Our skin insulates us, holds our organs, and is the body’s largest and fastest growing organ. It also protects us from an unfathomable number of collisions each second caused by air molecules—air pushes on our skin and our skin pushes back. In this Snack, you’ll approximate the surface area of your skin and the amount of atmospheric force pushing on it.
- Choose one person to be the subject.
- Carefully wrap each of the subject’s body parts in newspaper, taping the pieces together as you go. Totally cover your subject's body—head, feet, hands, everything! Try to tailor the paper suit to the shape of the person's body. It's okay to overlap the newspaper sheets, but be sure to leave a breathing hole for the person's mouth. (Note: Please ask your subject to cover and tape over any of their own body areas associated with gender and/or sexuality.)
- Remove all the paper from the person's body by carefully tearing the paper down each limb, leaving all overlapping paper intact. Do not use scissors, as you might cut the person's skin or clothing.
- Lay all of the paper pieces on the floor, moving them around to form a square or rectangular shape (you can rip some of the larger pieces of paper to fit into this shape). Remember to keep all multiple layers of paper stacked together.
- Measure the length and width of the arranged paper on the floor and calculate the total surface area: length x width = surface area.
The number you calculated is a reasonable approximation of the person’s skin surface area.
To get an idea of the force of air pressure on the skin, multiply the surface area you calculated by the pressure exerted on it: area x pressure = force.
If you took your measurements in inches and calculated your surface area in inches2, multiply this number by 14.7 pounds/in2. This will give you the total force in pounds pushing all over the subject’s body.
If you took your measurements in centimeters and calculated your surface area in cm2:
- For the kilogram equivalent of what your skin could support: multiply your surface area (cm2) by 1.03 kg/cm2.
- For the newton (or force) of what your skin is pushing against: multiple your surface area (cm2) by 10.1 N/cm2.
Were you surprised by the number for the surface area of your subject's skin? On an average adult, the skin is approximately 2,800 in2 (or about 18,000 cm2). Accounting for a little error here and a little too much paper there, the amount you estimated might be a little higher than this.
When you multiply the surface area by the pressure, the number you get is really huge (ranging from tens of thousands for pounds to hundreds of thousands for newtons). This really big number is caused by the force of air pushing on your subject’s skin.
All objects on the surface of Earth—including you—reside beneath a huge ocean of air: the atmosphere. The atmosphere is composed of colliding gas molecules. The push caused by each and every colliding molecule adds up to air pressure. Air has a pressure of about 14.7 lbs of pressure per square inch (psi)—that’s 10.1 N of pressure per square centimeter, or 101.35 kilopascals at sea level. Since air is smashing into every single square inch or centimeter of your skin, the equation area x pressure = force results in some pretty sizable forces.
An average adult's skin of approximately 2,800 square inches is under pressure of 14.7 psi—which makes a total of 41,160 lbs of force.
Have you ever had to inflate a car or bike tire? The gauges used to determine the air pressure of a tire are in units above and beyond that of the surrounding air pressure. Car tires usually need to be inflated to 32 to 35 psi; a racing bike tire, up to 135 psi. If you take the area inside the tire into account, that figure represents huge outward forces inside the tire.
Have you ever dived into water that is more than 33 feet (10 meters) deep? At this depth, the water surrounding you would have at least doubled the pressure and forces on the outside of your body.
The atmospheric pressure drops at higher elevations. Jet aircraft that travel high in the sky pass through air that has very low pressure—be thankful that there is a pressurized fuselage surrounding you!
This activity was developed during the 1997 UCSF/SEP lesson plan competition with students from Roosevelt Middle School in San Francisco.