The Science Notebook
Simple Machines - Part 1

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SIMPLE MACHINES

THE LEVER 
A Simple Lever - The See Saw
Making a Rubber Band Force Indicator
Making a Laboratory Lever
First Class Lever
Mechanical Advantage
First Class Levers and First Class Math
Second Class Lever
Third Class Lever
 
THE WHEEL AND AXLE
Using Two Wheels and Axle to Gain a Mechanical Advantage
Wheel and Axle in a Bicycle
Examples of the Wheel and Axle

SIMPLE MACHINES

In our everyday life, we usually think of work as something we have to do.  We have to read tomorrow’s social studies assignment, do some math problems, or mow the lawn. If it is something we have to do, or something that keeps us from doing what we want to do, we call it “work”.  

But to the scientist, work means using energy to move an object from one place to another. When you pick up a book, you do work.  When you ride a bike from your house to visit a friend, you use the bike to do work. When you play baseball, you are doing work as well.  If you are moving yourself or anything else, you are doing work.

By this definition, we do lots of work every day, and much of that work is made easier by the use of machines.  When we think of machines, we usually think of large pieces of equipment which perform specific tasks like washing machines or lawn mowers. But if you look closely at most any complex machine, you will find that it uses one or more of six simple machines, along with something that provides energy to operate the machine.

These six machines are the lever, the wheel and axle, the pulley, the inclined plane, the screw, and the wedge.  Each of these machines is capable of making work much easier to do, and when each is used by itself, or when one or more are combined together, they perform many of the tasks that we take for granted every day.

In this chapter, we will explore the six types of simple machines, and we will see how each makes work easier.  We will also see that while machines make the work easier to do, they really don’t decrease the amount of work required.


THE LEVER 

A lever is a very simple device that consists of a rigid length of wood, metal or other solid material which pivots on a point called the fulcrum.  At some point on one side of the fulcrum, there is a weight or load to be moved which is called the resistance. At another point on the lever, a force is applied to move that weigh called the effort. 

There are three types, or classes, of lever - first class, second class, and third class. The class of the lever is determined by where the fulcrum is located in relation to the resistance and the effort.


A Simple First Class Lever - The See Saw

The see saw is a simple first class lever.  The fulcrum of this lever is the pivot point at the center of the see saw.  In this experiment, the resistance will be the weight of the person you will try to lift.  The effort will be the force you apply to lift that person.  This is a first class lever because the resistance is on one side of the fulcrum, and the effort is on the other side.

Materials Needed: A playground see saw; a friend who weighs about the same as you.

Procedure:  If the see saw can be moved, make sure that it is centered.  Have your friend to sit on one end of the see saw while you push down on the other end.  When you push your end all the way to the ground, notice the distance from the part of the see saw under your friend to the ground.

Carefully lower your friend and have him or her to move half way between the end of the see saw and the middle.  Now push down on your end to lift your friend.  Do you have to push easier, harder, or about the same, to lift your friend? Observe how far he or she is from the ground directly underneath.

Now, have your friend to return to the end of the see saw.  Push down at a point half way between your end and the middle of the see saw to lift your friend.  This time, do you have to push easier, harder, or about the same?

What Happened: The first time you lifted your friend, you were able to lift him or her by applying a force that was roughly equal to his or her weight.  It was probably fairly hard to do (unless you cheated and put your weight on the see saw...). 

When your friend moved closer to the fulcrum, or pivot point, you probably found your friend much easier to lift.  However, if you observed carefully, you should have noticed that you did not lift your friend as high as before, because the lever was closer to the ground at the point where your friend was sitting when you pushed your end all the way down.  You didn’t have to apply as much effort as before, but you weren’t able move your friend as far, either.

When you moved closer to the fulcrum, you probably noticed that it was much harder to lift your friend from that point, but you didn’t have to move the see saw as far as you did when you were at the end.  In this case, you had to apply much more effort to lift your friend, but you didn’t have to apply that effort through as great a distance.

One advantage of a lever is that it allows you to move much more weight than you could otherwise move without the lever.  But can we measure just how much effort is saved, and what, if anything, we have to give up to save it?  We can, but before we do, we are going to have to make a couple of simple pieces of equipment.


