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.