Conservation of Energy 01

OPENING QUESTIONS: Take a moment to compare and contrast the Work done as a spring decompresses and the potential energy present when a spring is compressed

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WORDS O' THE DAY:

  • Work (Force through displacement)
  • Dot Product ("scalar product")
  • Work ("Newton meter (Nm)" or "Joule (J) ")
  • Hooke's Law (F = -kx)
  • Work done by or done on a spring = 1/2kix2 - 1/2kfx2
  • Kinetic energy of an object in motion = 1/2mv2
  • Potenial energy in a compressed spring = 1/2kx2

WORK O' THE DAY (continued)

Take a gander at this:

∆KE + ∆U = 0

Have a conservation with your group and suggest a written form that that equation

Howzabout this?

KEf + Uf = KEi + Ui

or (of course)

KEf - KEi =  - (Uf - Ui)


Which brings us to the main point for today:

In classical Newtonian mechanics energy is never created or destroyed. That's not QUITE the same with modern physics in which energy can turn to matter and matter can turn to energy.... but I digress.

There is a pretty big caveat to that rule, however, and that deals with systems.

If energy is added from OUTSIDE a system (think of a balloon for example. If we hold a lighter under a ballon, we add energy to balloon system) OR energy is LOST to an outside system we have to account for that increased energy.

The book goes into exquisite detail on how to identify a system... but I find that a tad hard to follow so I want to try something a bit different.

In any analysis of a problem, you should be able to determine whether energy is simply changing form (spring potential → kinetic → gravitational potenial) or whether energy is being added to a system:

(thermal energy from outside the balloon → kinetic energy of air molecules inside the balloon → increased potential energy of the balloon material)

Why do we include friction on the left side of the equation?

Now let's open the book and work through an easy example: 8.1

and a more challenging example: 8.3

    HOMEWORK

  • Read section 8.1 (gently). Try to gleen just what the author is going for, but keep in mind my suggestion of identifying the various types of energy as they come up (that is particularly evident in 8.3)
  • Work on examples 8.1 & 8.3 if we didn't finish those in class...
  • Do problem 8.3 on page 236 and then 8.6 and 8.7 on page 237
  • Please remember to write down a quick note after each problem emphasize the learning:

"What do I know now after finishing the problem that I didn't know before..."