Air Pressure and Elevation

December 29, 2014

Why do your ears hurt underwater or when you drive up in the mountains?  The reason is because of the change in pressure.  This episode explains why that happens.


Episode 13: An Ocean of Air

I’ve done several episodes about pressure.  Barefoot on a Bed of Nails introduces the idea of pressure in a painful way!….and The Vacuum Chamber  shows some cool demos you can do with pressure differences (including making your own mini-pressure chamber from a bottle).

In this episode I explain why air pressure changes with your elevation above sea level.

A little pressure pre-explanation

We all live in an huge ocean and we don’t even realize it (most of the time).  This particular ocean, however, is made of air and not water.  How deep we are in this ocean of air is what determines the air pressure.  If you were to hang out down at the bottom then you would have a lot of air above you and all of that air is heavy.  This would exert a large pressure on your body.  If you were to hang out near the top of this ocean of air then you don’t have very much air above you and so would have a very small pressure on your body.  The same idea is true for water pressure.  In fact, that’s probably an easier place to start anyway…


A fluid is  a substance that flows–so basically a liquid or a gas.  The depth of the fluid at a certain point is the main cause of the pressure at that point.  If you want to explore that more and get a little bit more mathy I recommend this video explanation.  It is by Paul Hewitt, a master lecturer and pioneer of teaching conceptual physics.  We use his textbook in one of my physics classes at school.  I am a big fan of his approach to teaching.

Let me know if you have any questions:)

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Stomp Rockets are cool!

February 10, 2014

Stomp Rockets are a ton of fun and also easy to make. This episode will show you how to make one out of PVC pipe and a 2 liter bottle. In about 15 minutes you’ll be launching rockets of your own!

Episode 6: Build a Stomp Rocket!

Stomp rockets also work because of pressure.  You are creating a high pressure region in the bottle by smashing the air inside into a much smaller space.  At the other end of the launcher the air in the pipe and paper rocket are at normal pressure.  Air naturally wants to move from high pressure to low pressure.  That moving air is what pushes on the rocket and launches it.  This is similar to the earlier episode with the marshmallow cannon.  In that case the air pushed a marshmallow as it went from high to low pressure.  In this case the air is pushing a rocket as it goes from high to low (for those really interested in more advanced details about pressure, here’s a link to the explanation of pressure  in fluids on hyperphysics).


  • 2 liter bottle
  • 1/2 inch PVC pipe (around 2 feet, a little more is better though)
  • board for support base (around 1 foot long–a 2×4 or 2×2 works well)
  • 2 hole conduit straps (from electrical area of hardware store–at least one, second one is optional for the holder)
  • screws for the straps (nails could also work)
  • duck tape (you don’t need much, just enough to tape the bottle to the pipe)
  • card stock (it’s a good combination of light and strong)
  • 90 degree PVC elbow
  • 45 degree PVC elbow (optional)
  • short length of 1/2 PVC pipe for the holder (optional)




  • make sure your rocket body fits snugly around the launch pipe, but not so tight that it can’t slide freely.
  • you may need to experiment with getting the fins on straight and how big to make them as well (you can do 3 or 4 fins)
  • if your 2 liter bottle gets a hole in it (and it will eventually), just tape on a new one.


How far can you get your stomp rocket to go?!  Measure the distance along the ground from where you launch to where the rocket hits the ground.  Make sure you are firing it from level ground (no launching off a hill or a second story window).

There will be prizes for the top three distances,  so send me a video or picture proof of your best launch.

Now, go get building the most EPIC stomp rocket you can!

PS: I’m serious about the prizes.





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the vacuum chamber

January 18, 2014

Hey guys,

Today we’re going to look at what happens when you lower the pressure inside a sealed container.  We’ll be using the vacuum chamber to do a couple of cool demos.  We’ll then show you how to make a mini version of a vacuum chamber yourself!

Episode 4: The Vacuum Chamber


Remember from the last episode (barefoot on a bed of nails) that Pressure = Force/Area.

Pressure differences create unbalances forces and those forces make things move.  (don’t forget about the marshmallow going through the soda can!)  There was a pressure difference between the inside and the outside of the balloon and that’s why the balloon expanded outward.  There was a pressure difference between the air in the shaving cream and the surrounding air so the shaving cream also expanded.  Pressure differences can sometimes create huge forces.  This You Tube video shows how pressure differences can crush a barrel…Check it out:

Isn’t that cool!!!

In this case, water was boiled in the barrel which pushed most of the air out of the barrel.  The lid was then screwed on and the water vapor was quickly cooled in ice water.  As it cooled, the water vapor turned back into liquid water and took up way less space and so the pressure became very low inside the barrel.  The pressure outside the barrel (normal air pressure) was bigger and so it pushed in on the barrel and crushed it.

The Challenge:

Remember the challenge was to make your own balloon bottle and get the balloon to inflate without blowing into it.  You’ll need a bottle that has stiff sides to get it too work.  Don’t forget to drill a small hole in the side at the bottom for air to escape.  You can also use the balloon bottle as a water pump; I’ll leave that to you to explore for yourself.  Good luck and email me a video of your finished product in action!



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barefoot on a bed of nails

January 8, 2014

Have you ever wondered if someone could stand barefoot on a bed of nails? Well, wonder no more!  This episode explores the pressure on your feet in that situation.  It also introduces the idea of pressure and how that relates to pounds per square inch (otherwise known as psi).

Episode 3: barefoot on a bed of nails


Quick Summary

Pressure = Force/Area

The pressure on my feet when I’m standing in shoes was found by taking my weight and dividing by the area of my shoes touching the floor.  I used this graph paper to to find the area (the 1/2″ option worked well).

Pressure = 175 pounds/66 square inches = 2.7 psi

When I’m standing barefoot on the bed of nails the total area of all the tips of the nails holding me up is around 0.35 square inches (that was an average of smaller nail tips and larger ones…the nail tips are actually all about the same size but some seem to “poke” into your skin more than others because they are slightly different heights)

Pressure = 175 pounds/0.35 square inches = 500 psi!

As you saw, that was a little too much for my bare feet!


Determine how much pressure is on your feet when you are standing on your tip-toes.  You’ll need to use graph paper to find the area.   Let’s say that you have to be able to stay on your tip toes for at least 10 seconds for it to count (none of this rocking up onto the very ends of your toes for a split second kind of stuff!).  Post your findings in the comments below.  Good Luck….I hope you can stand the pressure;)

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