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Building Height (HELP NEEDED)

  • #1

Homework Statement


Spider-Man steps from the top of a tall building. He falls freely from rest to the ground a distance of h. He falls a distance of h/4 in the last 1 second of his fall. What is h?


Homework Equations


All kinematic equations with free fall.


The Attempt at a Solution


I separated the motion into 2 cases where there was one part three times higher than the other. I made a bunch of equations that lead to nothing. Apparently I don't understand this.
 

Answers and Replies

  • #2
diazona
Homework Helper
2,175
6
Which equations did you make? How do they lead to nothing?
 
  • #3
Is it 276m?

What I did:

Let l = h/4

The Kinematics equation s = vit + 1/2 *at2

-l=v3l(1s) - 5*(1)2

gives (5-l) = v3

The distance spiderman moves before this is 3l, so using

vf2=vi2+2as

Vi is 0 since he starts from rest

so (5-l)^2=2*3*g*l

gives the quadratic equation l2-70x+25=0

Solving gives x = 0.358984 or 69.641

Multiply by 4 to get h.

I'm not sure if it's right though :uhh:
 
  • #4
LowlyPion
Homework Helper
3,090
4
I made a bunch of equations that lead to nothing.
I think you just need the right 2 equations.

Focus on the time to fall. It falls 3/4 of the way in T1.

3/4*h = 1/2*g*T12

and

h = 1/2*g*T22

where T2 = T1 + 1

Solve for the total time and it's all down hill from there.
 
  • #5
Thanks guys! That was a nice way to this problem LowlyPion. Also thanks physicsnoob93. I got 273m as my answer. I am pretty sure that's right after checking. Again thanks!
 
  • #6
diazona
Homework Helper
2,175
6
Thanks guys! That was a nice way to this problem LowlyPion. Also thanks physicsnoob93. I got 273m as my answer. I am pretty sure that's right after checking. Again thanks!
Not that it matters now, but I tried it both ways and they both give the same answer, 273m...
 

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