 7.35N. It represents the force that gravity exerts on the object. Load force can be directly measured using a spring scale.
• The load distance is the distance that a pulley lifts the object. Often, this is the distance from the ground to some height above the ground.
• The effort force is the force that an operator exerts on the other end of the pulley. It is also measured in newtons (N) and can be determined using a spring scale.
• Finally, the effort distance is the distance over which an effort force is exerted on the pulley. For example, if I grabbed the pulley rope with my hand at my eye level and pulled it down to my waist, that distance would be the effort distance.

Note: It is important to remember that neither the effort distance nor the load distance has anything to do with the distance between the actual pulley and where the load or effort are located. This may confuse some students when we discuss effort and load “arms” when talking about levers in the next Investigation.

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### SLIDE MACHINES-1-13

• Fixed pulley. This is essentially a diagram of the first part of the experiment in Investigation 1. The “fixed” pulley is firmly attached to an immovable surface. In this slide, it appears to be fixed to the ceiling. In the lab, it is fixed to the benchtop. Table A from the SDR is reproduced here as well.
• In the lab, three different load forces will be used and, in each case, the load will be lifted the same distance (load distance).
• On the effort side of the pulley (left side in this slide), you will pull down on the pulley with a spring scale and measure the distance the spring scale is pulled to lift the load the prescribed load distance. All of these data will be recorded in the Table.
• You will then use the data that you collect to determine if the fixed pulley provides a mechanical advantage and if so, what the mechanical advantage is.

Note: You may wish to review this slide to prepare for lab.

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### SLIDE MACHINES-1-14

• This is essentially a diagram of the second part of the experiment in Investigation 1. The “fixed” pulley is firmly attached to the benchtop while a moveable pulley is inserted between the fixed pulley and a second fixed point (a ring stand in lab).
• Once again, you will lift several different loads a specified distance and, using a spring scale and meter stick, determine the effort force and effort distance required to do so.
• You will use the data that you collect to determine if a mechanical advantage is provided by the fixed and mobile pulley system and if so, what the mechanical advantage is.

Note: You may wish to review this slide to prepare for lab.

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