Now, having read the first 5 questions of this assignment, the students are agai

Question

Now, having read the first 5 questions of this assignment, the students are again chasing Marc. This time, Marc decided it was wise (for some reason) to try to out run the students by running on the roof of a 2.8-m tall building. Having been chased to the edge of the roof, Marc must make a quick decision. To jump or not to jump, that is the question. Marc does some quick physics in his head and decides to jump off the 2.8-m tall roof. Of course when you land, you bend your knees to help "cushion" the impact. Most people do this naturally (see, most people's brains do know physics...), with the exception of humanities majors. If Marc's legs are able to slow down his torso (42 kg) over a distance of 0.7 m, with what velocity did his feet hit the ground? What is the force exerted on his torso by his legs during the deceleration (assume his legs provide a constant force over that period of time)?

Explanation / Answer

(a) height of building= h =2.8 m
from law of conservation of energy ,energy conservation, we have:

total energy at top= total energy at bottom

mgh + 0 = (1/2)mv2+ 0

mass m cancels on both sides

   velocity v = (2gh)
                  = {(2)(9.8)(2.8)}
                  = 7.41 m/s

(b) d = 0.7 m
v2 - v02 = 2ad
    0 - (7.41)2 = 2a(0.7)

a= -39.2 m/s2
F = ma
F= 42 (-39.2)

F=1646.4N upwards
  
weight W = mg
               = (42 )(9.8)
               = 411.6 N   (downward)

let Ft= force exerted onthis torso
net force F = Ft - w
Ft = F + w

Ft=1646.4+411.6

Ft=2058 N.
  

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