full-soln - Physics 105: Mechanics Fall 2009 Homework 7...

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Physics 105: Mechanics Fall 2009 Homework 7 Solutions 5.RQ.51 a. Same force applied over same distance same work done on each block. By the energy principle (Δ E SY S = W SURR ), this means the two blocks experience the same Δ E SY S . The only type of energy that changes in this process is the the kinetic energy, and since both blocks start at rest, Δ K = K f is the same for the two blocks (and equal to W ). To compare their Fnal magnitude of momentum, it is convenient to use the relation K = p 2 / (2 m ). Since K is the same for both, the one with smaller mass must have smaller magnitude of momentum. b. Same net force applied over same time same change in momentum (by momentum principle). Since both start at rest, this means they have the same Fnal momenbumt v p f . Again it is convenient to use the relation K = p 2 / (2 m ). Since p is the same for both, the one with the smaller mass must have greater K . 5.RQ.55 Initial state: positron (e + ) and electron (e ) at rest ±inal state: two photons moving in opposite directions SYS = all particles in initial and Fnal states SURR = assume nothing signiFcant Since the problem states that the potential energy of interation in the initial state (binding energy) is very small, we include only the rest energies of the particles in the initial state: E i = m e + c 2 + m e - c 2 In the Fnal state, we just have the kinetic energy of two photons. Since the initial total momentum of system is zero, the Fnal momentum must also be zero, which means the two photons have equal and opposite momentum. This also means they have equal kinetic energy.
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full-soln - Physics 105: Mechanics Fall 2009 Homework 7...

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