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jhyrman
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Homework Statement
From WebAssign:
One mole of nickel (6.02e23 atoms) has a mass of 59 g, and its density is 8.9 g/cm3. You have a bar of nickel 2.44 m long, with a square cross section, 1.8 mm on a side. You hang the rod vertically and attach a 45 kg mass to the bottom, and you observe that the bar becomes 1.6 mm longer. Next you remove the 45 kg mass, place the rod horizontally, and strike one end with a hammer. How much time T will elapse before a microphone at the other end of the bar will detect a disturbance? (Assume a simple cubic lattice for nickel.)
Homework Equations
Young's modulus, diameter of an atom, v=ωd
The Attempt at a Solution
tl;dr - I'm getting 0.000538 seconds, not the right answer.
Atomic Size and Mass:
1) convert given density to kg/m^3 = 8900kg/m^3
2) convert to moles/m^3 (kg/m^3 * mol/kg) = 150847 mol/m^3 (not rounding in my actual calculations)
3) convert to atoms/m^3 (6.022^23 atoms/mol) = 9.084e28 atoms/m^3
4) take the cube root to get the number of atoms per meter, = 4495309334 atoms/m
5) take the reciprocal to get the diameter of an atom, = 2.2245e-10 m/atom
6) find the mass of one atom (kg/mol * mol/atoms) = 9.7974e-26 kg/atom
Young's Modulus: Y=(F/A)/(dL/L)
1) F=mg = (45kg)(9.8N/kg) = 441 N
2) A = (0.0018m)^2 = 3.5344e-6 m^2
3) dL = 0.0016m
4) L = 2.44m
5) Y = 1.834e11 N/m^2
Interatomic Spring Stiffness: Ks,i = dY
1) From above, diameter of one atom = 2.2245e-10 m
2) From above, Y = 1.834e11 N/m^2
3) Ks,i = 40.799 N/m (not rounding in my actual calculations)
Speed of Sound: v = ωd
1) ω = √(Ks,i / m,a)
2) From above, Ks,i = 40.799 N/m
3) From above, m,a = 9.7974e-26 kg
4) ω=2.0406e13 N/m*kg
5) From above, d=2.2245e-10 m
6) v=ωd = 4539 m/s (not rounding in actual calculations)
Time Elapsed:
1) length sound traveled = L+dL = 2.44166 m
2) From above, speed of sound = 4539 m/s
3) T = (L+dL)/v = 0.000537505 s
which is not the correct answer.
Thank you for reading and for your feedback,
Josh