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March 21, 2026, 01:48:15 am

Author Topic: A few Specialist Problems  (Read 20636 times)  Share 

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GerrySly

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Re: A few Specialist Problems
« Reply #60 on: July 21, 2009, 02:36:39 pm »
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A container intially contains 500L of pure water. Salt water is being poured in at 5L/min with 10kg salt concentrate. It is leaving the container at 10L/min. Assume m kg is the amount of salt in the container at time t hours, find

That was a paraphrased question from our analysis task today (don't have the actual test with me)

Here is how I solved it



I am not sure if I went correct or not, because I had another equation to begin with but I scrapped it for this approach.

Note: My test is finished so you are not actually helping me complete a test, just helping me see my mistakes
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kamil9876

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Re: A few Specialist Problems
« Reply #61 on: July 21, 2009, 02:43:18 pm »
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Quote
10kg salt concentrate

if 10kg/L then your inflow is correct.

Your outflow is wrong however:




You assumed V is a constant 500, but really it changes over time: V=500+5t-10t=V=500-5t
Voltaire: "There is an astonishing imagination even in the science of mathematics ... We repeat, there is far more imagination in the head of Archimedes than in that of Homer."

GerrySly

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Re: A few Specialist Problems
« Reply #62 on: July 21, 2009, 03:05:29 pm »
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Quote
10kg salt concentrate

if 10kg/L then your inflow is correct.

Your outflow is wrong however:




You assumed V is a constant 500, but really it changes over time: V=500+5t-10t=V=500-5t
Hmmm, that sucks, thanks kamil
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GerrySly

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Re: A few Specialist Problems
« Reply #63 on: July 22, 2009, 05:02:08 pm »
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A particle travels in a path such that the position vector at time is given by

If the positive y axis points north and the positive x axis point east, find correct to two decimal places, the bearing of point , the position of the particle at from:

The origin

Alright I was able to find this one out fairly easily, I found the point and the quadrant the point was in, 4th quadrant point . Then I knew that the angle I find needs to add to it, so I went..



I then added to it and I got which is the answer.

Now here is the part I was having trouble with

The initial position

The initial position vector was which I then converted to cartesian form to find the bearing and I got . Now my assumption was because it was on the x-axis I just subtract from my previous answer to get the bearing but that gave me a different answer.

Appreciate any help :)
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dcc

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Re: A few Specialist Problems
« Reply #64 on: July 22, 2009, 06:56:30 pm »
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?

GerrySly

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Re: A few Specialist Problems
« Reply #65 on: September 02, 2009, 09:35:14 pm »
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Wow it's been a while since I've hit the Spec books heh, started to get into revision mode and hit a problem straight up with Vectors heh

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kamil9876

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Re: A few Specialist Problems
« Reply #66 on: September 02, 2009, 10:18:42 pm »
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tricky one. Use the cosine rule:

(i will denote dp as that dot product in the equation given).

Verify that the following two equations are true by drawing a diagram and applying the cosine rule twice:










hence two sides are of equal length, but the other pair is not, hence isosceles.
Voltaire: "There is an astonishing imagination even in the science of mathematics ... We repeat, there is far more imagination in the head of Archimedes than in that of Homer."

GerrySly

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Re: A few Specialist Problems
« Reply #67 on: September 18, 2009, 11:11:34 am »
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Ok I am fine with doing everything up to part c. I am just not sure about the wording. What does "The Spider decides to continue along AB to join MN" mean mathematically? Scalar multiples, perpendicular? Just not sure...
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GerrySly

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Re: A few Specialist Problems
« Reply #68 on: September 18, 2009, 06:27:38 pm »
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Also another question, consider the following shape...



If we were asked to find is it technically wrong if we expanded like so? because I keep going a different way to the book and get a different answer only because I take a different router (the book expands ) I realise why it is silly to do it my way in this case (we are told to find earlier) but in an exam if we have enough information for both ways will the examiners have two answers or only the one and if we go the wrong way they will mark it incorrect?
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TrueTears

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Re: A few Specialist Problems
« Reply #69 on: September 18, 2009, 06:38:41 pm »
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I don't think .

I think book is wrong as well

In any case even if you take a different route the final answer should always be the same.

Say you did

This would yield the exact same answer if you did
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biggzee

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Re: A few Specialist Problems
« Reply #70 on: September 18, 2009, 07:15:30 pm »
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(Image removed from quote.)

Ok I am fine with doing everything up to part c. I am just not sure about the wording. What does "The Spider decides to continue along AB to join MN" mean mathematically? Scalar multiples, perpendicular? Just not sure...

 allow "OA +mAB = OM + nMN" where m is any value of R, and n is between 0 and 1. solve in 3 dimensions i, j, and k
2008: Methods [49], Further [50]
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GerrySly

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Re: A few Specialist Problems
« Reply #71 on: September 18, 2009, 07:43:19 pm »
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I don't think .

I think book is wrong as well

In any case even if you take a different route the final answer should always be the same.

Say you did

This would yield the exact same answer if you did
Ok that makes sense now, I should be getting the answer no matter what (I think I typed it up wrong but you gave me a good explanation anyway thanks :))



Another one I am having trouble with, question a.ii in particular

Ok I can get as far as this...



Now there answer is but I not sure how they got the minus, aren't all angles within an equilateral triangle 60 degrees?  How are they getting 120? Wouldn't that be saying it isn't an equilateral triangle?

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bem9

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Re: A few Specialist Problems
« Reply #72 on: September 18, 2009, 07:50:08 pm »
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angle has to be tail to tail vectors so the angle is 120 degrees

GerrySly

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Re: A few Specialist Problems
« Reply #73 on: September 18, 2009, 07:53:53 pm »
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angle has to be tail to tail vectors so the angle is 120 degrees
I could kiss you right now, sitting here studying for 11 hours straight makes things seem that much harder :P
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GerrySly

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Re: A few Specialist Problems
« Reply #74 on: September 20, 2009, 02:37:47 pm »
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