Simple Machines Summary (#9)

Anna Cajiga

SME 301

Simple Machines Summary (#9)

April 21, 2005

Title

Experimenting with Levers

Benchmark

Area: Physical Science

Category: Motions of Objects

Benchmark: Design strategies for moving objects by application of forces, including the use of simple machines (Middle School).

Misconception

All levers are the same – no matter where the fulcrum is located on a lever, the input force, the speed and the distance over which the input force is applied all remain the same.

Background

There is a common misconception that all levers are the same. Many students believe that no matter where the fulcrum is located on a lever, the input force, the speed and the distance over which the input force is applied all remain the same. However, this activity disproves this misconception. A simple machine is a device that makes work easier for us to do. It does this by decreasing either the amount of force needed to move an object, or the distance over which this force is applied. A lever is a simple machine made up of a bar that rotates around a fixed point called a fulcrum. Levers make it easier for us to lift heavy objects. As the fulcrum of a lever moves further away from the resistance force, or load being lifted, the input force needed to move the object increases while the distance over which the input force is applied and the speed at which the work is done decreases.

Materials

1 Flat Wooden Ruler

Masking Tape

3 Large Washers

1 Pen

Directions

1. Stack 3 washers and tape them together.

2. Place the pen under the ruler at the 10 cm mark.

3. Place the washers on top of the ruler at the 1 cm mark.

4. Push down on the top of the ruler at the 30 cm mark and record your observations.

5. Move the pen to the 15 cm mark and, keeping the washers at the 1 cm mark, push down on the 30 cm mark. Record your observations and compare your effort to when the pen was at the 10 cm mark.

6. Move the pen to the 20 cm mark and, keeping the washers at the 1 cm mark, push down on the 30 cm mark. Record your observations and compare your effort to when the pen was at the 10 cm and 15 cm mark.

7. Move the pen to the 25 cm mark and, keeping the washers at the 1 cm mark, push down on the 30 cm mark. Record your observations and compare your effort to when the pen was at the 10 cm mark, 15 cm mark and 20 cm mark.

Results

Students will recognize that as the pen, otherwise known as the fulcrum, moves further away from the stack of washers, or the load, the amount of effort needed to raise the load increases, but the load is lifted higher into the air. It can also be observed that whenever the fulcrum is closer to the load, in order to raise the load to the same height as it is raised when the fulcrum is at the 20 cm mark, the student would need to increase the distance that the 30 cm end of the ruler is pressed down.

Discussion

Work is done (or performed) when a force is applied to a body and causes the object to move. A simple machine is a device that makes work easier to do. There is a very simple formula for work:

Work = Force x Distance

Work is measured in units of energy. The joule is a unit of energy. Part of the Law of Conservation of Energy states that the amount of work needed to complete a given task always remains the same, whether a simple machine is used or not. Since work is equal to the product of Force x Distance, this means that there is a tradeoff between the force needed to move an object and the distance over which the force is applied. By using a simple machine to do a certain amount of work, we can decrease the force needed to move an object, but the distance over which the force is applied will be increased proportionally. The opposite is also true, we can reduce the distance over which the force is applied, but the force must be increased in proportion. This is apparent in the above experiment, which involves using a lever, one type of simple machine, to lift a load of washers. When the pen, or fulcrum, is closer to the load, very little force is needed to move the load, but the load moves a small distance. When the fulcrum is further from the load, much more force is needed to move the load and the load moves a larger distance than when the fulcrum is closer to the load. In order to raise the load to equal heights, the distance that the opposite end of the lever would need to be moved in a downward direction would be much greater when the fulcrum is closer to the load. When the distance over which the force must be applied to move an object is decreased, it takes less time for the task to be completed. Therefore, when distance is decreased, the speed at which the work is completed is faster, but more effort (or force) is needed to complete the work. Likewise, when effort (or force) is decreased, the speed at which the work is completed is slower.

There are six types of simple machines. One of these, as discussed above, is the lever. A lever is a bar that rotates around a point called the fulcrum. There are three classes of levers. A first class lever is like the one produced in the above experiment, where the fulcrum is between the effort force and the resistance force, or load. Second class levers have the resistance force located between the fulcrum and the effort force. A wheelbarrow is a good example of a second class lever. Third class levers have the effort force being applied between the fulcrum and the resistance force. Tweezers are an example of a third class lever. In addition to the lever, there are five other types of simple machines – wheels and axles, pulleys, inclined planes, wedges and screws. The wheel and axle has a large wheel that is attached to a smaller wheel, called an axle. When one of these parts turns and makes a full rotation, the other part also turns and makes a full rotation. A pulley is made up of a grooved wheel that is able to turn freely, and a rope. Pulleys can be arranged in different ways to either be a fixed pulley that simply changes the direction of force from a downward force to an upward force, or a movable pulley that allows us to lift a heavy load by using less force. An inclined plane is simply an even, sloping surface at any angle above 0 degrees but below 90 degrees. Inclined planes make it easier for us to move a weight from a lower height to a higher height because they spread the applied effort out over a longer distance. Wedges are very similar to inclined planes. Single wedges are made up of one inclined plane, and double wedges are comprised of two inclined planes joined together back-to-back, with the sloping side facing outwards. Wedges allow us to change a downward motion into a splitting motion, for example, as an ax does. Screws are another form of an inclined plane, where the threads of the screw are like an inclined plane wrapped around a central core. Screws help us by changing a downward motion into a linear motion. Of these six simple machines, levers and wheels and are the only machines that are able to decrease either the amount of force needed to do work, or decrease the distance over which this force is applied. Inclined planes, wedges, pulleys and screws are only able to decrease the amount of force needed to complete a task. While there are many other, more complicated, machines that we use in our everyday lives, every machine is always made up of combinations of these six simple machines. Any combination of these six machines is called a “complex machine”.

