Comic-book superheroes are sometimes able to punch holes through steel walls. (

Question

Comic-book superheroes are sometimes able to punch holes through steel walls.
(
a) If the ultimate shear strength of steel is taken to be 2.70 109 Pa, what force is required to punch through a steel plate 1.70 cm thick? Assume the superhero's fist has cross-sectional area of 8.00 101 cm2 and is approximately circular.

My professor told me that I got the problem wrong because I used the wrong area. He told me not to use the area of the fist, but i'm still getting it wrong.

Explanation / Answer

from the area of fist, find out the radius Then A = 2*pi*r*t where, t = thickness = 1.7 cm = 0.017 m then, tau = F/A => 270109 = F/A Substitute to get F

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