Making a Rubber Band Force Indicator


When we study simple machines, we need to be able to weigh a given object and to measure how much effort is required to lift it.  To do this, we will need some sort of force indicator such as the one shown below, or a spring scale.  You may be able to borrow a spring scale from your school science lab if one is available.  If not, you can make this one to use in your study of simple machines.  The instructions given here will allow you to make one that will allow you to see whether a particular force is greater or less than 100 grams.  By changing the size of the rubber band, you can make one that will allow you to observe larger or smaller forces.

Materials Needed: Old Bic® or similar pen with a clear barrel (It does not have to be able to write); pliers; scissors; rubber band; string; tape; paper clip; permanent marker; 35 mm film canister or similar small container; homemade balance or other scale (see procedure); sand or powdered iron.

Procedure: Begin by placing the film can and lid on the balance or scale.  Add sand or powdered iron to the can until the total weight of the can, lid and iron or sand is 100 grams.  Label the can “100 g” and set it aside.  (You can use the homemade balance, so long as you have 100 grams of weights. If you didn’t make one, you can use a triple beam balance at school with the help of a teacher, or a small home “diet” scale that will weigh 100 grams.)

You may need to get an adult to help you with this step. Using a pair of pliers, pull the pen point and ink tube from the barrel of the pen.  Throw it away.  Then, remove the plastic piece from the top of barrel and save it.

Cut a rubber band to make a long strip of rubber.  Tie a 30 cm (12 in) string to one end and place a small piece of tape around the knot.  Trim any excess string and rubber band sticking out from the tape.  Run the string and the rubber band through the top of the barrel (string first) until the rubber band is about one fourth of the way down the barrel.  Place the plastic piece from the top back in the barrel to hold the rubber band in place.  Bend a paper clip into an “S” shape and tie it to the other end of the string.

Next, tie a small loop of string and place part of the loop inside the film can.  Snap the lid into place.  Lift the barrel of the pen up and hang the 100 g weight you have just made onto the paper clip.  Hold the pen so that the film can weight is suspended underneath. You want the rubber band to stretch to about halfway to the bottom of the pen barrel.  If it stretches farther than that, take the plastic top off of the barrel, and adjust the rubber band so that it goes half way to the bottom.  If the rubber band doesn’t stretch that far, you may need to try a thinner or longer rubber band.  If the rubber band stretches too far, then you may need to try a shorter or thicker rubber band. You may have to try several before you find one that works.

You will need to calibrate your force indicator by making a mark on the pen barrel with the permanent marker beside the end of the rubber band when the 100 gram weight is suspended from the scale.

Your indicator is now ready to use for some experiments with simple machines.

Going Further: As you can see, this rubber band force indicator is actually a very simple scale which will measure 100 grams when it is even with the mark.  If an object suspended from the pen stretches below that mark, it weighs more than 100 grams, and if the rubber band does not stretch that far, the weight is less than 100 grams. 

If you hold the barrel of the pen and pull down on the paper clip, you can see that this may also be used to measure force.  If less than 100 grams of force is required, then the end of the rubber band will be somewhere above the mark on the barrel.  If you pull on the clip with more than 100 grams of force, then the end of the rubber band will go below the 100 gram mark.


Making a Laboratory Lever



This next experiment will show you how to build a lever that you can use to investigate the properties of all three classes of levers.  If you have built the homemade balance, you already have most of the work done, since the base of the balance is basically the same as the lever support you make here, and can be used in its place.

Materials Needed:  Two small boards about 30 cm (12 in) long and 7-10 cm (3-4 in) wide. (Exact sizes aren’t too important.  You can probably use whatever you have on hand.); three small nails; ruler, meter stick or yard stick; three small binder clips; two paper clips; meter stick; pen or pencil.

Procedure:   Nail one of the boards to the other as shown below.  Use two nails so that the boards will not slip.  Nail the third nail about an inch or so from the top of the upright board.  This will be used for the lever support. 

Clip one of the binder clips on the center of your ruler or measuring stick.  Clip the other two clips at each end opposite to the middle clip.  Now hang the wire handles of the middle clip on the nail in the upright board.  If this one doesn’t exactly balance with the clip in the middle, adjust the middle clip to balance the two sides.

