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Homework Help: Stress and strain problem

  1. May 13, 2014 #1
    A 30kg sledge hammer strikes a steel spike of 2.30 cm diameter while moving at 20.9 cm/s. The hammer rebounds with a speed of 10.0 cm/s after 0.110 s. What is the average strain in the spike during impact?

    2. F = (mvf-mvi)/t
    S = (F/A)/(deltaL/initialL)
    Young's modulus for steel = 2 x 10^11

    3. I used (vf-vi)/t to find the force but apparently I'm supposed to use change in momentum/time. I don't understand why.
  2. jcsd
  3. May 13, 2014 #2
    nevermind, i got it.
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