Level: Elementary to H.S.
Time involvement: 10 to 15 minutes.
Materials/Procedure: Two, four inch suction cups, smooth top medium size table or school desk, and two adults strong enough to lift 50 pounds each. Suction cups can lift many pounds of weight due to air pressure on the outside compared to the almost vacuum pressure inside. The table lift demonstration requires a strong table with a smooth surface. Two adults are required. The weight of the student is limited to about 100 pounds or less. Invite the participating student to climb on the table. Shoes are optional. Try to center the seated student’s weight in the middle of the table.
Aligning the legs with the length of the table will help in the centering. The suction cups are placed near each end. It is essential that not only must the table surface be smooth; it must be free of scratches and dirt. If necessary the surfaces may have to be cleaned first. Explain to the student and observers that the lift will be only to a height of one or two inches. The reason is that there is a possibility of a leak in one or both of the suction cups.
Demonstrate with one suction cup how moving the lever handle will change the shape of the rubber disk. Although a perfect vacuum will not be created a very low pressure can be achieved under the cup. The suction cups should be firmly pressed down and the two lever handles brought together. It unlikely that the cups are perfectly aligned with the student’s weight. This condition will cause some tilting of the table during the lift. To prevent any alarm to the student due to some unsteadiness, use one hand to lift the handles of the suction cups and the other hand to steady the table. A little unsteadiness is not only OK, it will confirm to the student and observers that the table is actually being supported by the cups and not the floor. The lift need be only for a few seconds. When safely earthbound you may open the handles and show that there is no glue or fasteners holding the suction cups to the table.
Discussion: A few words of explanation are appropriate as the student dismounts. You might solicit a few questions. Make clear that although the upward force of the arm muscles are required the essential point of the demonstration is that atmospheric air pressure alone is firmly keeping the cups attached to the table.
Atmospheric pressure is the secret.
A drinking straw does not lift the liquid; a pressure difference caused by our expanding lungs allows air pressure to force the liquid up the straw! It may feel like we are “pulling the drink up”; the reality is that we are assisting the atmosphere to push it up! At sea level air pressure is almost 15 pounds per square inch. A 4-inch diameter “cup” may be able to lift over 100 pounds! If you chose to do a “tug of war” with the cups clear the area in all directions. An unexpected “leak” caused by dirt between the cups will cause a sudden “uncoupling”! Be careful with enthusiastic students. Large suction cups can often be purchased at hardware stores for about $5. Glass and mirror installers use them routinely. Older students like the challenge of attempting to break the suction with their arms. Most fail! A frequent student question is: “Would it be possible to walk across a ceiling using the cup?” This is great discussion starter.
Learn more: Although the above demonstration has the practical purpose of teaching a principle, suction cups and their pneumatic cousins are found everywhere. Suction cups operated with a small tube attached and connected to a vacuum pump with a computer-operated valve are frequent choices of equipment designers. Suction cups can quickly grip odd shaped objects, lift them, and release them without leaving any mark.
Robotic assembly lines such as the automotive industry install windshields easily and quickly using computer controlled suction cups. Granite counter tops with sink cutouts are very fragile. Battery operated vacuum pumps and suction cups are fastened to a long rigid brace that reaches across the weaken area. During transit, and installation, the pump operates, firmly holding the granite and preventing expensive damage. After installation the pump is turned off and the brace removed.
Pneumatic operated devices are a close cousin. Air brakes on railroad cars and trucks have been a important invention that allows brakes to be operated at a distance without the complication of levers, gears or wires. Although hydraulic excavators and related machines use fluids instead of air they operate on a similar principle. Hydraulic jacks use a clever force multiplying principle and can lift an automobile with mere hand pressure.
See http://www.powertransmissionworld.com/not-only-suction-cups/ for a clever window-climbing robot using suction cups.