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, 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.
Materials
You'll need:
- A large, plastic bag
- A portable vacuum cleaner
- An assistant.
NOTE: This experiment requires adult supervision! Never place a bag over
anyone's head.
Experiment
1. 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.
2. Plunge your arm all the way into the plastic bag.
3. Have your helper feed you to 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.
4. Have your assistant hold the bag and the vacuum hose tightly around
your arm to form a seal.
5. 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!
6. 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?
7. 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.
8. 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.
9. Wrap the plastic bag up and around his or her leg. Gather up the open
end and form a tight seal around the leg.
10. 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.
11. By this time, 4 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 in the meat packing business where cryovac
packaging is used to reduce spoilage of meats.
Additional Info
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 sphygnomanometer ( 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 reduced 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.
Experiment From Steve Spangler Sciene
 | |  | |  | |  | |  | |  |