Day 71: WB the Fg experiment and pound some problems

Advanced Physics:

I forgot to mention in the last post that I make it a point to write gravitational force on the object by the earth a fair number of times so that I ‘get sick of writing it’… as do many of the kids.  I explain that there is a common name for the gravitational force exerted on an object by the earth, we call it weight.

We WB’ed the gravitational force experiment results.  As part of the discussion, we realize the slope of the linear graph is not equal to 1.0.  Many of the students were probably thinking, big deal… but it is.  It shows that weight is not equal to mass; they are completely different quantities and can not be used as synonyms.  All of you know the slope of the linear graph turns out to be 9.8N/kg for all three substances.  I tie this back to what is really giving the substance its mass, protons, and neutrons (I’m waiting for a student to say Higgs Boson…). Like many modelers, I define this as the ‘gravitational field strength for the earth’.  This naturally leads to the gravitational field strength for other massive objects like the moon. If we did the experiment on the moon that value changes, but not the number of protons and neutrons in the sample… so mass is constant.

We also worked through a sample tension for problem that allowed them to see how we use Fg and the net force equations we have been writing as part of our force diagrams.  Here is the problem:


They were to determine the unknown mass and the tension force on the left.  They were given he tension force on the right and the angle it makes with the horizontal.  Once both problems were solved, we used a balance and the WDSS to check the answers.


General Physics:

We decided that we would allow the kids to work through a practice sheet on solving ca problems with the kinematic equations.  It was the same 5 problems they had solved graphically… again, they already know the correct answers and it allows them to see which method they prefer.  I usually dislike this type of lesson plan, but one student this is an awesome thing to do, just let the pound some problems and ask questions when they have them.

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