Friday, December 16, 2011

Test today; grades posting this weekend

A little fun about Santa from class today (not my work, but very amusing)
I will hopefully have grades posted this evening for where everyone stands at this point.

Wednesday, December 14, 2011

TGT review sheet

Get a copy of today's TGT review sheet here.

Tomorrow will be for independent review, setting up equation cards, reviewing notes, etc., and grade reviews with me for those missing work that can influence grades. 

Friday we'll have our exam  You may take it as a regular test (50pts) or a final (100pts) - let me know before the exam starts.  Next week (Monday OR Tuesday depending on your class) we'll have some fun - our "final" demonstrations. 

Monday, December 12, 2011

Thurs-Mon

Exam Friday.  You may elect to make it worth double a regular exam (finals credit) if you wish - all final grades (except this exam and the review game) will be posted in class Wednesday, including all labs.
Want to review?  Look to the chapter review sections in your book following chapters 3-5, and also parts of chapter 6 and 9 that we have studied the past week.  (p. 159&165 questions, plus content appearing on p. 254-257(top half), and p. 264-270.  )
If you would like to see answers from the textbook, my teacher's edition is at your disposal in class.  Just ask.  I'm also around to help if you need further answers/explanations.
Thursday and Friday last week we explored doing work and changing energy:  Thursday we worked a ranking task activity involving energy, and Friday we worked practice problems and answered homework questions/corrected answers to the p. 159/165 assignment. 

Today we had notes on heat (3 ways to transfter thermal E) and some demonstrations to match (most hours got to see them - some will be continued tomorrow.  In-depth examples were given for convection, and conduction and radiation were explained in limited detail. 
All classes got to see that water convects really well, but doesn't conduct thermal E hardly at all.
Tomorrow we will discuss energy in YOU and your food, where it and the matter accompanying it goes, and finish up demonstrations from today.  Wednesday we will have a TGT review game for bonus points, and Thursday will be independent review.  

Tuesday, December 6, 2011

Work, Energy, and Machines

Today in class we looked into the connection between work/energy and how machines work. 
We started off analyzing an inclined plane; its purpose is to lift an object to a greater height and change the object's GPE.  Either lifting the object straight up or sliding it up a ramp accomplish the same task, the same net change in energy, and (ideally) the same work is done in either case. 
(F*d input = F*d output)  if friction is ignored... so the ramp allows you to exert less force, but over a greater distance.  The ratio of input and output distance and force is explained by the term Mechanical Advantage (MA) and is given by two equations:
MA= input distance  / output distance              and   MA = Output Force / Input Force

Today in class we just looked at input and output distances; they're easier to measure than force.  A ramp that is 3m long and 1m tall has mechanical advantage:    3m/1m = 3   (no units)
This means it is 3 times easier to use the ramp, or you must apply 1/3 as much force to roll an object up the ramp that you would to just lift it.

I provided demonstrations of other devices with mechanical advantage; a bolt cutter's handle moves 50cm while the "cutting wedges" move about 1/2cm.     50cm / 0.5cm = 100, and that's not even considering the mechanical advantage of the cutting wedges.  Using the handles makes it 100x easier to cut a bolt thann shoving the wedges together by hand. 
Other examples:   shears for trimming trees - use the handles at the end vs. close to the fulcrum for higher MA,   jacks for lifting a car have MAs in the hundreds as well, making it possible for any adult to lift a car. 

**What about if the input and output distances are equal?  Then the MA would be 1, and the input and output forces would match.  This is the case for some levers and pulleys if they only change the direction of a force. 

Finally, a MA less than one means that the input side of a machine moves less than the output end, and the input force is greater.  The easiest example of this is a bicycle; starting out you use gears with a high MA, but at top speed, you want to apply a large force over a small distance relative to how fast your bike wheels are spinning.  Examples exist within your body as well; muscles can't always be placed in convenient locations, and they don't have the greatest range of motion, so most of our joints and muscles work with MAs less than one. 

Percent efficiency relates the input and output work of a machine in the real world (friction and thermal E included).  %eff = work output / work input       or   %eff = E out / E input. 
Because of friction and thermal E, 100% efficiency is possible, and although people have tried to make free energy for centuries, nobody's ever made a >100% efficient machine that generates useful energy. 
There's no such thing as a free lunch.

Monday, December 5, 2011

Thermal E & Work

Hmwk now due Thursday:  Read p. 154-165.  P. 159#1-7, p. 165#1-7  <-- I listed the wrong page # in class. 
Today in class: Defined Heat, Thermal E, Temperature, discussed research about near "absolute zero" temperatures, and related doing work on something to changing its energy - even directly giving it thermal energy, and had hands-on examples: syringes and compressed air changing temperature and producing mini-clouds of condensed water vapor when they rapidly did work on their surroundings, using their internal energy, and cooled.

Friday, December 2, 2011

12-1&2 Energy Forms & Work

HMWK due Monday:  Read p. 135-140.  P. 143#1-6
Yesterday we worked a GPE-KE - work marble lab where energy was calculated for a marble atop a ramp, then we rolled the marble down and allowed it to push a folded card until it came to rest.  We altered the energy input based on the marble's height and the number of marbles, and recorded the change in stopping distance.  It turns out that since Work (Force * distance) has the same units of energy, when the energy was doubled or tripled, the stopping distance was doubled or tripled as well - demonstrating that the Joule is a proportional unit for Work and Energy.
Today we had notes on Work and forms of energy / the fate of all E --> Thermal Energy.
Monday's class will be consumed by looking into Thermodynamics principles, and we'll discuss homework.  Later Monday and Tuesday will involve application /connection of the idea/equation of work to Simple Machines.