the delight of turning potential energy into kinetic energy

There have been too many weighty events this week - it is time for something fun.  

 

There is something wonderful about watching energy move backand forth betwen potential and kinetic energy even if there aren't many osciallations.  Swings, roller coasters, sailplanes, the arc of a tossed ball, chains of dominos ...

 

You've all seen videos of large domino chains.  By standing a domino on end you are increasing its potential energy.  Energy that can be released later when the domino tips, turning stored potential energy into the energy of motion - kinetic energy.  The key is to find an object that is usually stable, but can tip easily and move its center of mass a long distance in the process.  People make wonderful patterns using thousands of them - but there is another way to spend your day for a few seconds of delight ...

 

Wood is elastic and wants to return to its original position when bent - a property that allowed people to build bows that can propel an arrow at a high speed.  You can use this to store potential energy in a wooden stick.  When given a chance to release the stick will seek its normal shape and the resulting motion is a nice display of kinetic energy.  

 

With a bit of cleverness you can weave a pile of wooden sticks into a structure storing the energy that you used to flex them for later release.  Fifty sticks will give you and idea - a thousand will give you something wonderful.

 

A large number of weave patterns can be used - you just want to construct a regular pattern that bends the sticks enough without moving them past their elastic limit. Craft sticks are recommeded and are inexpensive at a few cents each in quantity.¹  A hundred bucks, a gym floor and a lot of kids would create a dramatic event. 

 

Like many things you can ask a few questions and gain a deeper understanding.  There is enough beauty without the questions, but new delights emerge when you look a bit more deeply.  How much energy is stored in a structure?  What is its energy density? What other materials could be used to improve it?  Could the shape and internal stucture of a stick be improved?   What weave patterns would be optimal? How much of the kinetic energy is turned into sound and heat and how much is goes into the bounce of the sticks?  Could you make a musical version? .. and so on.  Like so many interesting things, this is a rich playground for the mind.  

 

The artist and kid in you might try other weaves, adding some color or even making this part of a Rube Goldberg contraption.

 

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The energy density of mechanical storage systems is not very high compared with other forms of storage the industrial world uses.   Try calculating the energy density of a hydroelectric project as an example.  What about the maximum energy density of compressed air storage? How do these compare to electric batteries and hydrocarbon combustion or hydrogen combustion?  Are you likely to ever see a spring powered cellphone? 

 

On the other hand mechanical energy storage is very useful and even beautiful at the appropriate scale. That scale turns out to be our scale and it is vitally important to many biological systems.   Among other things it lets us run efficiently and makes nearly every sport possible.  To us even a modest amount can seem significant.  An average person on a six story building stores about the same amount of potential energy as the battery in his smartphone, but even that small amount (small for the purpose of running the machines in our lives) is enough be rather unfortunate in a fall.

 

stay curious

 

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¹ stick choice is important. I haven't done this since I was an undergrad, but remember we tried tongue depressors and popsicle sticks - these days you can also buy craft sticks, which I've seen recommended, but I haven't tried.  The amount of "spring" in each of these, their elasticity, varies and will produce very different results.  It probably pays to try small units with a few dozen sticks each before settling on a material for a huge project.

We ended up with tongue depressors mostly because we found a cheap source and made a 500 stick weave.  It was addictive enough that we rebuilt it three times.

 

Comments

[NurtureGirl]

Thank you Steve! Off to get some sticks so my kids and I can try it out this weekend. Weeeeeeee!!!!!!!!!!!

[Johna Till Johnson]

Steve, this is totally AWESOME! Thanks for posting! --Johna

[plumbing]

The faster the body moves the more kinetic energy is produced. The greater the mass and speed of an object the more kinetic energy there will be.

[kinetic energy]

this is so cool, just imagine if these sticks were bigger and made into a full display that went on for ages.