(30 points) A guy (100kg, 2m) jumps off a 5-meter wall a. (10 points) If he rema
ID: 1879273 • Letter: #
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(30 points) A guy (100kg, 2m) jumps off a 5-meter wall a. (10 points) If he remained vertical while landing and his shoes are compressed 1cm, how much force he experienced from his shoes? (10 pts) If he fully bended his knees while landing and the distance from his hip to ankle is 1m, how much is the average force he experienced now? Assume that the weight of the body is uniformly distributed over the height; Assume that the knee is at the middle of hip and ankle; Hint: consider the change of center of mass. (10 pts) If he not only fully bended his knees but also fell down to lie on the ground, how much is the force now? (similar consideration as b) b. c.Explanation / Answer
(A) speed just before toucing ground is v1.
then v1^2 - 0^2 = 2(-9.8)(-5)
v1 = 9.90 m/s
it speed v1 becomes zero in 1cm (0.01m)
0^2 - 9.90^2 = 2(a)(-0.01)
a = 4900 m/s^2
F = m a = 100 x 4900 = 4.9 x 10^5 N .......Ans
(B) his center of mass's displacement = 2.5 - 1.5 = 1 m
{ final height of center of mass = (5 - 1) - 2.5 = 1.5 m
now 0^2 - 9.90^2 = 2(a)(-1)
a = 49 m/s^2
F = m a = 4900 N ......Ans
(C) now center of mass comes down by 2.5 m
0^2 - 9.90^2= 2(a)(-2.5)
a = 19.6 m/s^2
F = m a = 1960 N .......Ans
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