Homely Scientist

My daughter A told me last week, while I was at the conference, that she had a homework assignment to do this weekend and wanted us to do it together.  No matter what it was, I was going to say yes.  Single dads don’t get that option often.  Regardless, then she said it was to build a simple machine.  W00t!  Science fun!

When I picked the kids up on Friday morning I found out the details.  She had to design and build a device that used two simple machines and would go at least 2 meters (6.5 feet).  Bonus points were given for coming back and/or carrying something.  Oh and once she let go or gave the device its initially energy, she couldn’t touch it.

No sweat.  I knew what to build in seconds.  The Can Car.  This is a super easy project and would be great for rainy day races.  Before I get into the Can Car, and the fun I had with the project (because it didn’t work the first time), let’s talk simple machines.

From Wikipedia a simple machine is:

In physics, a simple machine is any device that only requires the application of a single force to work. Work is done when a force is applied and results in movement over a set distance. The work done is the product of the force and the distance. The amount of work required to achieve a set objective is constant, however the force required to do this can be reduced provided the lesser force is applied over a longer distance. Increase the distance and reduce the force. The ratio between the two is the mechanical advantage.

What are the simple machines?  Here’s the list:

• The inclined plane
• The wheel and axle
• The lever
• The pulley
• The wedge
• The screw

 When I took physics I think the screw was considered an inclined plane, no matter A was using the same list …

For the can car you see that the wheel and axle are there, but what is the second one?  That was something I was pondering too, and wanted to make sure I was on solid footing if challenged by a teacher (sorry a grade 4 teacher isn’t a science authority in my book, but I wanted to have a good argument in case A got dinged on the project).  If you look at the PBS site there is a pencil used to wind up the rubber band to provide power.  I’m calling it a lever.  To me it is the same as the crank on a bike or winch.  The lever lets you transfer energy.  Works for me.

Okay making it was fun.  Had some challenges like a poor supply of rubber bands (I wound up using an elastic from an old conference nametag holder–good thing I’m a packrat!) and a few other improvisations.  And the biggest challenge was the first couple versions didn’t work.

See our initial design was based on a spool racer.  Which uses all the same principles of physics, but the major difference is that a spool is the wheel, not a can.  Oh and at the lever end, it’s a washer not a bead.  These two things combined made for a very frustrating first try.

See I used an improvised washer and a very strong rubber band (broccoli bands rock).  The tension was too much.  The chop stick couldn’t turn.  Bad news.

A and I did what any good scientist would do, we went back and did more research.  Okay she researched and I experimented.  I had a vague memory of a different version of the can car that would come back to you, so I tried that one from memory (couldn’t find it online).  After a lot of tinkering we got it to work.  The success factors were two-fold.  First was using a metal nut between the lid and the chop stick.  Second was using liquid soap to grease the lid on the chop stick end.

And I tell you, that thing when wound up tight, flies across the floor.  And bonus, because it’s a can with covered ends, it can carry stuff!

Bonus points!

For A, this was a good experience in research, trial and error, and experimentation.  For me it was just fun to play with science and share that with my kids.

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