Bend the two paper clips to make “S” shaped hangers.  These will be used later.

Your lever is now ready to use for the following experiments.


First Class Lever



You should remember that a first class lever is one that has the fulcrum between the weight and the effort.  The see saw is one practical example of a first class lever.  Two others are a crow bar and the claw end of a hammer used to remove nails.

Up to this point, we have seen how a first class lever works to make loads easier to lift.  With a spring scale or the force indicator, we can measure just how much easier.

Materials Needed: 100 gram weight; laboratory lever; homemade force indicator or spring scale (may be borrowed from school).

Procedure: You should have binder clips on the lever at each end.  The clip that is your fulcrum should be balanced on the support.  Hang the 100 g weight from one of the binder clips (the resistance clip).  Hook your homemade force indicator or spring scale to the other clip (the effort clip) and pull downward to lift the weight.  If you are using the force indicator, where is the rubber band?  If you are using a spring scale, what is the reading on your scale?

Next, move the resistance clip about half way from the end to the fulcrum.  Again, pull down on the effort clip with the scale.  Is more or less force required?  If you are using a spring scale, can you tell how much force is required now?

Now return the resistance clip to the end and move the effort clip so that it is halfway from the end to the fulcrum.  Again pull down with the force indicator or scale.  How much force is required now?

What Happened: You already know from the see saw experiment that the fulcrum is between the resistance and the effort in a first class lever.  There, you saw that as your friend moved close to the fulcrum or center, it was easier for you to lift him or her (required less effort), but you also saw that the closer your friend moved to the center, the less high he or she was lifted.

You should have seen the same thing here.  When the effort and the resistance were the same distance from the end, the effort should have equaled about 100 g. As you moved the weight closer to the center, the effort required was less than 100 g.  However, when you moved the effort end closer to the fulcrum, more than 100 g of effort was required.

Going Further: If you are using a spring scale, try changing the distance between the fulcrum, resistance and effort.  Use your spring scale to determine exactly how much effort is required at the different distances.  Also, does the weight of the lever itself make a difference when you move the fulcrum?  Why or why not? For the time being, make sure that the resistance and effort are on opposite sides so that you continue to have a first class lever.


Mechanical Advantage

When simple a machine, such as a lever, takes a small force (or effort) and increases the effectiveness of that force, a mechanical advantage has been produced. For example, a lever may require a force of only 50 g to lift 100 g.  If so, it has a mechanical advantage of 2, because it doubles the effectiveness of the 50 g effort.  While the rubber band force indicator may give you some idea of how much the mechanical advantage increases, a laboratory spring scale borrowed from school will help you to see this much better.

Materials Needed: Laboratory lever; rock or other small weight; laboratory spring scale.

Procedure: Weigh the rock (or other weight) and record the weight. 

Set up your lever so that the fulcrum is in the middle, and the weight and effort are at opposite ends an equal distance from the fulcrum.  Use the spring scale to measure the effort needed to lift the weight. It should be about equal to the weight.  You can use the following formula to calculate mechanical advantage:


                                           Resistance (g)
Mechanical Advantage =   --------------------
                                          Effort (g)


Suppose your weight is 200 g and the spring scale reads 200 g when you pull down. Then,



                                       Resistance (g)      200
Mechanical Advantage        =   --------------------   =  -----  = 1
                                           Effort (g)           200


Now move your weight half way to the fulcrum ad measure the effort needed to lift it.  Suppose the effort is now measured at 100 g.  Now,

                                       Resistance (g)      200
Mechanical Advantage        =   -------------------    =  ------  =  2
                                          Effort (g)            100   


Depending on you measurements, the mechanical advantage of your lever will be different, but your answer should be greater than 1.  Calculate the mechanical advantage using your numbers.

Now return the weight to where it was and move your effort halfway to the fulcrum.  Measure the effort needed to lift the weight now.  Suppose it is now 200 g.  (Again, it probably won’t be exactly this, but it should now be more than the weight.)  Now,

                                       Resistance (g)            100       1
Mechanical Advantage =   -------------------   =   ------ =  ---
                                               Effort (g)               200       2   


Calculate the new mechanical advantage using your numbers.  In the example above the mechanical advantage is 1/2. Your answer may vary, but it should now be less than 1.

