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Remember: This Week Read Ch. 6 (Work & Energy): En6re chapter. Read Ch. 7 (Momentum): Ch. 7.1 – 7.±, 7.8. CAPA assignment #9 is due Friday evening at 10 PM, to be done on‐line. Lab this week : Energy & Momentum. Do on‐line and print Prelab prior to the Lab Session. Includes Momentum, which we’ll only begin to discuss this week. L25 W 3/10/10 1 Physics 2010

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L25 W 3/10/10 Physics 2010 2 General Statement of Conservation of Energy Δ E mechanical + Δ E thermal = W external E mechanical = KE + PE Δ E thermal = thermal energy generated = -W frict W external = external work done on the system (work done by sliding friction)
L25 W 3/10/10 Physics 2010 3 General Statement of Conservation of Energy Δ E mechanical + Δ E thermal = W external E mechanical = KE + PE Δ E thermal = thermal energy generated = -W frict W external = external work done on the system Isolated System With No Dissipation: W external = 0, Δ E thermal = 0 (KE f + PE f ) – (KE i + PE i ) = 0 KE i + PE i = KE f + PE f Conservation of mechanical energy (work done by sliding friction)

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The effect of some forces can be expressed as a Potential Energy: e.g., gravity, elastic forces produced by springs (later) These forces are said to be Conservative . They have the property that the work done depends only on the starting and ending points, not the path taken while the force is being applied. The work done by some other forces depends on the path taken: e.g., friction. Under the action of these forces, mechanical energy is lost.
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