Shrink Wrap Science
People often think of shrink wrapping as a process by which air is "sucked" out of a bag. Newton never used the word "suction" in any of his writings, but instead he referred to the forces that acted upon an object. This demonstration created by Wayne Goates helps to eliminate this misconception in a most unforgettable way.
- Large plastic bag
- Portable vacuum cleaner
- An assistant
- In this experiment, you’ll need the assistance of another person to help you shrink wrap either your arm or your leg. Remember, one of the two people on your shrink wrap crew must be an adult.
- Plunge your arm all the way into the plastic bag.
- Have your helper feed you the nozzle of the portable vacuum cleaner. Cup your hand around the end of the nozzle so the bag does not get stuck in the hose. In other words, use your hand to shield the bag from getting pushed up into the nozzle.
- Have your assistant hold the bag and the vacuum hose tightly around your arm to form a seal.
- Turn on the vacuum and remove the air from inside the bag. Any leak in the bag will keep it from shrinking tightly around your hand. In just seconds, the bag has molded around your hand and arm… and the feeling is most unusual!
- With your hand shrink wrapped, try to move your fingers. It’s next to impossible. What would Sir Isaac Newton have to say about this?
- Tell your assistant to stop yelling, “Let me try! I want to shrink wrap my arm!” Release the seal and repeat the process… but this time you’ll want to shrink wrap your assistant’s leg.
- Have your friend take off his or her shoe, but leave on the sock. Stick the end of the nozzle down inside your friend’s sock.
- Wrap the plastic bag up and around his or her leg. Gather up the open end and form a tight seal around the leg.
- Turn on the vacuum cleaner and remove the air. The sock will keep the bag from being pushed into the nozzle, but you might need to experiment with the best placement of the nozzle around the foot.
- By this time, four other people are in line to experience the effects of atmospheric pressure on their bodies… and you’ve started a new business.
How Does It Work?
When presenting this demonstration in a science class, students have the misconception that the air is "sucked" out of the bag. This may be a good time to remind students that Newton talked about forces as a "push" or a "pull" and did not refer to any "sucking" motion. Here are a few important things to remember:
- Air occupies space. Prior to starting the experiment, the bag was filled with air.
- Before turning on the vacuum cleaner, the air pressure inside the bag was the same as the pressure outside the bag.
- The vacuum cleaner removed the air from the bag and created a situation where the pressure outside the bag was greater than the pressure inside the bag. This allowed the bag to shrink or be molded around your hand. In light of your new discovery, explain what happened to your arm using the terms “push” and “pull” in place of the word “suction”.
The practical applications for this experiment include vacuum packaging of food such as coffee and peanuts or cryovac packaging in the meat packing business to reduce spoilage of meats.
Wayne Goates is credited as the creator of this science demo in 1969, and he’s been using it to illustrate the dramatic effects of atmospheric pressure. Wayne’s original demo consisted of an arm being placed in a small plastic bag to simulate how a sphygmomanometer (blood pressure machine) works and was later expanded to the one with the student’s arm being "shrunk." He revived the demo when his wife purchased a plastic bag from QVC that was to be used for storing sweaters and blankets so that several articles could be placed in the bag and the bag and the contents could be reduced in size by pulling the air out of the bag. He explained that she had spent too much money on the bag and her reply was to come up with something better… and he did.
Reference: Wayne Goates. NMLSTA Level Line. "Shrink Wrap With A Vacuum," Winter, 1996.