Question

Superman lived on Krypton where the acceleration due to gravity is 23.6 m/s2. If a woman falls from a building that is 88.3 m tall, how long does he have to save her before she hits the ground?

Im confused as to what equation I am to use. I know of the acceleration, initial velocity, as well the displacement. But what equation would I use?

Answer 1

@Astrophysics

Answer 2

Do you know your kinematic formulas?

Answer 3

Yes I do, but not by heart.

Answer 4

Alright so can you list them please :)

Answer 5

Sure.

\(\Large{d=v_{i}t +\frac{1}{2}at^{2}}\)
\(\Large{v_{f}^{2}=v_{i}^{2}+2ad}\)
\(\Large{v_{f}=v_{i}+at}\)
\(\Large{d=\frac{v_{i}+v_{f}}{2}}t\)

Answer 6

Great ok, so what are we solving for here?

Answer 7

Superman lived on Krypton where the acceleration due to gravity is 23.6 m/s2. If a woman falls from a building that is 88.3 m tall, how long does he have to save her before she hits the ground?
——————————————————————
a = 23.6 m/ss, Vi = 0, d = 88.3 m, Solve t.

Answer 8

Well we are trying to find the time it takes before the woman hits the ground. We are well aware of the acceleration (23.6 m/s^2), the displacement (88.3m), and of the initial velocity (0 m/s).

Answer 9

Since Vi = 0, d = t(ta)/2, Solve t.

2d = t(ta)

2d/a = t(t)

t = +/- sq(2d/a)

Note: Since t is time we will accept only the positive value.

Answer 10

So we would change the first equation into a function of t?

Answer 11

ab = c tells us what c is, and also tells us a = b/c or b = c/a.

We use which depending on if we want a, b, or c.

Our formula tells us distance d, but we rearrange it to tell us time t = +/- sq(2d/a)

Answer 12

Ok. So my equation would be...

\(\Large{t=(\frac{2(88.3)}{23.6})^{2}}\)

right?

Answer 13

sq = square root, I understand the confusion. Let's call it

t = root(2d/a)

Answer 14

ooo ok i square root it...

\(\Large{t=\sqrt{\frac{2(88.3}{23.6}}}\)

Answer 15

calculator time ... time t is expressed in the unit seconds s.

So t = sqroot(2d/a) seconds

Do you have the answer to cross-reference what you obtain on your calculator?

Answer 16

I got 2.73 seconds. Would that be correct?

Answer 17

calculator doesn't lie

Answer 18

Thank you so much! I was stuck on this for a while, but you got me out of it :D

Answer 19

don't sweat it and stay studying

Answer 20

Will do!

Answer 21

If we did have an initial velocity Vi not equal to 0, our equation is

d = tVi + at(t)/2

In which case, to solve for t, we would have to set the equation to 0 so that

at(t)/2 + tVi - d = 0

Giving us a solvable quadratic equation

t = quad. form.

in which Qa = a, Qb = Vi, Qc = -d

Quadratic Form of an Equation:
Qa x^2 + Qb x + Qc = 0