What Happened: A mechanical advantage of 1 means that the load is no easier or harder to lift with the lever than without it.  If the mechanical advantage is more than 1, the lever (or other machine) will make a load easier to lift.  However, it the mechanical advantage is less than 1, the load is harder to lift than it would be without the lever.

Going Further: If you understand how to calculate mechanical advantage, try changing the distances of the fulcrum, resistance and effort, and calculate the mechanical advantage.


First Class Levers and First Class Math

There is one more bit of math we need to know about levers, but it isn’t hard at all.  In any lever, the distance of the resistance (or weight) from the fulcrum times the resistance will always equal the distance of the effort from the fulcrum times the effort. This can be written:

Distance x Resistance = Distance x Effort

Let’s see how this works.

Materials Needed: Setup from the last experiment.

Procedure: Repeat the last experiment, first with the resistance (weight) and the effort the same distance from the fulcrum, then with the weight halfway from the end to the fulcrum, and finally, with the effort halfway from the end to the fulcrum.

At each position, measure the distances from the fulcrum to the resistance and to the effort.  You can use either cm or in.

Using these numbers and the formula above, see if the numbers really are equal.  For example, suppose the weight is 100 grams and is 10 cm from the fulcrum, while the effort is 25 cm from the fulcrum, and is 40 g.

Distance x Resistance = Distance x Effort
10 x 100            =          25 x 40   
   1000               =           1000      


What Happened: Your numbers may not be exactly equal, but they should be fairly close.  The reason they may not be exactly the same include errors in measuring the length from the fulcrum to the effort and resistance, and errors in measuring the weight and effort (due to the accuracy of the scales).  However, the numbers may also not be exactly the same because of something else that has to be considered that may be difficult for you to measure. The weight that is being lifted includes not only the weight itself, but the weight of the resistance arm, as well.  If that is difficult to imagine, think of how your lever would be different if the lever were made of a piece of iron instead of wood or plastic.  Then the lever arm would probably weigh much more than the 100 gram weight, and it would definitely affect your measurements.


Second Class Lever



A second class lever is one that has the resistance (weight) between the fulcrum and the effort.  An example of a second class lever is the wheelbarrow.  The fulcrum is the wheel, the effort point is at the handle, and the resistance or weight is between the two. If you have ever used a wheelbarrow, you know that it makes a load much easier to lift. Let’s see why.

Materials Needed: Laboratory lever; spring scale; 100 gram weight.

Procedure: Set up the lever with clips on each end of the lever and the third clip in the middle.  This time, the two end clips should be on the same side, as shown. 

Place one end on the nail.  This will be the fulcrum.  Hang the 100 g weight from the middle, and use your spring scale to lift the other end.  How much effort is required to lift the weight?

What Happened: In this second class lever, the effort required to lift the weight was less than the weight, but notice that you had to lift the end of the lever much higher than the weight was lifted.


Third Class Lever


A third class lever is one that has the effort between the fulcrum and the resistance (weight).  An example of a third class lever is your forearm.  The fulcrum is your elbow, the effort point is where your muscle attaches to the bone of your forearm, and the resistance or weight is at the end in your hand.  As you will see, there is no mechanical advantage gained with this type of lever, since the mechanical advantage will always be less than one. However, this type of lever still has some practical uses.

Materials Needed: Laboratory lever; spring scale or homemade force indicator; 100 gram weight.

Procedure: Set up the lever as in the previous experiment with two clips on each end of the lever and the third clip in the middle as shown. 

Place one end on the nail.  This will be the fulcrum.  Hang the 100 g weight from the other end, and use your spring scale to lift the middle.  How much effort is required to lift the weight?

What Happened: In this third class lever, the effort required to lift the weight was greater than the weight itself.  However, you should have seen that the weight moved a greater distance than the effort.   There is no mechanical advantage here, since greater effort is required to lift the weight, but if the force is available to do the work, then the weight may be moved a much greater distance.  This is the principle behind many large cranes. 


