A wheel and axle may be used as a pulley. When two or more pulleys are combined, a mechanical advantage is gained.
In this experiment, and in several that follow, you
will use sewing thread spools as simple pulleys.
Materials Needed: Empty sewing thread spool; round pencil; wire or heavy string; 2 meter strip of wide ribbon (See procedure); 2 paper clips; 100 g weight; spring scale or force indicator; meter or yard stick; a friend.
Procedure: If you don’t have an empty spool, you can borrow a spool with thread already on it, since the thread will not be damaged. You should use a small piece of tape to tape the end of the thread down so that it won’t unwind.
You will need a spool with a hole large enough to allow it to turn freely when a round pencil is fitted through the hole. Run the pencil through the hole and tie a piece of wire or heavy string on each end of the pencil to make a loop as shown. If you can’t find a pencil small enough to allow the spool to turn freely, use a large nail.
If you can locate a large used gift bow, carefully remove the staple or plastic piece from the back. When you unwrap the bow, you should have a single long piece of wide ribbon. Two or more may be taped together as needed to make a 2 meter strip. Otherwise, you can use a piece of new ribbon. Regardless of which you use, the ribbon should be at least half as wide as the spool just to keep everything stable. Place a small piece of tape on both ends to reinforce the ribbon and keep it from fraying.
Bend two paper clips into an “S” shape. Punch a small hole in the tape at each end of the ribbon. Hook an “S” shaped clip in each hole. Hang the weight on one end of the ribbon by the paper clip, and run the ribbon up over the spool. Have your friend to hold the pencil and spool while you attach the spring scale or force indicator to the other paper clip.
Lift the weight and note how much effort is required to lift it. How far do you have to pull the string down to raise the weight 15 cm?
What Happened: The spool, pencil and string (or wire) combine to make a simple pulley. When you lifted the weight, you should have seen that the effort required to lift the weight was about the same as the weight. There is no mechanical advantage gained by using a single pulley. You just reversed the direction of the effort used to lift the weight. To lift the weight 15 cm, you had to pull the string down 15 cm. How far you have to pull the string becomes important in determining whether a mechanical advantage is gained, as we will see.
Needed: Spool; round pencil; ribbon (from the last
experiment); wire or heavy string; paper clip; tape; 100
g weight; force indicator or spring scale; meter
or yard stick; ruler; a friend.
Procedure: Make a pulley using the spool, pencil, and wire or string, just as you did in the last experiment. However, this time, secure the weight to the loop in the pulley by using a paper clip bent into an “S” shape.
Securely tape one end of the ribbon to the edge of a table. Place a piece of tape on the other end of the strip to reinforce it, and punch a small hole through the tape and ribbon.
Run the ribbon under the spool pulley and back up as shown. Then, hook your spring scale or force indicator through the hole on the free end of the ribbon.
Raise the pulley and weight and notice how much effort is required to lift them.
Next, move the free end of the ribbon so that it is even with the tabletop. Have your friend to hold the measuring stick beside the weight and record the height off the floor. Lift the weight 15 cm (6 in) above where it was. When it has been raised, measure how much of the ribbon is above the tabletop.
What Happened: You saw that less force was required to lift the weight when the pulley was set up in this manner than was needed in the last experiment. If the system were perfect, it would take exactly half the force. However, the weight of the pulley has to be taken into account, as does friction, so you actually may have observed that it took a little more than half the force of the fixed pulley. A mechanical advantage of about 2 is gained because the part of the ribbon on each side of the pulley supports half of the load of the weight. In the fixed pulley, all the weight was supported by the side from which the weight was suspended. The only thing that the ribbon on the other side did was to change the direction of the force, and no mechanical advantage was gained.
But there is a trade off. You also observed that to raise the weight 15 cm, you actually had to move 30 cm of ribbon through the pulley.
Needed: 2 spools; 2 round pencils; ribbon strip;
wire or heavy string; paper clip; tape; 100 g weight; force indicator or spring scale; meter
or yard stick; ruler; a friend.
Procedure: Assemble two pulleys from the spools, pencils and wire or string, as you did in the previous experiments. Fasten the weight to the loop of one of the pulleys. Also, prepare a 2 meter strip of ribbon as you did in the last experiment.
Refer to the illustration. Tape one end of the ribbon to the pencil of the upper pulley as shown. Next, while your friend holds this pulley, run the strip under the bottom pulley with the weight, and back up over the top pulley.
Hook the scale to the free end of the ribbon and pull. Notice how much force is required to lift the weight. (NOTE: Using two spool pulleys is harder than it looks, so you may have to work at this a bit to get it to work just right.)
Move the free end of the ribbon so that it is even with the top of the top pulley. Have your friend to measure the height of the weight above the top of the floor. Raise the weight 15 cm. As you do, measure the length of the ribbon that is required to be moved to raise the weight 15 cm.
What Happened: Using these two pulleys to lift the 100 g weight required about 50 g of force. As before, it was probably a little more, due to friction and the weight of the pulley. The mechanical advantage is roughly double that of the fixed pulley, but about the same as that of the moveable pulley.
