GPE, KE, Work. All E --> thermal E! HMWK for Friday

Monday we discussed the expected results from the Cut Short lab activity; indicated that no matter what, the pendulum had enough energy to come back to the same height (or thereabouts) that it was released from.  We also started notes on Stored (potential) energy and discussed other ways of storing E.
At the end of class we got to see how well people really understood the law of conservation of energy; if they really trusted that a bowling ball pendulum would not swing back with more energy than it was given.  Most people still flinched.  :)

Tuesday we worked some GPE practice problems, basically solving for GPE, mass, and height, after being given other measurements about an item.  (see notes link from last week) (we could also solve for g, if the problem involved a location somewhere other than Earth).
We showed that when work is done on an object, it's energy changes; the equation for work is simply:
W = change in E on an object.

Since we just learned about GPE, I showed how lifting a 2kg book from the floor to a 1-m tall table we wouldn't think about its energy, we would think about its weight (the force we have to apply to lift it) and how far it's lifted.  For this book, W = m*g, so weight is 20N and we moved it 1m.
 (20N*1m = 20N*m or 20J)
This spawns a new way to define work:  Force * distance.  (there are some qualifiers we'll learn later)

So Work = change in E  OR Work = F*d

Finally we did work on paperclips... not by giving them GPE or throwing them, but bending them.  The work went into the paperclip but appeared to go nowhere.. .until we felt them and they got HOT!  The work went into thermal E, which is actually where all E ends up.

HMWK for Friday:  p. 148#1-25  (chapter review)

Wednesday, we used ramps and cars to calculate both GPE at the top of the ramp and the work done in pulling the car up the ramp.
(GPE = m*g*h)           and                  Work = change in E           OR               Work = F*d
After all was said and done, the GPE calculations were very similar to the Work calculations!
This showed that they were the same; work done on an object is equal to its change in energy.  Sometimes the energy is hard to see (Thermal) but the amount is the same; energy is conserved... Energy is used to do work, and work involves a change in energy.
This concept will be used in several other applications.

Thursday we'll start off with a video about the Physics of Car Crashes - I really like it - and at the end of class we can demonstrate the work - GPE relationship and tie it into KE.  Then I'll show you how it relates to Force, acceleration, change in velocity, etc... and then many of you will be happy to see that you finally "get" many of the motion and acceleration problems from the beginning of the year.