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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Nerf Gun Dart Speed–part 2

November 8, 2014

I decided to take a second look at the speed of a dart coming out of a nerf gun.  In this experiment I used photogates to time how long it took the dart to cover a short distance.

Episode 12: Nerf Gun Dart Speed–part 2

When I analyzed nerf dart speed six months ago,  I  measured the dart launch using some high speed video.  You can find that episode here.  I came up with around 19 m/s.  This go round I used photogates to time how long it took the darts to cover the first 10 cm.  I then discuss how certain (or uncertain) I am about the measurement.   That is called uncertainty.  Even though it sounds weird, I am actually more “certain” or confident in my measurement in this experiment than I was before.  Partly  that is because I have a lot more data (30 trials shooting the darts) but because I checked it with a radar gun and against some high speed analysis my student did.

Check it out:

Here is my nerf gun:

Nerf N-Strike Maverick

Uncertainty is a topic I usually save for my AP Physics class, but it seemed like I should include a little in this episode.  It really is at the heart of every measurement we make.  Here is a site that does a pretty good job explaining the basics if you want to know more.

I was pleasantly surprised that my radar gun could track the dart and that it agreed nicely with the photogate data.  That added to my confidence in the photogate measurement.

A Student’s Data

One of my physics students (Derek H) did a project on finding the speed of his nerf dart gun.  He used a high speed camera shooting at 240 fps and Logger Pro to determine the firing velocity.  He got some good data:

dart1 redo


graphs of nerf dart speed


If you are interested, here is his video:   Derek’s nerf dart video

I was shocked when I saw that his results were practically identical to mine!…and Derek wasn’t using the same type of nerf gun.  That has to be partly coincidental.  Physics Professor/Blogger Rhett Allain did an analysis and got a much slower velocity from his nerf gun.  Here is his post about it.

I was also surprised that Derek’s results were a lot more consistent too; his uncertainty was more like +0.5 m/s.  So, nice job Derek and thanks for letting me use your video and results in this episode:)

If you want to do your own video analysis you can use a free program called Tracker.   It does the same sort of thing that the Logger Pro software does.  If you have Logger Pro already (mainly educators I would imagine) but don’t know how to do video analysis yet, you should learn–it is an awesome tool for analyzing motion.  I made this screencast to help my students.  The audio is a little too low, but maybe it can help you get started too.


I have no idea what happened in my last attempt at finding the dart speed.    Those results now seems way too high.  I must have messed up the analysis.  However, I now feel confident in saying that my nerf gun fires at a speed just under 14 m/s which is about 30 mph.

Now go figure out how fast your nerf gun fires!  Email me your results.





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Waterslide Experiment

August 16, 2014

Here is a great waterslide experiment to squeeze in while the weather is still nice.  Don’t miss out on this great excuse to go to the waterpark one more time! You get to investigate how your weight affects your speed going down a waterslide.

Episode 11: The Waterslide Experiment

I’ve always wondered about the physics of waterslides.  They don’t seem to work quite like we learn in school.  Well, what I should say is what we learn about friction doesn’t seem to fit.  Whenever my family has visited the waterpark it seems like heavier people go faster down the slides.  But according to the simple model of friction taught in physics, heavier people should have more friction (if the surfaces are the same) and so go slower.

There doesn’t appear to be much on the web that investigates this.  There was a video that went viral about 5 yrs ago that had a guy with a huge waterslide and ramp that launched him into the air and landed him in a kiddie pool hundreds of feet away (It was a fake). Mythbusters did an episode on it.  It was pretty cool.   You can see part of it here.  But it doesn’t address the speed issue.

This waterslide experiment is an attempt to figure out how your speed is affected by your weight.  We first need to come up with a lot of data (this is where I need your help) and then second to figure out a better model for how friction works on waterslides.

Here’s the waterslide experiment video:


OK, this is a chance for you to have a great time and do some science at the same time.  What could be better than that!!!!!! (don’t answer that).  This is the perfect opportunity for you to convince your parents that you need to go to the waterpark one last time. Or if you don’t live near one, you can convince them to buy you that really long sheet of plastic you’ve secretly wished for all these years.

After all, this is for science.  You would be learning stuff…even better than that you would be doing science….you would be a scientist….and what better way to spend your free time then job training, right??!!! Tell your parents it is an investment in your future career.