You should have also seen that you had to move about 30 cm of ribbon to lift the weight 15 cm, just as with the single moveable pulley.
When the pulleys are arranged in this way, there is no advantage to using two pulleys over one moveable one. If you examine the setup carefully, you will see that only two of the three parts of the string are supporting the weight. The portion taped to the pencil, does not support any of the weight. In the next experiment we will use all three strings to support the weight to gain a greater mechanical advantage.
Needed: Same as the last experiment.
Procedure: Set up the two pulleys as shown. This time, you will connect the ribbon to the bottom pulley instead of the top.
Again, raise the weight and measure the amount of force required to lift the weight. Also, measure how much ribbon has to be moved to lift the weight 15 cm.
What Happened: This time, all three parts of the ribbon are supporting the weight. It takes about 33 grams of force to lift the weight, but you must move about 45 cm of ribbon to lift the weight 15 cm. The mechanical advantage is about 3.
Going Further: Looking at these experiments, you may have noticed that the mechanical advantage is related to the number of parts of the ribbon in a pulley system that actually support the weight. Ignoring friction and the weight of the pulley attached to the weight, the mechanical advantage is equal to the number of parts that bear the load.
You can also determine mechanical advantage for the pulley just as for a lever, by dividing the weight lifted by the effort required to lift it.
A third way to determine the mechanical advantage is to divide the distance the ribbon has to be moved by the distance the weight moves. In this experiment, the ribbon had to be moved 45 cm to move the weight 15 cm.
When pulleys are combined to produce a mechanical
advantage, the combination is called a block and tackle.
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.
Materials Needed: Four spools; two round pencils; two coat hangers; wire cutters; ribbon; small pail; spring scale or force indicator.
Procedure: Place two spools on each pencil. Cut the two coat hangers, and bend the ends around the pencils as shown.
Secure one end of the ribbon to one of the pencils and run the ribbon over and under the pulleys as shown. Hang the top pulley over a low tree limb, and hang a small pail on the lower one.
Place a heavy object in the pail and lift it using the block and tackle. Can you tell the difference?
Refer to “Going Further” in the last experiment. Use the spring scale to determine the mechanical advantage. Also find the mechanical advantage by the number of load bearing sections and by the length of cord required to lift the weight a certain height.
Spool pulleys will help you understand how pulleys work, but they can be a little awkward to use, since you have to use a strip of ribbon with them. If you want better pulleys for your experiments, there are a couple of ways to make them. Both require a little extra effort, but either one could be used to provide pulleys or a block and tackle for a science project.
Always use sharp objects such as knives or scissors with
adult supervision only! Hold any sharp point away from
your body, particularly your eyes.
Materials Needed: Heavy corrugated cardboard; compass; scissors; nail; strong wood or paper glue; small nuts and bolts (see procedure); wire cutter; pliers; coat hanger.
Procedure: For each pulley you want to make, draw and cut out 2 circles with a 4 cm (1.5 in) radius. Draw and cut a third circle that is a little smaller than the first two. The sizes are not critical, but they should be the same for every pulley you make.
Use the nail to punch a hole through the center of each circle. Place all three circles on the nail with the smaller one in the middle. Glue the smaller disk between the two larger ones. Allow the pulley to dry and remove from the nail.
Cut a section of clothes hanger wire about 20 cm (8 in) long and bend it as shown. If you are going to put two or more pulleys on the same bolt, you will need to cut this a little longer.
The size of the bolt and nut you
use will depend on how many pulleys you want on each
axle. Select a bolt and nut that is about twice as long
as the total thickness of all the cardboard pulleys.
Push one end of the wire onto the bolt. Push the
cardboard pulley through the bolt as shown. You may need
to work the cardboard pulley a little to get it to spin freely
on the bolt. If you are using more than one pulley, place the
others on the same way. Finally, place the other loop on
the bolt and place the nut on the bolt. Tighten the nut,
but not so much that the pulley cannot spin freely.
Going Further: If you want to place two or more pulleys on a bolt to make a block and tackle, you will need to choose a bolt that is long enough to fit all the pulleys and that has enough extra room for the wire support. Obviously, the wire support will have to be longer, too. You may also want to place a metal washer between each pulley to reduce friction between the individual pulleys.
Cardboard pulleys are fine for lighter weights, but
heavier weights require stronger pulleys. This
activity shows you how to make pulleys from thin plywood or
CAUTION! You should get an adult friend to do the wood cutting. Never use any power tool without the supervision of an adult who knows how to use it safely.
Materials Needed: Piece of thin plywood (less than 1/4 inch) or fiberboard (scraps are fine); electric drill and hole saw set; sandpaper; nail; wood glue; clamps; an adult friend.
Procedure: A hole saw set is used with an electric drill to drill holes to mount door knobs and locks. It is available at most hardware stores. You should be able to find an adult friend that has such a hole saw set and a drill who will help you with this.
For each pulley you want to make, have your friend to cut two holes in the wood the same size, and a third one that is slightly smaller. Each time a hole is cut, there will be a circle of wood inside the bit. Remove and save the circles.
