Hands on Science Sunday: Feeling pressured?

by Stephanie Chasteen on May 31, 2009

Here’s today’s science classroom activity.  We’re surrounded by the crushing weight of layers of atmosphere above us, but we don’t feel it.  Why?  Our perception is tuned to differences, not absolutes.  If we were in a completely pink world, we would notice anything that wasn’t pink, but (I’m pretty sure) after a few minutes, we would become blind to pink itself, just like you don’t hear the noise of a fan in the room until it stops.

Similarly (though through different mechanisms), we’re not constantly aware of the intense pressure pushing in on our bodies.  (Would we really turn into mush in a vacuum?  No… read more about the effects of a vacuum on the human body here).  Thank goodness, because it’s quite startling.  I know, because I’ve felt it, in this wonderful science classroom experiment.

All you need is a big trash bag and an industrial strength vacuum cleaner, and a willing victim (er, “faithful subject of science.”) The victim (aka “subject) gets inside the bag, and once you suck all the air out of the bag with the vacuum cleaner, they’ll feel an intense pressure.  SAFETY FIRST!  Read this PDF writeup of the activity (from the Exploratorium’s Eric Muller) for all the ins-and-outs and safety factors in doing this with your kids.  (Words to the wise — don’t put your head inside the bag!)  It’s stunning — try it if you can.

Courtesy of Eric Muller - http://www.exo.net/~emuller
Best skin-tight prom dress.  Courtesy of Eric Muller – http://www.exo.net/~emuller

Why do we feel this pressure?  Stop and think about it a moment.  What changed when we sucked the air out of the bag?  There’s the same atmospheric pressure outside the bag (14.7 PSI at sea level), that didn’t change, there’s still the weight of the atmosphere pressing down on you.  What changed is the pressure inside the bag.  What does that have to do with anything?

The high pressure outside the bag pushes the bag’s surface against your body, and the bag stretches against your skin.  We feel this stretching of the bag as it pushes on our skin and the little hairs on our body.  We don’t feel the pressure without the bag, because though the air pushes against our skin, it pushes the same in all directions.  The bag lets us feel what is already there — the weight of the air!

You can extend this activity a bit by measuring the pressure inside the bag (at the Exploratorium it was 1 PSI). Eric says:

Paul Doherty and I used a barometer watch to measure the pressure inside the bag when doing research on this activity. Some one that goes to the mountains a lot might be able to loan you one or you can buy one. You can also just get a barometer.  I found a bunch for sale on eBay. Lastly, you can make a home-made barometer.  If you do a Google search, there are a variety of easy to make barometer designs (but you still might need a good barometer to calibrate your homemade one)

Paul D. has a similar activity to let you feel the pressure in water with a plastic bag.  Stick your hand in a pail of water.  You don’t feel any pressure.  But stick your hand in a plastic bag and stick it in the water, and you’ll feel an intense pressure (that gets stronger with depth) as you put your hand in the bag.  Paul D. explains:

Why do you need the glove or the bag?
Human sensors detect differences or changes in a signal. When you stick your ungloved hand into the water the water exerts a uniform force on your hand. It flows around every hair and every wrinkle in your skin. Now a single hair is bent to the side. When this happens you cannot feel the pressure exerted by the water.

However when you wear the bag or the glove they will bend down the hairs on your hand, and the glove and the bag may have folds that exert uneven forces on your skin. So that you can “feel” the force exerted by the water.

Cocktail Party Physics has a great old post on the history of measuring pressure (fascinating stuff, really).  And Eric has more activities here.

A few relevant science toys from Arbor Scientific

One of Paul Hewitt’s favorite demos to show that suction cups stick because of the air pressure pushing (rather than a “sucking force”) is this atmospheric pressure mat.  You can lift a whole lab stool once you stick this down on it.  A similar but smaller version is these atmospheric pressure cups.

A vacuum chamber/pump will let you reduce the pressure on anything (they suggest marshmallows), explore gas laws, etc.    See the website for some example class activities. They also have a class set.

They’ve also got a durable hand-held vacuum pump for expelling air from any other kind of container.  (Good for the “coin and feather fall at same rate in vacuum” kind of demo).


Tess June 2, 2009 at 9:23 pm

Thanks for an awesome post! Another great relevant toy is the marshmallow masher ( http://www.stevespanglerscience.com/experiment/00000106). My students never get tired of pressurizing the bottle and then releasing to watch the marshmallows puff back up. Entertainment and learning all rolled – or should I say compressed- into one! 😉

Sebastian January 28, 2011 at 3:28 pm

Crazy picture and awesome idea. Thanks for the post!

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