- #36
lioric
- 306
- 20
Are you asking me to subtract the new coordinates of P and Q or the initial coordinates given in the question?
OK, never mind about the i and j comments.lioric said:I m very confused
In post #25 it seems that the vectors are in the format that you asked (...)i+(...)j and you asked me to simply them. So I expanded them and now your asking me to revert them back again.
That's what I was clarifying in post 36LCKurtz said:OK, never mind about the i and j comments.
This thread is getting very jumbled. There are multiple arithmetic errors and/or typos. In post #25 you have an arithmetic mistake in the new position of P. Then in post #32 you carried the same mistake through. Also in post #32 you dropped the ##t## in the calculation of Q-P, as I told you in post #33. Fix those. Once you finally get Q-P correct, with a ##t## in it, it is a simple one more step to finish your problem.
Think about the problem you are trying to solve. You are given original positions P and Q and their velocities. They are moving. Then you are asked at what time ##t## is Q northeast of P. So you need their positions at time ##t## don't you? So you need to know for what ##t## does Q-P point northeast. Fix your arithmetic and show Q-P as a function of ##t## and you are almost done.lioric said:That's what I was clarifying in post 36
The initial P and Q position vectors given in the question has no t in it
So which P and Q are you referring to?
Is it the answers on post 25?
LCKurtz said:Think about the problem you are trying to solve. You are given original positions P and Q and their velocities. They are moving. Then you are asked at what time ##t## is Q northeast of P. So you need their positions at time ##t## don't you? So you need to know for what ##t## does Q-P point northeast. Fix your arithmetic and show Q-P as a function of ##t## and you are almost done.
I am not going to go off on another tangent with you. You have understood the problem well enough to draw a picture long ago in post #19. Just finish the problem as I have suggested to you.lioric said:Ok
Let's think here
I'm just trying to clear my confusion please do tell if I'm thinking right
Let's keep the vector situation and go to a simpler linear one
Let's say P is on (1,1) on a grid
And Q is on ( 5,-5) on the same grid
P is moving on positive x-axis (to the right) at let's say 1 unit/s
And Q is moving in the positive y-axis (upward) 1 unit/s
Let's say the question is asking to find the time that P moves east of Q
Is this the same or similar sort of thing?
LCKurtz said:I am not going to go off on another tangent with you. You have understood the problem well enough to draw a picture long ago in post #19. Just finish the problem as I have suggested to you.
It should point to somewhere in the first quadrant meaning it should not have a bearing exceeding 90 degreesLCKurtz said:Yes. Finally! Now, for what ##t## does that point northeast? What do you require about the components for a vector to point northeast?
So that means as I said before bearing of 45 degreesLCKurtz said:I would assume in this problem that the meaning of northeast means exactly northeast, that is midway between north and east.
Yes. And the question asks at what time does that happen? So...lioric said:So that means as I said before bearing of 45 degrees
LCKurtz said:Yes. And the question asks at what time does that happen? So...
Ya the 3i+3jLCKurtz said:Do you know how to tell if any vector points NE? Which, if any, of the following vectors point NE?
3i - 5j
2i +3j
-i - 2j
3i + 3j
i - j
-2i + 2j
Explain how you can tell.
Multiply by sin45?LCKurtz said:OK. So what, if any, value of ##t## makes Q-P point NE? That answer is the solution to your problem. Although the problem may want to know what time of day is it, given it started at noon.
That answer makes no sense at all. You are asked for a value of ##t## and you reply "multiply by sin45".lioric said:Multiply by sin45?
sorryLCKurtz said:That answer makes no sense at all. You are asked for a value of ##t## and you reply "multiply by sin45".
lioric said:Now Q-P
From post #43:lioric said:(2+t)i + (10t-15)j
What relationship has to exist between the i and j coordinates for a vector to point northeast?LCKurtz said:Yes. Finally! Now, for what t does that point northeast? What do you require about the components for a vector to point northeast?
They have to be positive and same magnitudeMark44 said:From post #42:From post #43:
What relationship has to exist between the i and j coordinates for a vector to point northeast?
You could have answered that way back when I asked it in post #43.lioric said:They have to be positive and same magnitude
lioric said:Now Q-P
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(2+t)i + (10t-15)j
Mark44 said:What relationship has to exist between the i and j coordinates for a vector to point northeast?
So what do you get?lioric said:They have to be positive and same magnitude
Mark44 said:From post #42:So what do you get?
I get it nowLCKurtz said:Put ##t=1## in your vector. Does it point NE? How about ##t = 2## or ##t=3## etc. The question is, is there any value of ##t## that gives a vector that points NE.