Question

A rock is dropped from a cliff. How high is the cliff if the rock falls (1/3) of the total height of the cliff in the last second of it's fall?

Answer 1

2h=gt^2

Answer 2

now distance travelled in the last second (t) is h/3

Answer 3

2h/3= g[(t^2 - (t-1)^2)}

Answer 4

that should do it

Answer 5

got it?

Answer 6

I'm not realy sure where you are getting the {t^2-(t-1)^2} to be honest. I understand 2h=gt^2.....but then it looks like you divided that by 3 and then didn't do the same to the otherside ha......wait...maybe I do see what you did..you constrained t so that however long it takes for rock to fall 2/3 the height, it's always 1 less than the total fall time....but i'm still not sure how this will give me the cliff's height ha

Answer 7

from the first eqn get t, which is is the total time traveled

Answer 8

then the distance in the last second, is the distance in t seconds - distance in t-1 seconds right?

Answer 9

so h/3 = [gt^2 - g(t-1)^2] / 2

Answer 10

so now uve got 2 eqns and two variables t and h

Answer 11

eliminate t from the system of equations and get h

Answer 12

got it?

Answer 13

the answer is around 173 meters

Answer 14

and him, your method is incorrect, btw.

Answer 15

how com?

Answer 16

2h/3= g[(t^2 - (t-1)^2)}
how did you get this?

also, in the last second of its fall, the rock already has an initial velocity. You are discounting that.

Answer 17

tht gets subtracted neway

Answer 18

linalg009, is that the correct answer in your book?

Answer 19

learn to read..i explained it above

Answer 20

I almost feel like I'd have to trial and error it until I get around the correct time for the last second of the fall. ha....It doesn't have an answer in the book :/

Answer 21

oops....I mean get the right distance for the last second of the fall ha....i ration of 1/3

Answer 22

ratio*

Answer 23

so now the qustn is?

Answer 24

The question is...can I just plug in different values for the initial height of the drop until I get around 1 sec. for the last third of the fall ha..somehow

Answer 25

wtdu hav to find out temme clearly

Answer 26

It's the same question. I'm trying to find out the height of the cliff. I was just trying to think of a different method

Answer 27

ive told u the method..wat part do u not get? im positive its correct

Answer 28

What are the two equations? 2h=gt^2 and 2h/3= g[(t^2 - (t-1)^2)}..?

Answer 29

yeah right

Answer 30

t=sqrt(2h/g)

Answer 31

or first u divide the two to make a quadratic eqn in t

Answer 32

then get the value of t and plug it into the first eqn to get h..got it?

Answer 33

ya...solving now ha

Answer 34

yes, him is right. I did not read what he had posted correctly.

Answer 35

the answer is around 145-150 meters.

Answer 36

I get 145.52 meters

Answer 37

yes, thats right.

Answer 38

Perfect! Thanks for the help him1618 and dhatraditya ha..it makes perfect sense now! ha.