Key concepts
Physics
Simple machines
Forces
Energy
Friction
Introduction
Before the industrial age people largely had to rely on muscle power to move and lift heavy objects. Simple machines such as pulleys, levers and ramps made it easier for humans to lift heavy objects like rocks and logs. In this project you will use simple household materials to explore one of these classic machines: the pulley.
Background
Pulleys are simple machines that can make the job of lifting objects easier. They are made from another type of simple machine—the wheel and axle. A simple pulley has one more part added—a rope wrapped around the wheel. One end of the rope can be tied to a load, for example, a bucketful of water at the bottom of a well. You can pull on the other end of the rope and the tension in the rope will lift the load. The advantage of a simple pulley is that it allows you to change the direction of the force required to lift it. For example, to lift the bucket up out of the well, you can pull down on the rope.
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What if you want to change the force required to lift a load? With a simple pulley the force you must exert is equal to the weight of the load. With a compound pulley—two or more pulleys combined together—you can reduce the force required to lift the load. This reduction doesn’t come for free though—due to conservation of energy, it increases the distance over which you must exert a force. (Energy equals force times distance.) For example, in order to reduce the force, you must exert to half the load's weight, then in order to lift the load one meter, you will need to pull the rope two meters.
There’s one more factor you have to consider when using a pulley—friction. An “ideal” pulley—the kind you see in a physics textbook or homework problem—doesn’t account for this. Real-world pulleys will always have some friction, and you need to exert a little extra force to overcome it. In this project you will investigate simple and compound pulleys—and how they change the force required to lift a load.
Materials
Two empty cereal boxes
Two pencils
Four paper clips
At least 10 metal washers or hex nuts of the same size
String
Scissors
Ruler (optional)
Empty spool of thread (optional)
Preparation
Stand the two cereal boxes up parallel to each other on a table or desk.
Carefully poke two holes across from each other toward the top of the cereal boxes (on their inner faces) so you can push a pencil through the holes in a way it will be supported by the boxes. (If necessary, have an adult help with this part.)
Cut a piece of string that is slightly longer than one cereal box is tall.
Tie a paper clip to each end of the string.
Procedure
Hang the string over the pencil with one paper clip dangling off each side. This is your simple pulley.
Hang two washers from one of the paper clips. (Bend the paper clip slightly so you can easily use it as a hook.) These washers are your “load.” Let them fall down to the tabletop.
One at a time, start hanging washers from the other paper clip. Place the washers onto the paper clip carefully, do not drop them. These washers are your “effort” force. How many washers do you have to hang on the paper clip until the load starts to lift up off the table?
Next you will build a compound pulley.
Poke two more holes (one in each box) so you can insert a second pencil parallel to the first one; they should be a few centimeters apart.
Cut a new piece of string, roughly twice as long as one box is tall.
Tie one end of the string to the second pencil, and drape the other end over the first pencil.
Hang a paper clip from the part of the string between the two pencils so it dangles down between them. Hang two washers from this paper clip and let them fall down to the ground. This will be your load.
Tie the free end of the string to another paper clip. This paper clip should be up off the ground, close to the pencils. If not, you might need to shorten your string.
One at a time, start hanging washers from the second paper clip. How many washers do you have to hang on the paper clip until the load starts to lift up off the table?
Compare the results for the two different types of pulleys. There should be enough space to set both types of pulleys up at once. If you have enough washers, you can do the tests side by side. Does it take more or fewer washers to lift the same load using a compound pulley compared with a simple pulley? Why?
Extra: There is a lot of friction in these pulleys because the string rubs directly against the pencil. (There is no true “wheel” that can rotate.) Can you try to reduce friction in your pulley? For example, try using an empty spool of thread as a wheel. How does this change your results?
Extra: Use a ruler to measure the distance traveled by the load and the effort for each type of pulley. How far must you move the effort force in order to lift the load 10 centimeters with a simple pulley? What about with a compound one?
Observations and results
You should find that it takes fewer washers to lift an equivalent load using a compound pulley compared with a simple pulley. For example, ideally with a simple pulley you would be able to lift two washers using two other washers. (The effort force would be equal to the load force.) In reality, there is some friction between the string and the pencil that you must overcome, so it might take three or four washers. (The effort force is greater than the load force.) With the compound pulley, ideally you would be able to lift two washers using only one other washer. Again, there is some friction to overcome, so it might take two or three washers. But remember, this decrease in force required is not “free”—you have to move the effort force twice as far. You can confirm this if you measure the distances with a ruler.
More to explore Pulleys, from ExplainThatStuff!
Give Yourself a Lift: Lightening the Load with Pulleys, from Science Buddies
Heavy Lifting with a Lever, from Scientific American Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