The pulleys made in the lab of The Science Notebook were cut with the two largest saws, the 64 mm (2 ˝ in) and 54 mm (2 1/8 in) diameter saw bits, but you can use most any size you want. The circles will probably be rough around the edges, so you may need to sand the edges smooth.
Each circle will also have a hole in the center. You should find a nail that fits snugly through this hole. Place a large circle, a small circle, and the other large circle on the nail. Glue the three circles together with wood glue and press the circles firmly together. If you can borrow a wood clamp, clamp them.
When the glue dries, remove the nail. By selecting the right size nut and bolt, you can now use this wooden pulley just like the cardboard pulleys in the previous experiment.
An inclined plane is a sloping surface that allows
objects to be raised with less effort than would be required
to lift them straight up. Among the uses for the
inclined plane are ramps, stairs and screws.
Needed: Flat board or piece of thick corrugated
cardboard about 1 meter (3 ft) long; several books; toy car;
force indicator or spring scale; weights (see Procedure);
Procedure: Make an inclined plane by leaning the cardboard or board against a stack of books about 30 cm (1 ft) high.
Find a small toy car. Try to find one rolls easily and, if you are using the homemade force indicator, one that weighs about 100 grams or less. If it weighs less, tape small nails, coins or other weights, or add modeling clay to it until it weighs as close to 100 grams as possible. (HINT: You can use your force indicator to weigh it!) If you have a laboratory spring scale, use any small car that rolls easily and that can be weighed accurately.
Measure the length and height of the inclined plane.
Attach the scale to the front of the car and pull the car up the inclined plane. Note how much effort is required to pull the car up the plane as indicated by the force indicator or spring scale.
Lower the inclined plane to 15 cm (6 in) high by taking away some of the books. Again pull the car up the plane and measure the effort required to get it to the top.
What Happened: You should have observed a mechanical advantage, but how much you observed depends on how long and how steep the plane was. The mechanical advantage increased even more when you lowered the plane. In both cases, to lift the car to the top of the plane, you had to move the car the entire length of the plane. The trade off that produced the mechanical advantage was that the total distance you had to move the car was greater than the height you actually raised the car.
Going Further: Can you calculate the mechanical advantage of each inclined plane? Remember:
We usually think of an inclined plane as being
flat, but this experiment show that many useful inclined
planes are actually spirals.
Materials Needed: Paper; ruler; scissors; 2 pencils; tape; large screw.
Procedure: Cut a rectangular piece of paper about 23 cm (9 in) long and 13 cm (6 in) wide. Fold the paper diagonally and cut along the fold to make two triangles. Tape the 13 cm (6 in) side one of the triangles to the edge of the pencil with the 23 cm (9 in) side on the bottom and roll the paper on the pencil. Tape it down when you finish.
Hold up the other triangle so that the 23 cm (9 in) side is on the bottom. You should immediately recognize this as an inclined plane. Measure the length of the inclined plane (the longest side). You should realize that if you were to move something up this inclined plane, you would have to move it the length of the plane.
Now take your pencil. You have just wrapped and taped an inclined plane to the pencil, and it is now spiral shaped. To prove this, take another pencil and place the point at the bottom of the paper spiral. Next, place the point of this pencil on the edge of the paper at the bottom of the screw and trace your way around the spiral.
Finally, take the screw and
place the pencil at the bottom in the groove. While
holding the pencil point in the groove, turn the screw.
You should see the point of the pencil make its way up
What Happened: The pencil was moved to the top of the paper along the edge of an inclined plane. You should have immediately noticed that the paper spiral looks just like the spiral on the screw. It is.
It takes a lot of energy to push a 6 inch nail into a piece of wood using a hammer, but by cutting a spiral inclined plane into metal to make a screw, much less energy is required to push it into the board using a screwdriver. As with all machines, there is a mechanical advantage, but the advantage is at the expense of the distance that the object has to be moved. The screw has to be turned a number of times to get it into the board.
Going Further: With the screw, the inclined plane (the thread) is being forced into the board. But in order for the thread to cut into the board, It also depends on another simple machine that we will study next - the wedge. After doing the next experiment, try screwing a wood screw into a board and watch how this works.
A bolt, which is another inclined plane, works by moving a nut along the inclined plane. Observe what happens as you spin a nut up a bolt.
The last of the simple machines is the wedge.
A wedge is an object that has a sharp edge or point.
Wedges are used to cut or to split. Examples of wedges
are axes, scissors, knives, pins and nails.
Always use sharp objects such as knives or scissors with
adult supervision only! Hold any sharp point away from
your body, particularly your eyes.
Materials Needed: Vegetable knife; apple.
Procedure: Carefully examine the knife blade. Notice that the cutting edge is narrow and sharp, but that the opposite side is wider.
Using the blunt side of the blade, CAREFULLY try to cut into the apple. Now use the sharp side to cut the apple.
What Happened: You were not surprised to see that the thick side of the knife didn’t cut the apple, or if it did, it required a lot of effort. However, the sharp side cut the apple much easier. Why?