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Suction Science: How to Break a Ruler Using Air Pressure

Perform an apparent feat of strength with this impressive physics demonstration
bsh ruler break air pressure


How strong is air?: Learn just how much pressure a gas can exert with this exciting rule(r)-breaking activity!
George Retseck

Key concepts
Physics
Gas
Pressure
Suction

Introduction
Do you think you could break a wooden ruler using just the air around you? What about if you added a newspaper and just one hand? In this cool physics demonstration you'll use the sheer force of our atmosphere's pressure to break a ruler with nothing but newspaper and a single hand.

Background
Our atmosphere is a blanket of gas nearly 125 kilometers thick, and just like all matter in the universe the air in our atmosphere, which is made up of molecules, has mass. Gravity pulls on anything that has mass—even when it's as "light" as air! And because we're standing underneath most of this mass, we experience this pressure as a result—we just don't notice it because it has always been there.

So how does your body deal with the sheer force of this pressure? It produces its own internal pressure to push back. If you've ever noticed a buildup of pressure in your sinuses or inner ear while traveling to a higher altitude, that's because your body had initially filled these spaces with the right amount of air to match the atmospheric pressure you experience at a lower altitude. You may also know that you can hold your nose shut and forcibly exhale to "pop" your ears (which is not recommended!); this helps equalize the pressure in your skull with that of the surrounding atmosphere as the latter changes with altitude.

More pressure also causes a higher air molecule density—that is, more molecules are forced into a smaller space. This is why athletes tend to perform better at sea level: each breath of air contains more oxygen molecules. This pressure–density relationship works in reverse, too: changes in density affect pressure. As the density of a gas rises, the pressure it exerts rises; as density drops, the pressure drops. Keep this relationship in mind when conducting the following experiment.

Materials
• Large, broadsheet newspaper
• One to two thin wooden rulers or meter sticks that can be broken. (Avoid using plastic, and avoid using rulers that have a strip of metal embedded in them.)
• Sturdy table

Procedure
• Place your wooden ruler or meter stick on the table so that slightly less than half of its length extends over the edge of the table. (For example, if using a ruler, allow about five inches to extend over the edge of the table.)
• Find a spot on the ruler located about three inches beyond the edge of the table.
• Without bracing the other side of the ruler give this location on the protruding end of the ruler your best knife-hand, or karate, chop, making sure to strike with the soft part of your palm. (And watch out for the flying ruler!) Don't brace the ruler with your nondominant hand. What happens when you strike the ruler? Why do you think this is? Try to identify the various forces at play.
• Pick the ruler up off the ground and set it on the edge of the table just like you did in the first step.
• Next, unfold the newspaper and cover the portion of the ruler touching the table with two overlapping sheets. Smooth the newspaper out to reduce the number of air pockets present in the space between the table and the newspaper. (You definitely won't be able to create a totally airtight seal, but do your best!) Again, make sure the appropriate length of ruler extends over the edge of the table.
• Find a spot on the ruler located about three inches beyond the edge of the table. What do you think will happen to the ruler now after you try "chopping" it?
• Give the ruler your best strike. (Again, no bracing other than the flattened newspaper is allowed!). What happened to the ruler this time? Why do you think that is?
• If the ruler didn't break, try it again, being sure to smooth out the newspaper well and to use a swift, strong chop. If it hurts your hand, ask an adult to volunteer to do the chopping.

Observations and results
If you struck the ruler firmly and sharply enough during the trial that utilized the sheets of newspaper, you should have been able to break it! Why? You likely inferred that it had something to do with the fact that the end of the ruler that lifts up off of the table has to lift the large sheet of newspaper with it. The newspaper itself isn't very heavy, but it has to push against a lot dense air, which resists changes in its motion (a concept known as inertia).

As you saw (and felt!) from this activity, gases in our atmosphere aren't total pushovers! You'll know this if you've ever stuck a cupped hand out the window of a moving car and tilted it against the air. The inertia of its individual molecules and the continuous collisions between those molecules prevent a gas from just flying out of the way when a solid object passes through it. In fact, scientists describe gases as viscous—they resist being deformed by other forces. Air isn't exactly molasses in this regard, but it's a force to be reckoned with, nonetheless.

An even more powerful force that counteracts the chop you exerted on the ruler is suction. Ever wonder how suction cups are able to stick so firmly? "Suction" can be a misleading concept. Instead of a "pull" you might want to think of it as a "push" that comes from atmospheric pressure. Think about it this way—at a given altitude, air pressure exerts the same amount of force on all exposed surfaces of an object. Air presses on every inch of our skin—it doesn't just press down on the tops of our skulls. This means every inch of your newspaper in contact with the air experiences the same amount of air pressure—that is, until the newspaper lifts up off of the table. Because air isn't able to move very quickly into the expanding space between the newspaper and the table, the molecules in that space have to temporarily occupy more room. So, the density in this space drops and the pressure decreases as a result. In effect, you're creating a partial vacuum (an area of low pressure) in the space between the newspaper and the table. When atmospheric pressure pushes back against this area of lower pressure, it counteracts the ruler’s rotation, causing it to snap in two!

More to explore
Why Your Ears Pop (and What to Do if They Don't),” from Gizmodo
Amazing Strength of Air, from Brusspup
More Great Science Experiments to Conduct at Home, from Education.com
Break a Ruler Using Newspaper and Atmospheric Pressure, from Education.com

This activity brought to you in partnership with Education.com
Education.com

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