Hello, I don't understand how to do part "d" of the question. Can someone please explain it to me step by step? Do I use the 2nd equation to solve it?

Question

Using one of the following formulas:

1. Vy = g * t + Voy
2. y = 1/2 * g * t^2 + Voy * t
3. Vy^2 = Vo^2 * g + 2 * g *t
4. (avg) V = (Voy + Vy)/2

A ball thrown down by 4 m/s from 80 meter height.

Find:
(a) How long takes to reach ground?

(b) What is velocity of ball just before hit the ground?

(c) What is velocity of ball after 3 second from thrown point?

(d) At what height from the ground velocity is 30 m/s?

I calculated the following:
(a) t = 3.65
(b) Vy = 39.8 m/s
(c) Vy = 33.4 m/s
(d) = ?

Henry · Answer

a. Correct.

b. Correct.

c. Correct.

d. h = ho-(V^2-Vo^2)/2g
h = 80-(30^2-4^2)/19.6 = 34.9 m

Or
V = Vo + gt = 30 m/s.
V = 4 + 9.8t = 30
9.8t = 26
t = 2.653 s.

h = ho - (Vo*t + 0.5g*t^2)
h = 80-4*2.653 + 4.9*2.653^2 = 34.9 m