Please help with Motion Problem, looks easy but is confusing (to me)

[accesskb]

Hi,
I've spent a long time and wasted lots of paper on this question. It might be very easy to do but I have tried/used every one of the motion formulas and don't seem to get a right answer. This is one of those challenge question that our teacher has given us. He hasn't worked out the answer himself yet but one of the student who was able to get it says the answer is about 172.5m

Please could help me out with this Motion problem. Appreciate all the help. thnx

A hiker sees a boulder drop from the top of a cliff and notices that it takes 1.5seconds for the boulder to fall the last 1/3 of the way to the ground. What is the height of the cliff (in meters)?

here is a rough pic I drew:
http://www.clubbersnation.com/math/physics_problem.jpg

 

[Ambitwistor]

No, the distance is about 327 m. You can tell that 172.5 m is wrong, because you can find out how long it takes to fall 172.5 m and 2/3 of that, and their difference doesn't work out to be 1.5 s.

You know the formula for how far it falls in a given time, y(t). Turn it around and solve it for how long it takes to fall a given distance, t(y). You know that t(y=h)-t(y=2/3 h) = 1.5 s, so you should be able to solve that for h.

 

[accesskb]

thnks Ambitwistor for the explanation :)
I have never tried any problems like this before. I shall work it out now.

 

[Luong tu Khanh]

My answer is about 356m. There's no difference between my solution and Ambitwistor's .


LTK

 

[HallsofIvy]

Luong tu Khanhwrote
My answer is about 356m. There's no difference between my solution and Ambitwistor's .

Looks like a difference of 29 meters to me! I get Ambitwistor's answer (well, actually I get 327.7 m which I would round to 328).