uh, SHM. mistake in solution? (1 Viewer)

[Grey Council]

On a certain day in Fremantly Harbour the depth of high tide is 32 metres. At low tide 6.5 hours later the depth of water is 21 metres. If high tide is 12.10am, what is the earlist time at which a ship needing 28.5 metres of water can enter the harbour? Assume rise and fall of tide in SHM.

Now, what is completely stuffing me up is the fact that it seems to start off at HIGH tide. "If high tide is 12.10am" blah blah.

In the solutions, the answer is given as 4.12am. Thats absolute bs. The only way I could get that answer is if I assumed that it is LOW tide at 12.10.

:-\

Am I doing something wrong?

 

[CM_Tutor]

The question doesn't make sense if 4:12 am is the answer. If it is HIGH tide at 12:10 am, then the depth is 32 m, and the ship can safely enter the harbour at that time.

 

[Xayma]

No, the question is wrong, High tide should read low tide.

 

[:: ck ::]

wtf i get 2:39am

did every1 else get this?

 

[CM_Tutor]

wtf i get 2:39am

The first of those times is the latest time the ship can enter if 12:10 am is high tide. I have no idea what the second one is, as the period is 13 h and so the ship can enter again some time before 1:10 pm (about 10:41 am, I think, but that's calculating it in my head, so i may be off).

Edit: And the second one is now gone.

 

[:: ck ::]

hmmm i get 10:41pm as another answer.. but i reread question and it sed earliest time...

well anyway for the value of "t" i get 2:28:47.92

edit : yer i deleted it cm

 

[Grey Council]

lol, so I wasn't wrong. I got all confused while doing that trial paper, spent most of my time on that question. :-\

I figured that what it MIGHT mean (since the paper hadn't been corrected) was that we had to take 6 and a half hours away from 12:10am, ie start from low tide, then figure out when ships could begin to enter the harbour.

but, all's well that end's with me being right.

PS, is it me or do these new emoticons just suck.

 

[Xayma]

PS, is it me or do these new emoticons just suck.

I like them, except for looks like its wearing lipstick

 

[kpq_sniper017]

The question doesn't make sense if 4:12 am is the answer. If it is HIGH tide at 12:10 am, then the depth is 32 m, and the ship can safely enter the harbour at that time.

by using 12:10am as *low tide*, i got 4:12pm as the earliest time. earliest time is 4.02 hours after low tide at 12:10am.
and (i think) the latest time is 10h 32m after low tide i.e. 10:42pm.

 

[CM_Tutor]

You mean 4:12 am, not 4:12 pm, and 10 h 32 min doesn't sound right. If you take 12:10 am as low tide, then the times should be symmetrically distributed about the high tide at 6:40 am, so the second time should be 9:08 am.

 

[kpq_sniper017]

You mean 4:12 am, not 4:12 pm, and 10 h 32 min doesn't sound right. If you take 12:10 am as low tide, then the times should be symmetrically distributed about the high tide at 6:40 am, so the second time should be 9:08 am.

sorry, took 12:10 as being in the afternoon (stupid me). so yea, 4:12am.
with the 10h 32m, i wasn't too sure if that was right, but i used a formula to do it. i dunno if everybody used the same one as me:

x=x₀cosnt + v₀/n*sinnt.
where x₀ and v₀ are the initial velocity and displacement.

i took the first solution of the equation and found "t" to be 4.02. i then took the next value of "t" (i.e. in the next quadrant which satisfied the conditions) for which the equation worked. and i got 10.xx (can't remember exactly). so i'm not too sure where i went wrong. but, nevertheless, it wasn't asked in the question so it's not of great concern.

 

[Grey Council]

which school do you attend?

 

[:: ck ::]

so is 2:39 the same as wut u got grey?

 

[Grey Council]

i get 4:12

hrm, think about it, how could it be 2:39?
period is, what, 13 hours was it?
half that, 6.5 hours. It starts off at low tide, so at 12:10, it is lowest.
it has to rise above middle height, so it can't possilbe be before 12:10+3.25 or whatever.

 

[:: ck ::]

for the non-corrected question =.=

 

[Grey Council]

my apologies.

lol, I can't remember what I got. hrm, depends on how you approached it. :S
I thought they had given us high tide, so take AWAY 6.5 hours to find low tide, then find from what time the ship could enter harbour. So my answer was like before 12, not after. prolly 12 - (14:39 - 12:10)
heh Shoulda known a fortian maths freak like yourself couldna make a silly mistake like that.

 

[:: ck ::]

mm ill post it up tmrw =]

i did it on a scrap piece of paper so i mite have done sth wrong..

 

[Grey Council]

-.-"

Why are so many people wasting so much time on a simple waves question? One thats worded wrong too.

:-\

I'm thinking of deleting this whole thread. humph. Don't worry about it Ryan, I know what you did. Now, go do those induction questions.



hard, aren't they?
:-\
damn CM_Tutor. Or should I say, I love CM_Tutor. Best questions.

 

[CM_Tutor]

If we take low tide as 12:10 am, then the ship can enter after 4:12 am.

If we take high tide as 12:10 am, then the ship can enter up until 2:39 am.

Grey Council, please don't delete threads - they may have information that is useful to others.

 

[Grey Council]

fair enough, won't delete