A 75.8 kg man steps out a window and falls (from rest) 1.08 m to a sidewalk. Wha

Question

A 75.8 kg man steps out a window and falls (from rest) 1.08 m to a sidewalk. What is his speed just before his feet strike the pavement?

= 4.60 m/s

If the man falls with his knees and ankles locked, the only cushion for his fall is a 0.458 cm give in the pads of his feet. Calculate the average force exerted on him by the ground in this situation. This average force is sufficient to cause cartilage damage in the joints or to break bones. I keep trying to do this question myself and keep getting 17509 N, which is incorrect. I also keep getting 3.67 which is incorrect, as well. HELP!!!

Explanation / Answer

mass of the person, m=75.8 kg

height of the wall, h=1.08 m

a)

use energy relation,

1/2*m*v^2=m*g*h

==> v=sqrt(2*g*h)

v=sqrt(2*9.8*1.08)

v=4.601 m/sec

b)

displacement of the person is, s=0.458 cm

use,

v^2-u^2=2*a*s

4.601^2-0^2=2*a*0.458*10^-2

====> a=2311.05 m/sec^2

acceleration, a=2311.05 m/sec^2

average force, Favg=m*a

=75.8*2311.05

=175177.6 N

=1.75*10^5 N

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