Explain that what they are about to witness is a magical can with a mystery force inside of it. First show the can as being still and unable to move. Then roll the can away from you. As the can stops rolling, say, "Come back can!" After the can reaches you, say, "Good job, can!" as you pick it up.

You can explain it if you choose to, let them figure it out, or let it continue to be a mystery! (A rubber band, attached to a weight inside the can, stores up energy as it is twisted. The kinetic energy of the can's motion is changed into potential energy. When the rubber band unwinds, the potential energy is changed back into kinetic energy. The rubber band stores energy as the can rolls away and then releases that energy by returning.)

Have several students try the Come-Back Can as part of your demonstration.

Have a student volunteer come up. Ask if it is possible to lift a book into the air without touching it. Brainstorm: Let students come up with a variety of answers, discuss, compare and contrast.

Uncover lever with fulcrum and students will probably get the answer to the question. Place the book on one side of the teeter-totter at the far end. Have the student push on the opposite end and it will put the book into the air.

Talk about the force that was exerted to get the book airborne.

Re-adjust with the end to be lifted further from the fulcrum. Is it easier or harder to raise the book into the air? (Harder) Re-adjust the fulcrum again with the end to be lifted closer to the fulcrum. Ask: Is that easier or harder than last try? (Easier)

Ask: Does anyone know the name of something that uses a fulcrum as in the teeter-totter? (Lever) Simple machines that use a lever can make work easier for us. Show the lever with a fulcrum. There are other objects that help make work easier too, such as a pulley.

Single pulley

This demonstration uses pulleys to lift an object. Each object has the same mass, but using the pulleys we can change how "heavy" the object feels.

Find the system with only one pulley.

Lift the weight with your hand and get a sense for its heft.

Now lift the weight by pulling down on the end of the rope that rides over the pulley. The heft of the weight in these two cases should feel about the same.

The advantage of using the machine in this case is that rather than having to bend over and lift the weight you can simply pull down on the rope to lift the weight. Imagine trying to lift a 100 kg hay bale up to the second story of a barn using your legs and back to carry it up the stairs. Certainly it would be much easier to just haul it up on a rope using a pulley.

The Mechanical Advantage of a pulley system is approximately equal to the number lines that are attached directly to the load or attached to the pulley that is attached to the load. These lines are known as "support ropes". The number of support ropes is about the same as the mechanical advantage of the system.

Now consider the two pulley system. This set up contains what is known as two support ropes. When you lift the mass by pulling on the rope in this case, the weight feels much lighter than on the single pulley system.

By using two support ropes this system has a mechanical advantage of two.

This means that whatever force you apply to the rope gets multiplied by 2 in the machine. Note that the mass of this system "feels" about half as heavy as the single pulley system. In this case the weight stays constant, but your effort force is cut in half.

If we had a fixed effort force, we could use the pulley system to double the amount of weight we could lift without changing the "feel" of how much force we apply to lift the weight. For example, if you weigh 50 kg you could lift a mass of 100 kg by using a pulley with two support ropes (50 x 2 = 100).

Using this machine to double your lifting ability does have a drawback. When using machines nothing is free, there is always a price to pay. The price in this case (a 2x mechanical advantage) is that you must move your end of the rope twice as far as the object being moved. For example, lets say our 50 kg person wants to lift the 100 kg object to a height of 5 meters. The machine with a mechanical advantage of 2x will multiply the 50 kg effort force by 2 and allow the person to lift the 100 kg mass, but the person will have to pull their end of the rope 20 meters in order to lift the object 10 meters.

Predict how the three pulley system will work. Ask students for their thoughts. Can the class come to a consensus? Have a student demonstrate the three pulley system and describe the experience as compared to the one and two pulley system.

If we adjust the fulcrum so that the effort arm is 6 feet long and the resistance arm is 3 feet long, the mechanical advantage is now 6/3, or 2x. This means that whatever force we place on the effort arm will be multiplied by two on the resistance arm. For example, if a student weighs 30 Kg, she would be able to move 60 Kg on the other end of the lever. Once again, there is a price to pay for being able to lift twice your weight; that price is distance. If our student wants to lift the 60 Kg object 50 cm off the ground, she has to move her end of the lever 100 cm (two-times as far). If our student wanted to lift a 50 Kg object 30 cm high, how could she use a lever to do it? What will the lengths of the effort and resistance arms have to be?

Take a few minutes and have students observe your classroom looking for simple machines that are part of your room. Make a list of these simple machines found, post them on the board along with a simple drawing of the machine.

Discuss how your classroom might function, or not function, if these simple machines did not exist. Ask students how other simple machines could be used in your classroom to make the room more functional. Again, list ideas on board with simple drawings.

Now we will get some hands-on experience with levers and other simple machines. Explain game rules and lever activity before dividing class into two groups. Group #1 will play Machine Muncher, Group #2 will divide into sub-groups of 3 and view the sample simple machines in the unit.

1. This game is to be played in pairs.
2. It works best to play it on the carpet.
3. Make a lever using tongue depressor and fulcrum.
4. Place lever in front of container.
5. Students take turns trying to catapult chips into container.
6. Each person gets one chance at catapulting the #5 and the #10 chip on a turn.
7. Each one keeps track of his/her points.
8. One chip has a #5 for 5 points and the other one has a #10 for 10 points.
9. If a student misses the container with a chip, no points are given.
10. Game continues until a student reaches 50 points or highest number of points when time is over.
11. Students can experiment by changing fulcrum position with lever to see which position works easiest.

Your task as a group is to design on paper a machine, using as many simple machines as needed, to complete ONE of the following tasks:

1. Shoot a basketball into the hoop without touching the ball with any part of your body.
2. Dig a tunnel in a sand hill that measures 4 feet across large enough to put a "matchbox" car through without directly touching the sand.
3. Clear off dirty dishes from the kitchen table and put them in the sink without touching any part of the dishes with your body. The table is permanently located 10 feet from the sink.

You will not actually build the machine but will draw it on paper clearly labeling all parts and telling what the machine does and how the machine works.