The amount of force that is gained by using a machine rather than doing the work on your own can be measured by computing the Mechanical Advantage, or MA, of a simple machine. The general formula for determining MA is:

MA = Output Force/ Input Force

The output force is the amount of force the machine delivers, and the input force is the amount of effort (or force) put into the machine. To compute the MA for each particular simple machine, we must use different formulas. For a lever, the MA is equal to the ratio of the distance from the fulcrum to the applied force to the distance from the fulcrum to the load being lifted. To determine the MA of a wheel and axle, we compute the ratio of the radius of the wheel to the radius of the axle. Determining the MA of a pulley is simply done by counting the number of rope ends that support the pulley. This number is the MA. For an inclined plane, the MA can be found using the following equation:

MA of an Inclined Plane = Length of Slope / Height of Inclined Plane

Similarly, to determine the MA of a wedge, the following formula is used:

MA of a Wedge = Length of Either Slope / Thickness of Larger End

Lastly, computing the MA of a screw requires the use of the following formula:

MA of a Screw = Circumference of the Screw / Pitch of the Screw

The pitch of the screw is another word for the distance between each thread of the screw. All of these formulas can be used to determine how much each simple machine helps us complete a given task. A higher MA means that less effort is needed on our part.

Understanding these concepts about simple machines explains many observations in our everyday lives. For example, most everyone has surely noticed that it is easier to walk up a long gentle slope than a shorter steeper slope. This is because the effort needed to walk up the hill is spread out over a longer distance when the slope is longer and gentler. Also, when using a pair of scissors, it is easier to cut something very thick if you place the object near the pivot point, or fulcrum, of the scissors instead of near the tip. This is because the distance between the load and the fulcrum is less, so, as we observed in the above experiment, the work becomes easier. We can also make connections between the six simple machines and objects in our everyday lives. For example, before learning about simple machines, we would have never known that a light bulb is really a screw. We are able to put it into light sockets and receive light because the threads of the screw allow us to turn a rotating motion into a linear motion. Lastly, we can understand what people mean when they talk about “mechanical advantage”. We now know that if someone says that a lever being used to lift a large stone has a mechanical advantage of 10, they mean that the lever magnifies the amount of input force by 10. We would also understand that a lever with a mechanical advantage of 10 would make the large rock much easier to move than a machine with a mechanical advantage of 2.

Classroom Resources

1. Simple Machines (Starting With Science) by Adrienne Mason and Deborah Hodge

This easy-to-understand nonfiction children’s book guides students through thirteen experiments that deal with different aspects of simple machines so that they have an opportunity to explore concepts related to the lever, wheel and axle, pulley, inclined plane and screw, and are able to form connections on their own.

2. All About Simple Machines Video

All About Simple Machines begins by addressing misconceptions about the term “work” and then introduces students to ideas about force, work, and simple machines by making clear and interesting everyday connections that even young students will understand.

3. Simple Machines Kit from Nebraska Scientific - Buyer's Guide for Science Education

This classroom kit includes in-depth guides that assist the teacher in leading engaging lessons that teach students through hands-on activities about work, how simple machines make work easier, how force and distance are related, and encourage students to form everyday connections to their newfound knowledge.

Credits

I found this activity at http://www.ed.uri.edu/SMART96/ELEMSC/smartmachines/lever.html>. I slightly adapted it by adding an extra step where students move the fulcrum (pen) to the 25 cm mark, in hopes that this would further emphasize the fact that as the fulcrum moves further from the object to be lifted, both the effort needed and the distance that the object is raised are increased. For my research about simple machines, I used the following resources:

Encarta Encyclopedia at <www.encarta.msn.com>, “Simple Machines” entry by Odis Hayden Griffin Jr., BS, MS, PhD

Heath Physical Science Teacher’s Edition, Chapter Four by Louise Mary Nolan and Wallace Tucker

Improving Teaching and Learning Using assessment in Middle School Science: Force and Motion by James J. Gallagher