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Physics 1A, Section 6

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

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  1. Physics 1A, Section 6 October 30, 2008

  2. 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.

  3. Quiz Problem 49

  4. Quiz Problem 49 • Answer: • {[k3 + (k1-1 + k2-1)-1]-1 + k4-1]-1

  5. 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.

  6. 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.

  7. 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

  8. Quiz Problem 50

  9. 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

  10. 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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