Force to accelerate a RipStik

Until spring 2013 I had used the acceleration of the RipStik to graph the force theoretically accelerating the RipStik. In the spring of 2013 I thought up another way to attempt to show how force produces an acceleration of a mass. The string I used was too light, which limited the mass I could to accelerate the RipStik.

This led into a horribly complex momentum calculation on Wednesday 13 February 2013 that bears not repeating ever again.

I used heavier cordage in June 2013, but the speed gain was small, the acceleration distance was too small. During the summer a set of new spring scales arrived, with various maximum pulls up to 50 Newtons.

The idea crept into my head that I might be able to use the new spring scales to pull myself with a known force over a measured distance. By timing the distance I could either work out the change in velocity and thus calculate the acceleration. With the acceleration calculated I could multiply by my mass and obtain an estimate of the force, which should be comparable to the force displayed by the spring scale.

My first plan was to tie off the line to a post and pull myself with the spring scale towards the post. I realize in retrospect that I would have needed an elastic line that would generate a fair amount of constant pull for this to have worked.

With the clothes line tied off, I put a knot in the other end.

A 50 Newton spring scale was hooked into the rope end.

I used a covered walkway pillar to hold myself stationary while I tensioned the line to 50 Newton of force, a stopwatch in my right hand, spring scale in the left. What could go wrong? The students murmured something about a thousand ways to die.

I let go and briefly accelerated. Briefly. All too briefly. The line went slack, the force dropped to zero.

I probably should be using a four wheel cart, maybe a standard four wheel skateboard. The distance I was pulled was far too short. So I thought about the set-up for a moment, and then re-rigged. This time I held the end of the line. Note that there is a very slight up gradient that helped tension the line.

Dwayne has the scale. We tried a 20 Newton max but it was clear that would not start me rolling. I had Dwayne switch up to a 30 Newton max, but I was not sure that would roll me either. So he wound up using the 50 Newton scale.

As Dwayne backpedaled to hold tension at 50 Newtons, I accelerated. Dwayne would later note that the force fell to the 20 to 24 Newton range, which suggests I should probably stay with the 30 Newton spring scale. Starting acceleration requires a jerk (change in acceleration), which took 50 Newtons. Once the board is accelerating, only 20 Newtons is necessary to continue the acceleration that is established. Thought of another way, there are static friction forces to be countered at the start, once I am rolling the frictional forces are kinetic or sliding friction in the bearings.

At 600 cm the line started to slack as Dwayne could no longer backpedal fast enough, so I bailed off of the board at 700 centimeters and 8.78 seconds. Given that distance = ½ * acceleration *time², then the acceleration = 2*distance/time². 2*(700 cm)/(8.78)² is 18 cm/s² or 0.18 m/s². The board and I are roughly 69 kg of mass, which generates an estimated force of about 12.5 Newtons.  The actual force was somewhere between 20 and 50 Newtons. The forces are of the same magnitude (within a factor of ten).

I think the above approach could be refined to produce better results. I was on the sloped part of the sidewalk, maybe getting up onto the flat might help. I think using a 20 or 30 Newton scale should be considered to keep the acceleration low, make the backpedal more manageable. I might be able to use a small push off to overcome the initial inertia of being at rest with zero acceleration. Note that 20 Newtons does not even pull the slack out of the line, so 20 might be too little. But if it could work, it could extend the run and potentially improve the numbers.

Note too that although I had planned to used the change in velocity to calculate the acceleration, I abandoned this approach and when straight to ½ a t²

Adding to the complications, in class I somehow calculated 24 N instead of 18 N. I do not now know how I obtained the 24 N. I also forgot to factor in the mass of the RipStik, and used only 67 kg, my own mass. For a first attempt, not so bad, not so good. Hard to plan something one has never done before.

The plan for spring 2014 is to run the same basic run but use a 20 N or 30 N scale, move up onto the flat, try for a slower, steadier acceleration. My hope is to use an 800 cm slow acceleration, distance and time to calculate acceleration, run against the mass. Spring 2014 has featured a singular focus on centimeters, grams, and seconds. Even energy was done in ergs with my mass being 67000 grams. My potential energy was up around 2.2 billion ergs. So I ought to use dynes spring 2014. The catch is one Newton is 100,000 dynes, which will complicate math on the sidewalk given that the spring scales are calibrated in Newtons and gram-force. Gram-force will come up on Thursday in the pulley lab, but I am unsure how to handle gmf on the sidewalk. When I solve a=F/m what will the units of acceleration be if F is in gmf? And I want to show that this is equivalent to a = 2d/t². No, I will surely have to work in meters, kilograms, and Newtons to keep the results in that human comprehensible number range of 1 to 100.

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