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E is always constant, but KE and PE can change

E is always constant, but KE and PE can change If PE and KE change, they must change in such a way as to keep E constant Example Consider the 1D free-fall of an object of mass m from a height of y 0 =h. The initial energy of the system defines the total energy. y. m.

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E is always constant, but KE and PE can change

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  1. E is always constant, but KE and PE can change • If PE and KE change, they must change in such a way as to keep E constant Example Consider the 1D free-fall of an object of mass m from a height of y0=h The initial energy of the system defines the total energy y m y0=h, t0=0, v0=0 y<h, t, v>0 system

  2. System – the collection of objects being study to the exclusion of all other objects in the surroundings, in this example, we consider the object of mass m only Some time later, KE and PE have changed, but E has not Energy E KE PE=mgy y 0 What is KE, PE, and v when y=h/2? h

  3. For y=0 (just before the object hits the ground)? Note: we have neglected air resistance and what happens when the object hits the ground.

  4. Example A hockey puck slides across the ice. Its speed slows from 45.00 m/s to 44.67 m/s after traveling a distance of 16.0 m. Determine the coefficient of kinetic friction between the ice and the puck. Solution: Given: v0=45.00 m/s, vf=44.67 m/s, x=16.0 m=s Method: Use work-energy theorem FN s fk mg

  5. Conservative and Non-conservative Forces Conservative Force: a force for which the work it does on an object does not depend on the path. Gravity is an example.

  6. y m s s mg s h mg mg B C A A B C • Non-conservative Force - a force for which the work done depends on the path - friction - air resistance

  7. If both conservative and non-conservative forces act on an object, the work-energy theorem is modified For the case of gravity

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