One Tuesday in June, an athletic physics student decides to jog to campus. When

Question

One Tuesday in June, an athletic physics student decides to jog to campus. When she's about halfway there, she stops to catch her breath at a construction side where a 1.70 m wooden fence post with a squair cross-sect ion (14 cm on each side) has been left hanging vertically from a crane. Conveniently, the fence post has a hook on the bottom, so the student uses it to hang her backpack, The backpack ha a mass of 1 kg along with its contents of 300 physics textbooks, each 1.3 kg, What is is length of the fence post when the very full backpack is hanging from it? (Young's modulus for wood is 11 GN/m^2

Explanation / Answer

s = F / A = mg / side² = (1 + 300(1.3))(9.81) / 0.14² = 195.7 kPa I'm assuming you mean 11 Giga N/m², this is reasonable for wood... E = s / e e = s / E = 195700 / 11 x 10^9 = 1.779 x 10^(-5) e = ?L / L ?L = e*L ?L = [1.779 x 10^(-5)](1.7) = 3.02 x 10^(-5) m The final length is 1.7 + ?L = 1.7000302 m

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