Making the Coffee Can Mobile
Monday, April 23rd, 2007My 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.
