Feeling Pressured

1. Place the garbage bag on a soft surface such as a carpet. Have one person remove their shoes and step into the bag. We’ll refer to this person as the "bag-person"—and for the duration of the activity, you can, too! Be careful not to puncture the bag.
2. The bag-person should sit in the bag with legs crossed.
3. Have the bag-person hold the end of the vacuum hose in his or her lap and run the hose up alongside his or her neck. The bag-person should cup one hand around the vacuum nozzle opening without blocking the opening—this prevents the bag from being sucked into the vacuum hose.
4. The second person should use his or her hands to help the bag-person hold the bag around their neck. The plastic from the bag should be in contact with as much skin around the neck as possible. The bag should have no obvious openings to the outside air.

When everyone is ready, the third person, who is in charge of the on/off switch, should turn on the vacuum cleaner. This person must always be ready to turn off the vacuum in case any problems arise. The second person can also act as a "spotter," as the bag-person can sometimes tip over during this demonstration. Never leave the vacuum turned on and unattended.

Observe what happens once the vacuum is turned on.

Continually ask the bag-person about any sensations he/she may be experiencing. Turn the vacuum cleaner off after a few seconds. 

As the vacuum cleaner decreases the pressure inside the bag, the bag-person should feel a tightening or squeezing sensation on the body. Observers will notice that the bag conforms to the bag-person’s body. This happens because higher air pressure always wants to move toward regions of lower air pressure.

The squeezing sensation can be quite substantial. The squeeze is caused by the plastic’s reaction to the difference in the external air pressure outside the bag and the internal air pressure inside the bag.

When the bag-person first gets inside the bag, the external and internal air pressure are the same. At sea level, air pressure is usually 14.7 pounds per square inch (psi), 1 atmosphere (atm), or approximately 1 bar. You can see this in the diagram below (click to enlarge).

Depending on the vacuum cleaner, the pressure inside the bag can be reduced from 5–20%. When we tried this experiment at the Exploratorium, we averaged a little over 1 psi (about .1 atm or .1 bar) reduction in air pressure in the bag using a 12 amp vacuum—close to a 10% reduction in internal air pressure.

Once turned on, the higher external air pressure pushes the bag's outside surface towards the lower air pressure regions inside the bag. After a few seconds, the bag will come into contact with the bag-person’s body. However, there are also spaces where the collapsing bag doesn’t touch the person’s body, such as nooks and crannies where the legs are crossed, near the armpit, where the arm bends, or where the hand is holding the hose—even between the fingers. You can see this in the diagram below (click to enlarge).

Once the bag touches the bag-person’s body, the plastic wants to stays in place. External air pressure will force the plastic to stretch and wrap around various body parts where the bag is stuck. At the spots where it doesn’t touch the person’s body, the bag will try to force its way into the gaps. The stretching plastic causes the squeezing sensation around the bag-person’s body.

On a large scale, weather systems are driven by high-pressure areas moving towards and into lower-pressure areas.

On a smaller scale, some food packaging and storage systems use a similar process as this experiment to preserve and seal food. Pressure is lowered inside a bag of food so that the outside air pressure tightly molds the bag around the food.

Heated plastic can also be molded into a variety of shapes by using differences in air pressure. This process is called vacuum forming.

You and your pets should always be careful around swimming pools with covers because if you fall on a cover in a pool, the higher water pressure can press and tightly hold the cover around your body.

Be careful! Never try this experiment alone, never try it without an adult, and don’t try it with young children.

If done as a demonstration, this activity can be done in approximately 15 minutes.

A version of this activity was first seen at the 1995 NSTA International Convention, Jamaica.