I would love to crowdsource the waterslide experiment.  Get as many people doing science as we possibly can.  Sure, my 9 data points hint at a interesting connection, but we need 100’s or even 1000’s of data points to really nail down the full story. How does weight really affect the time it takes to go down a slide? Is the trendline  really a straight line, or is there a curve to it? Does it only apply to tubes?–what about those mats you can use at the waterpark?–What about those little blow up cushions you get with some store-bought water slides?–what about going down a slide without a tube or mat or cushion?

austin doing his own waterslide experiment

Are you convinced to help yet?  Are you STOKED about science yet?

Let’s do this.  The great waterslide experiment of 2014.  It will be awesome!

Post your results in the comments or send them to me at

Now go do some science:)

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water rocket launcher

August 5, 2014

Water rockets are loads of fun! This episode will show you how to build a water rocket launcher out of parts from a hardware store.  It can be made for around $10 in just about an hour.

Episode 10: Build a water rocket launcher

This summer I ran three classes called Rocket Science in the Park for the Passport to Boise Program (Which as been a smashing success in its first year–If you haven’t checked it out you should click here).


rocket science sign


Kids came to my class and learned a little about the physics behind rockets and then built and launched their own water rockets made out of 2 liter water bottles.


demo with the kids


filling the rocket with water



Check out this awesome video that Tobe Brockner filmed for us:

We had some serious fun!

The water rockets are super easy to make (just tape some triangular fins on a water bottle, maybe add a nose cone if you are really ambitious and fire away).

The launcher is a little more tricky however.  I had a lot of people want to know how to build one.  There are tons of plans and videos on the web about building launchers, but I wanted to make a version that was as straightforward and easy as possible.

So, here’s my design…

Supply List:

  • Two 1/2 inch threaded pvc pipes 24 inches long (I used the dark gray pipe-schedule 80)
  • 1 threaded 1/2 inch 90 degree elbow
  • 1 threaded 1/2 inch end cap
  • pipe thread tape
  • pack of cable ties (zip ties)–you’ll need 8, I used the 8 inch ones
  • duck tape
  • adjustable clamp
  • tire valve stem (from walmart or autoparts store)
  • string
  • scrap board(s)

a pic of water rocket launcher

Project video

You may have noticed that his water rocket launcher I showed you how to build is not the same as the ones used at our Rocket Science in the Park Class.  Those launchers both use a scissor system to hold the bottles in place, a rubber stopper with a hole, and hose instead of PVC.  It is a little safer (remember the warning from the video: PVC can shatter under pressure) but costs a little more and it’s tougher to get your hands on rubber stoppers without ordering them online.  If you are interested in that system you can find similar plans here.


  • It seems that about 1/3 the way full of water works the best for the water rockets.
  • You’ll probably want a large open space to launch your water rockets.  I used our backyard some (which is fairly small) and we were constantly having to go into the neighbor’s yards to chase down our rockets:)
  • The launcher make take some tinkering to get to work properly.  Sometimes certain 2 liter bottles seal a little better or worse and you’ll have to adjust your rocket launcher a little.
  • There are better systems than using duck tape on the white pipe to make it a tighter fit on the zip ties to prevent it from releasing too soon.  Sciencetoymaker has a good solution for that here.


Now go build a water rocket launcher and Get Stoked About Science!


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Why does the string dance?

May 10, 2014

If you watched the previous episode you’re probably wondering how those weird loopy wavy things work (if you haven’t watch it yet click here).  This episode explains it.  The type of wave that is being created is called a standing wave and it occurs when two waves interfere with each other in just the right way.

Episode 9: Standing Waves part 2

If you want a little more detail about standing waves, The Physics Classroom has a nice explanation with some animations here.

Stay tuned for the next episode which will show you how to make a demo of this yourself and amaze all your friends!


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Make a string dance

April 13, 2014

Wow your friends with this great demo!  It looks like this string is dancing in slow motion and you can control the speed of its dance moves.  It’s not just cool, it’s science:)

 Episode 8: Dancing String

(Standing Waves part 1)

Isn’t that Great?!!!

