Modified reference to _PhysicsSolver2. Removed some random whitespace that Eclipse was having none of. Changed the Linear Layout in activity_main.xml to a FrameLayout, since eclipse repeatedly would not compile giving the error that TabHost requires a FrameLayout with id 'android:id/tabcontent'.
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Updated the Pendulum activity to include working units selection that change based on what is being solved for, and a nicer dropdown menu for selecting the solved for variable. I also changed the layout settings of the the activity to be easier to manage. This activity can be considered a template for other problem activities.