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Observational Markers that Force(s) are Happening

March 26, 2013

In teaching physics, our plunge into forces has been:

#1 Qualitative observations with hover pucks, consolidating observations into generalizations about what effect doing nothing, tapping, and pushing has, to some degree following the activities and discussion outlined by Kelly O’Shea.

#2 Introduction and practice with system schema diagrams, followed by readings and discussion about the ontology of force, interactions, and force pairs, and what this has to do with understanding / learning the force concept.

#3 Yesterday, we formalized our ideas into specific observational markers that force(s) are happening. We then worked our way through reasoning, discussing, and collecting observational evidence that a table exerts in upward force on a book, largely following the instructional sequence outlined by Clement.

Here are our observational markers for force(s) happening

Squishing or scrunching–visible deformations like when you stand on a carpet, or lie down on pillow top mattress.

Stretching or elongating–like when you are stretching a rubberband, or using pulling back a slingshot.

Sound + contact: Two surface in contact with accompanying sound–a baseball bat hitting a ball and making a knocking sound, or scratchy sound of sandpaper over wood,

Tightness or taughtness–like when a string is pulling you can feel the string is tight and see the string is more straight than it would otherwise be.

Bending–standing at the edge of a diving board, you can see the board bending.

An object is speeding up, slowing down, turning around, or changing direction.

 

Tomorrow, we try to quantify operational quantify amount of force, largely following the thinking and experimenting outline by Arons, which is really trying to bootstrap back and forth off observational marker #2 (about stretching) and observational marker #6 (about speeding up). Here is the gist:

  • Use an uncalibrated spring to tug on an low-friction cart of certain mass. First, try to keep the amount of “stretching” constant and pull. See what effect that has on mass using motion detector. Verify that this is repeatable–same stretching of string attached to cart always results in the same acceleration. And verify that it doesn’t just speed up but it’s speed up with a relative constant acceleration when you have a relatively constant spring stretch.
  • Stretch the spring to different amounts, and see how the effect changes, using same cart. We don’t assume linearity or spring force–i.e., don’t assume twice as much stretch = twice as much force. Instead, see what acceleration happens with different stretchings. This allows us associate the spring stretch with a specific acceleration for known mass, without making assumptions about the spring.
  • Motivate and invent a unit of force– for example “1 Robert” could be an amount of force that will accelerate the known mass at a rate of 1 m/s/s. Label various spring stretches in terms of their Roberts.
  • Now, vary the mass to see what effect that has–what effect does a “1 Robert” force have on different masses. Construct various plots of Acceleration vs. Roberts for various masses.

Curious to see how this goes.

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