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Physics 1A, Section 2

Physics 1A, Section 2. October 28, 2010. Quiz Problem 49. Quiz Problem 49. Answer: {[k 3 + (k 1 -1 + k 2 -1 ) -1 ] -1 + k 4 -1 ] -1. Energy Conservation. Energy is conserved: K + U + heat + … = constant Sometimes, mechanical energy is conserved:

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Physics 1A, Section 2

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  1. Physics 1A, Section 2 October 28, 2010

  2. Quiz Problem 49

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

  4. 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.

  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. • 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.

  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. • 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

  7. Quiz Problem 50

  8. 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

  9. Monday, November 1: • something to do with potential energy

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