RipStik Learning Curve

My youngest daughter wanted to learn to ride a RipStik. She had made a couple of attempts on two prior days without any success, she would fall off almost as soon as she started moving. This evening I took her out to the movie theater parking lot and worked with her on learning to ride the RipStik.

Those are socks for knee guards - she skinned a knee on a prior day's fall - and gloves for her hands. The lack of shoes might seem an obvious oversight, but she runs and plays barefoot every day and the bottom of her foot is quite tough. Kids who play barefoot find that learning to ride barefoot is far easier than learning to ride with shoes on.

I also used the opportunity to gather some data to play with in statistics class. The data in the table indicates her attempt number and the number of seconds she remained up, rolling, and successfully riding the RipStik [rolling time (s)]. Her first attempt ended as soon as it began, her second lasted a scant half a second, but by her third try she was up and riding for almost a second.

Attempt	Rolling time (s)
1	0
2	0.5
3	0.92
4	2.58
5	8.54
6	3.64
7	5.75
8	4.81
9	12.25
10	7.88
11	8.31
12	10.4
13	9.28
14	10.59
15	10.13
16	16.58
17	11.95
18	6.31

With each attempt she was generally, but not always, able to ride longer. Except for the 18th and last attempt, I steadied her on the board and she pushed off from my arms. On that last attempt, she mounted and launched the board without my assistance. Although she was unsteady and the ride only lasted six seconds, she had successfully launched the board.

I realize the data is not a true "learning curve" nor is that my use for the data. My use is in a linear regression example for statistics class. Note that the scatter graph is based on data rounded to the nearest tenth of a second.

No child was hurt in the production of this data.

RipStik Evening Session

All that remains of the building is the cement pad. In a town with limited recreational opportunities, an open cement pad is the next best thing to a playground. Especially for caster boarders.

The rule is that if and when you fall off, the next kid sitting on the sidewall gets to ride. Despite the hard falls, the local rule appears to be no complaining, no crying.

If a rider is good enough to remain up indefinitely, then the local rule is apparently that zoris can be thrown at the bottom front of the rider's RipStik in order to precipitate a fall. As a result of this, the local kids are generally proficient at sudden maneuvers to avoid colliding with a zori. 

Occasionally riders would attempt to knock other riders off their boards. The kids all seemed to be able to withstand these shoves and remain up and riding despite hard pushes and thrown slippers.

Meanwhile, over in a quieter and less competitive lot, my son works out the rudiments of a new sport. Some form of street hockey or polo while mounted on a RipStik. 

He has also demonstrated dribbling a basketball and playing catch while riding. Somewhere in far off suburbia some kids are using caster boards to invent some unique new sport. Caster boards have none of the limitations of a skateboard - the rider never needs to touch the ground to move over level ground. The boards can turn on a literal dime, and a good rider can put herself or himself anywhere on the field of play fairly quickly, maintaining stability despite occasional knocks or field objects.

RipStik Forces

Noting that I seem to topple off of my Stik more often and harder than my son, I ran a crude check on the minimal force required to achieve linear acceleration of the RipStik. The force data is based on earlier acceleration measurements.

time (s)	d (m)	velocity (m/s)	acceleration (m/s²)	Dana (kg)	Marlin (kg)	Fdana (kg m/s²)	Fmarlin (kg m/s²)
0	0	0	0	65.91	27.27	0	0
3.43	4.6	1.34	0.39	65.91	27.27	25.77	10.66
6.02	9.2	1.78	0.17	65.91	27.27	11.07	4.58

The force to accelerate me is larger by the proportion of my mass to my son's mass. I am exerting far larger forces through the caster wheel system. 

The result is that when I do hit a crack, rock, or hole in the cement, there are far larger forces at play. With increasing force, things go wrong both faster and with more destabilizing consequences. Or at least this is my excuse for spending more time getting acquainted with the ground.

Saturday morning

Saturday morning is marking papers. Outside. On the porch. In February.

Saturday morning is getting out the entire air fleet.

The air fleet is made possible by a plane-a-day calender, a Christmas gift that keeps on giving each and every day.

Flight ready to launch.

Saturday all-day is teenager-stare-at-small-screens day.

Conservation of marble momentum

Laboratory four begins with a line of marbles on a ruler. Before I roll one marble into a line of five marbles, I ask the students to predict what will happen. Some guess that all of the marbles will move, others guess that one will move. Then I roll the marble. I follow-up with predictions and observations of two marbles rolling into four, three into three, and so forth. I also demonstrate that speed in equals speed out. 

