# Day 11: Equality of Normal Forces

College-Prep Physics: I’ve been using the normal force vote/discussion lessons from Preconceptions in Mechanics. This is the target scenario for the second lesson. Although you can’t tell, the buggy is turned on and the wheels are spinning with the buggy in place.

Which force, if either, is larger: the buggy pushing on the bolt or the bolt pushing back on the buggy?

The rest of the lesson leads students through several different scenarios (including the Dueling Rubber bands from Day 10) carefully selected and sequenced so that they can return back to the target scenario and have a model/mechanism for the answer.

FYI: That board with the bolt came in very handy today.

• I put a Pasco matter model over the bolt so I could show how the compression/normal force changes direction as the board changes angle from horizontal to vertical. The bolt kept the model from sliding down the board.
• Later, I took the matter model off the board and put a book on the board, tilted the board, and the bolt prevented the book from sliding — and we talked about the directions of the normal forces between the book & board and the book & bolt.
• And finally I used it to introduce the Hooke’s Law lab by hooking a spring on it and pulling with a spring scale (“What do you notice?”).

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The previous normal force lesson dealt with the nature of the normal force and introduced the ball and spring model. Here are some responses to the HW for the first lesson:

##BFPM