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Physics 1A, Section 6. October 30, 2008. Section Business. Section 6 for Monday, Nov. 3 is canceled . Visit another section if you wish. Office hour for Nov. 4 is unchanged. Look for graded Quiz 2 in Section 6 box in Bridge, noon Monday at the earliest. Quiz Problem 49. Quiz Problem 49.

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Physics 1a section 6

Physics 1A, Section 6

October 30, 2008


Section business
Section Business

  • Section 6 for Monday, Nov. 3 is canceled.

    • Visit another section if you wish.

    • Office hour for Nov. 4 is unchanged.

    • Look for graded Quiz 2 in Section 6 box in Bridge, noon Monday at the earliest.



Quiz problem 491
Quiz Problem 49

  • Answer:

  • {[k3 + (k1-1 + k2-1)-1]-1 + k4-1]-1


Energy conservation
Energy Conservation

  • Energy is conserved: K + U + heat + … = constant

  • Sometimes, mechanical energy is conserved:

    • K + U = mechanical energy = constant

      • Example: ½ mv2 + mgh = constant

    • This often allows a quick solution of a difficult problem.


Energy conservation1
Energy Conservation

  • Energy is conserved: K + U + heat + … = constant

  • Sometimes, mechanical energy is conserved:

    • K + U = mechanical energy = constant

      • Example: ½ mv2 + mgh = constant

    • This often allows a quick solution of a difficult problem.

  • However, in other cases, mechanical energy is not conserved, so K + U  constant:

    • friction: Energy is lost to heat.

    • inelastic collision: Energy is lost to heat.

    • This is the same thing as saying the force can’t be described by a potential energy; the force is a function of some variable other than position.


Energy conservation2
Energy Conservation

  • Energy is conserved: K + U + heat + … = constant

  • Sometimes, mechanical energy is conserved:

    • K + U = mechanical energy = constant

      • Example: ½ mv2 + mgh = constant

    • This often allows a quick solution of a difficult problem.

  • However, in other cases, mechanical energy is not conserved, so K + U  constant:

    • friction: Energy is lost to heat.

    • inelastic collision: Energy is lost to heat.

    • This is the same thing as saying the force can’t be described by a potential energy; the force is a function of some variable other than position.

    • In some of those cases, one can resort to using the force to calculate the energy added to the system:

      energy input = W = ∫F•ds



Quiz problem 501
Quiz Problem 50

  • Answer:

  • v = sqrt(2gh)

  • F = -kx – mmg, to the right

  • W = -kxs2/2 – mmgxs

  • Wf = 2mmgxs

  • h’ = h – 2mxs

  • xs = [-mmg + sqrt(m2m2g2+2kmgh)]/k


Thursday november 6

Monday, November 3:

Thursday, November 6:

  • Section 6 is canceled.

  • Tuesday office hour as usual.

  • Quiz Problem 38 (momentum/collisions)

  • Optional, but helpful, to look at these in advance.


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