Stacy and Krystal consider their calculations

 

I usually focus the student's attention on the penultimate marble in the stationary line. I ask, "How does this marble know whether to stay or to go?" The students start throwing out terms such as "force" or "energy." This term one student suggested "magic." Another said, "The marble can feel whether one or two marbles have hit the other end." This usually leads to a discussion of what marbles can feel, and whether they have those feelings hurt. "Can marbles fall in love?" 

 Nancyleen records data on the mass of a marble

I note that the marbles not only have no feelings, but the marbles also do not know about force or energy. The marble do what they do. The attaching of words to why the marbles do what they do is a peculiarly human enterprise. All of the words and concepts are simply human constructs applied to the marbles, none of them - not at some deep level - actually explain why the marbles do what they do. 

Midion masses a marble

I tie this back to a Freeman Dyson quote - nature is fundamentally mathematical, and this is mysterious. 

For a physicist mathematics is not just a tool by means of which phenomena can be calculated, it is the main source of concepts and principles by means of which new theories can be created... ...equations are quite miraculous in a certain way. I mean, the fact that nature talks mathematics, I find it miraculous. I mean, I spent my early  days calculating very, very precisely how electrons ought to behave. Well, then somebody went into the laboratory and the electron knew the answer. The electron somehow knew it had to resonate at that frequency which I calculated. So that, to me, is something at the basic level we don't understand. Why is nature mathematical? But there's no doubt it's true. And, of course, that was the basis of Einstein's faith. I mean, Einstein talked that mathematical language and found out that nature obeyed his equations, too.

 – Physicist Freeman Dyson

 Joan and Lorry-Ann prepare to release a marble down the ramps

As noted in class, "We can calculate how the marbles ought to behave and when we run the experiment, the marbles know the answer."

 Rosthom and Sepe calculate momentums

In the second part of the laboratory the students collide three size of marbles, determining the momentum before and after the collision. This term the students noted that the tiny duck are called sinsai and the taws are called palas on Pohnpei.

A view of the equipment and layout for the momentum experiment

The experiments collide one moving marble into a single stationary marble of nearly equal mass. This minimizes the complexity and calculations. The result for the three marbles is a linear regression which theoretically would have a slope of one if there was no friction nor loss of energy in the collision.

Ethnobotany healing plant presentations

The ethnobotany class students gave individual presentations on healing plants. The following are some images and information from a sample of these presentations.

Yvonne Sue of Pohnpei presented on the use of eight inner shoots of the grass reh padil to treat diarrhea.

Jessica of Pohnpei presented on the use of kareretik (a small, very tart likely variety of key lime found here on Pohnpei) to treat kopokop (cough) and inta laud (high blood pressure). For cough, the fruit juice is consumed uncut with water.

Jeffrey presented the Pohnpeian use of topwuk (Premna obtusifolia) in combination with ilau (Clerodendrum inerme) and konok (Piper ponapense) for umwlap (medicinal steam bath). Jeffrey also detailed the resolution of boils, umpwos, using leaves heated on hot rocks and then applied with marekeiso (coconut oil) to the boil.

Qdelia of Pohnpei covered the use of koramahd (coleus) for herpes zoster infection.

Marcia noted the use of i (Morinda citrifolia) to treat diabetes on Kosrae.

Piulyn shared the Mortlockese use of Senna alata to treat skin diseases. The Mortlockese name of the disease and the plant are the same, kushuwa. Senna alata is broadly known for its ability to treat fungal skin diseases and is also known by the disease name on Pohnpei (tuhken kilinwai) and Kosrae (sra kihto).

RipStik as demonstrator of potential converting to kinetic energy

In a rather overly simplistic physics demonstration of gravitational potential energy and kinetic energy, I used a RipStik at the bottom of a hill to show I had no potential for motion - no energy due to my gravitational position. Then I moved up slope and mounted up onto the caster board without pushing off with my foot. I then accelerated down slope, trading gravitational potential energy for kinetic energy. I then carved around on the board and noted that I could not gain speed going uphill. I was now trading kinetic energy, speed, for gravitational potential energy. I also demonstrated that I could continue uphill beyond the energy available from my speed only by swizzling the RipStik. Going uphill requires energy. Crude, primitive, and lacking in measurements, but as a basic demonstrator of the concepts, the RipStik was again useful.



Earlier demonstrations included constant linear motion and accelerated motion.