Day 67: Force Diagrams and Solving ca Problems Graphically

Advanced Physics:

Today (and probably tomorrow)was spent WB’ing a set of force diagrams.  We only made it through 3 or 4 of the eight I assigned.  It took long for a few reasons. First of all, not all the students completed the  practice (which really does not make me happy… I have not found a way to convince some of them that the MUST do the practice when they do not understand the topic) so there were many questions as they prepped the WB’s.  Second of all, it takes them a bit to explain the diagram and answer the audience questions.  Finally, I always ask a few extra questions that require some extra explanation.  For example, on this one:

Screen Shot 2014-12-08 at 8.31.52 PM

I asked how the tension forces would change if everything was the same (so still the same angle) EXCEPT the right side string was attached to a ceiling that was higher so the string was longer.  Or if the angle was different.  We then use equipment to set up the new scenario and record the forces.

Here is another example:

Screen Shot 2014-12-08 at 8.31.39 PM

I ask what would happen to each of the three forces (Fg, Fn, and Ft) if the ramp were made steeper.  To demonstrate this I use a set-up shared a number of years back by Dale Basler at a Phox Share group meeting.  It’s a bit hard to describe, but it uses the WDSS as the box and the 3-axis accelerometer.  Each one is displayed using an animated vector representing  one of the forces.  Fg is the y-axis that is calibrated to -9.8m/s/s; the Ft is the  x-axis and the Fn is the x-axis. It’s all wrapped up in a pretty like Logger Pro file, but here is a picture of the set-up:

WDS Demo set-up

So as the right side is elevated, the animated vectors show that Fg does not change, but the Ft and the Fn do.


General Physics:

We discussed the kinematic graph assessment they took on Friday and then started WB’ing the graphical solutions to some constant acceleration problems.  Tomorrow there will be more of the same.


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