Like I said in the video, it’s really just an illusion.  That string is moving back and forth really fast and the strobe light just makes it seem like it’s moving slowly–or like it’s dancing.  A strobe light is a light that flashes on and off really fast.  In the dark room the video camera only “sees” the string when the light flashes on.  I can adjust how quickly it flashes.  So I can adjust it so that the light blinks on exactly when the string is back in the same spot it was at the last flash (even though it moved down and then back up).  That makes it look like the string hasn’t moved.  I can also make it flash a little slower so that on the next flash it looks like the string is just below where it started (the string moved down  and then up and then down just a little more).  That makes it seem like the string is slowly moving down.  Or I can make it flash so that the string looks like it’s barely moving upwards.  It’s the same sort of effect as when it looks like the wheels of a car on TV are spinning backwards.  The first part of this video shows that illusion (you can watch the whole explanation if you want, but it does get a little math-y).

Even though the dancing part is an illusion the fact that the string doesn’t move back and forth in the middle is not–that is just cool physics…but more on that next time.

This is the first video of a series of three about standing waves.  Look for the next video soon explaining the science behind them.  The third video will be how to make a standing wave jiggler to impress your friends. 

Get Stoked About Science!

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Nerf Guns: How fast is the dart shot?

March 4, 2014

Who can resist a Nerf gun?  You just can’t help but pull the trigger if one is nearby.  But have you every wondered how fast the darts are actually moving?  Well, in this episode I answer that.

 Episode 7: How fast is a Nerf Dart?


Speed Formula

Speed = distance/time

I could have just measured the total sideways distance the dart traveled and divided it by the time it took to hit the ground.  But that would have given me the average speed of the dart while in the air.  Since our Nerf darts are made of foam, the air slows them down while they are flying.  That means that they are moving faster in the beginning and slower at the end.  That makes the averages speed somewhere in the middle.  But I want to figure out the speed (or velocity) that the Nerf gun fires them.  So, that’s why I need to use the slow motion video.  That way I could see how far the dart was traveling in the first 30 cm or so before the air had a chance to slow it down much. I used Logger Pro to analyze the slow motion video.  Here are a couple of screen shots from the video analysis.

dart with dots

In the Logger Pro software, I clicked on the tip of the Nerf dart for each frame of video.  So the blue dots represent where the dart was for every video frame.  It does look like those dots are slightly closer toward the end.  That would mean that the dart isn’t travelling quite as far between each frame of video.  Or in other words, air resistance slowed the dart down a bit.


So I found that my nerf gun shoots the darts at around 16 m/s or 35 mph.  (I did find 19m/s when I analyzed the video one way–but I think there must have been an error with that).


I revisited this. You can find the most recent experiment here.

The short version of my second attempt is that I measured the speed with photogates and got just under 14 m/s or 30-31 mph.  I then tripled check it with a radar gun and also got 30-31 mph.  In addition, my student measured his own nerf gun’s speed and also came up with just under 14 m/s.

So, I’m feeling a lot more confident about 14 m/s or 31 mph.

The newer episode also talks more about uncertainty in my measurements, so if you’re interested check that out with the link above.

*Newest Update*

I tried a different gun, the Nerf Sharpfire.  It was a little slower at around 11 m/s.  I also investigated how air resistance slows the dart down and what percentage of its energy is “lost”.  It was pretty interesting.  You can think about air resistance being like riding in a car and sticking your head out the window–the faster you go the more the air pushes back on you.  I shot a whole video lesson around the concept of nerf guns and air resistance for a learning website called Here is a short clip from that lesson.

It was a lot fun making the lesson and I was surprised at the results.  You can sign up for a free trial subscription at and watch the lesson.  If you like it then watch all 12 of the video lessons I made about Energy.  They are a great mix of concepts, demos, experiments to try and some cool DIY projects to build as well (like racquetball poppers and rubberband cars).  Here is the link to the lesson: How to Calculate Air Resistance.  Check it out:) 

Next, I want to test this little speed demon (the Elite Firestrike)…my son got it recently and it seems super fast.


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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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splat ball: get yourself one!

January 30, 2014

Yes, splat balls are fun, but if you film them in slow motion they are AWESOME!

When a regular ball is dropped the majority of its motion energy  transfers into the “springy-ness” of the ball and then transfers back into motion energy after the bounce.  A ball filled with watery stuff is different.  When it hits the ground it splats.  That is why it’s called a splat ball:)  This episode looks at where the energy goes and how much of it goes there.

Episode 5: the splat ball


Our green splat ball didn’t last too long.  Maybe none of them do, but they are fun for a while.  You can probably find them at a dollar store, I’ve never looked.  Here are some cool looking ones from Amazon if you are interested:



Here is another slow motion video from You Tube that captures the motion nicely.

So, get yourself a splat ball and have some fun!

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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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