Day 148: Coulomb’s Law and The Biggest Horse

Advanced Physics:

We finished WB’ing the Coulomb’s Law Practice problems.  I really like this problem:

Screen Shot 2015-04-28 at 7.35.19 PM

because it provides a chance to ask a few really nice follow-up questions.  For example, (once the two forces are drawn in), sketch the net electrostatic force (a nice chance to quickly  review adding vectors tip-to-tail).  Another follow up is to ask for the position of the little charge +q (assuming A and B are anchored) an instant later…. then another instant later.  The majority of students think the +q follows the net force line…. forgetting that now the two individual electrostatic forces are now different meaning another different net force.  I think it provides a really nice preview for electric fields, where we are headed next.

We also discussed the Charge assessment they took yesterday.

General Physics:

We purple penned the work(ing) problems they solved in small groups yesterday.  After this, we predicted one more graph from what we did yesterday… the Work(ing) as a function of time.  You fully realize that it is linear:

Screen Shot 2015-04-28 at 7.43.38 PM

and that the slope of this graph is the energy transferred each 1.0 second… or power.  We wrote the EOL that, when rearranged, gives the usual power equation.  We talked about the different units for power, watts ft.lbs/s and of course horse power.  We even converted the slope for the student into horsepower… quite a few laughs about how small it is… the perfect segue into  the activity called the Biggest Horse.  This is just a simple activity that we use to have the students practice working, and power calculation by determining their power output.  Here a clip of two students doing the simple activity:

It uses old 10 pound weights.  I’d really love to find a set of 5 lb weights to give the students the chance to debate lower mass (force) in a shorter time or big mass and possibly slower time.  We enter all the data into a spreadsheet to check the calculations and to rank the students.  The Biggest Horses (make and female) in each class are given a prize… a Power Bar !!


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