fisica1-youn-e-freedman-exercicios-resolvidos

2.90: a) Suppose that Superman falls for a time t, and that the student has been
falling for a time t 0 before Superman’s leap (in this case, t 0 = 5 s). Then, the height h of
the building is related to t and t 0 in two different ways:
1 2
− h = v
0 y t − gt
2
1 t 2
0
= − g +
2
( t ) ,
2
where v 0y is Superman’s initial velocity. Solving the second t gives t = − t .
gh
h g
Solving the first for v 0y gives v 0y = − + ,
t 2 t and substitution of numerical values gives
t = 1.06 s and v 0y = –165 m s , with the minus sign indicating a downward initial velocity.
b)
0
c) If the skyscraper is so short that the student is already on the ground, then
1 2
h = 123 m.
2
gt = 0