THE WHEEL AND AXLE

You may already know that wheels can reduce friction, but did you know they can be used as simple machines as well?  It is possible to connect two wheels together by a belt, a chain, or by gears to either gain a mechanical advantage, or to else to gain additional speed.  Such wheel and axle combinations can be seen in devices such as bicycles, mechanical clocks and watches, and some parts of a car engine, just to name a few.


Using Two Wheels and Axle to Gain a Mechanical Advantage


Materials Needed: Corrugated cardboard; compass; scissors; tape; pencil with an eraser; straight pin; string; 100 g weight; force indicator or spring scale.

CAUTION!  Always use sharp objects such as knives or scissors with adult supervision only!  Hold any sharp point away from your body, particularly your eyes.

Procedure: Draw a circle with a 15 cm (6 in) radius and another with a 8 cm (3 in) radius on the cardboard.  Cut out the two circles.  Using the sides of the scissors, go around the edge of each circle and press down on the middle layer of cardboard. You want to make a thin groove all the way around each circle.

Line up the centers of the two circles and tape them together. Carefully punch a hole in the center of the circles just large enough for the pencil to go through.  Push the pencil through the hole.  The two circles should be able to turn easily on the pencil, but they should not be too loose.

Tape a 1 meter piece of string to the edge of the large circle and wrap three or four turns around it.  Leave about 20 cm free.  Tape another piece of string to the edge of the small circle. Wind three or four turns around the small circle in the opposite direction. 

Tape the pencil to the edge of a table with the circles hanging over the edge. 

Attach the 100 g weight to the end of the string on the small circle, and the force indicator or spring scale to the end of the string from the large circle.   Pull down on the string from the large circle and note how much effort is required to lift the weight.

Next, attach the weight to the string on the larger circle and the scale or force indicator to the string on the smaller one.  Again, lift the weight and see how much force is required now. 

What Happened: When you first attempted to lift the weight, the effort required was less than 100 g.  The mechanical advantage was greater than 1.  However, when you reversed the positions, more than 100 g of effort was required to lift the weight, and the mechanical advantage was less than 1.   Can you figure out why this was so?


Wheel and Axle in a Bicycle

Materials Needed: A bicycle; tape.

Procedure: Turn the bike upside down.  Place a small piece of tape on the back tire so that you will be able to count the turns of the wheel.  (If you are using clear tape, tape a small piece of paper to the tire.)

Note the size of the sprocket wheel and axle connected to the pedals and the one connected to the back wheel.  If you are using a multiple speed bike, notice the size of the sprocket wheels the chain is on when you do the experiment.

Now, slowly turn the pedal one complete turn.  As you do, count the number of turns the back wheel makes.

If the bike has multiple speeds, go for a ride.  Start with the lowest gear.  Notice the position of the sprocket wheels in that gear.  As you ride, shift into higher gears and observe the sizes of the two sprockets.  Also notice how easy or hard it is to pedal in each gear.

What Happened: Regardless of gear, one turn of the pedal produces several turns of the wheel.  The higher the gear, the greater the difference in size between the front and rear sprocket wheels and the more turns of the wheel for each turn of the pedals.  However, with the higher gears, more effort is required to pedal.

In a bicycle, the wheel and axle provide a mechanical advantage.  Remember from the last experiment that the mechanical advantage was gained when the effort was on the larger wheel and the resistance was on the smaller one.  This advantage is provided in distance moved. You can tell this as you shift into a lower gear because, although it is easier to pedal, you don’t move as far for each turn of the pedal.


Examples of the Wheel and Axle

Materials Needed: A car; an adult friend.

Procedure: With the help of an adult who is familiar with car engines, look under the hood of a car.  See if you can identify one or more wheel and axle combinations connected by rubber belts.  Ask the adult to explain what each one does.

What Happened: You should have seen several examples.  Among them would be the alternator.  This device uses power (effort) from the engine to turn a shaft in the alternator to generate electricity to charge the car battery. The turn of the engine will  turn of the alternator by means of the connecting belt.


There's lots more to learn about machines in Part 2 of Simple Machines!


"The Science Notebook"  Copyright 2008-2014 - Norman Young