RipStik Gravitational and Kinetic Energy

Although I have long demonstrated qualitatively that gravitational potential energy can be converted to kinetic energy by a RipStik on a slope, I have never tried to attach numbers to this exercise. The covered walkway and the walkway pillars, however, sparked the idea to try to attach numbers to this exercise.

The above is all "guess and golly" with the students eyeballing whether the surveyors line is level. Yes, I could have used a level. No, I did not possess a level. The concept was a thumbnail scratch calculation that would be correct only to an astronomical order of magnitude. In astronomy a cow is a sphere as a first approximation. The drop height h was estimated at 34 centimeters. 980 cm/s² was used as g, and my mass is roughly 67000 grams.


I used the last gap on the east side to the north of the walkway branching off to the A building. I started about three or four pillars up slope from a starting speed of zero. I used the pillar to remain upright at the start. The line above runs off screen to the start. I used the branch to the A building as my run out. I wore running shoes and found my maximum speed was well within my board dismount speed range.


The last set of pillars were 300 cm apart and I covered this gap in 1.55 seconds. I left the students with these raw numbers and made as homework the calculation of my gravitational potential energy and my kinetic energy at slope bottom. There are all sorts of errors inherent in the measurements made. The line was probably not level. The slope is not constant. The bottom speed is only an average for that last 300 cm and that stretch is still a slope, so I am accelerating all the way down.

I told the class that the GPE should exceed the KE, should be equal to or greater than due to losses to other forms of energy (frictional heat being a big loss component).

I did warn them that the numbers would be very big. I noted to that the units are called an erg, and that one erg is an ant push-up. This term I have consistently and solely used cgs to date. I think this use of cgs to the exclusion of any other measurement system reduces complexity for the students. Previous terms have included some labs and other measurements in mks, this term I have studiously avoided that usage. The result is that today was done in ergs, but even astronomers sometimes use ergs for very energetic objects. Maybe this will lead naturally to side coverage of scientific notation.

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