- #1
kingwinner
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Transformation/change of variables in this differential equation!?
Maybe my background is just weak...I was thinking about this for almost 1.5 hours already, but I still end up totally confused. Perhaps this is because I was never able to understand the ideas of a function and change of variables completely...perhaps I have a serious conceptual flaw.
Consider the following partial differential equation with initial values(IV) and boundary conditions(BC):
ut - k uxx = x + 2t, 1<x<7, t>0
BC: u(1,t) = u(7,t) = 0
IV: u(x,0) = x+5
Our goal is to transform the above to the interval [0,6].
Let w = x-1.
Transform the whole problem to the interval w E [0,6]. (write in terms of w)
2. Homework Equations /concepts
No knowledge of partial differential equation is needed. Anyone with a solid background in mutlivariable calculus and change of variables should be able to answer this question. (I think)
ux=uw dw/dx = (uw) (1) = uw
uxx = ...(apply chain rule again) = uww
[On the left side, think of u as u(x,t). On the right side, think of u as u(w,t)]
BC:
x=1 <=> w=0
x=7 <=> w=6
So the boundary conditions get transformed to u(0,t)=u(6,t)=0 [here think of u as u(w,t)]
IV:
We know u(x,0) = x+5
=> u(w,0) = w+5
[I believe the logic in this step cannot be wrong, consider e.g. f(4z)=cos(4z), now how do we find f(4z-y)? Of course, f(4z-y)=cos(4z-y). How do we find f(z)? Surely, f(z)=cos(z). Right??]
So my final answer is: [here think of u as u(w,t)]
ut - k uww = w+1+2t, 0<w<6, t>0
BC: u(0,t) = u(6,t) = 0
IV: u(w,0) = w+5
However, I really have some bad feeling that the result u(w,0) = w+5 is wrong, but I don't know where the mistake is.
I tried to calculate it in a different way and the answer is the same.
u(w,0)
= u(x-1,0) = (x-1)+5
=> u(w,0) = w+5
Can someone please kindly explain why and where my mistake is? What is the correct answer?
Any help is greatly appreciated!
Maybe my background is just weak...I was thinking about this for almost 1.5 hours already, but I still end up totally confused. Perhaps this is because I was never able to understand the ideas of a function and change of variables completely...perhaps I have a serious conceptual flaw.
Homework Statement
Consider the following partial differential equation with initial values(IV) and boundary conditions(BC):
ut - k uxx = x + 2t, 1<x<7, t>0
BC: u(1,t) = u(7,t) = 0
IV: u(x,0) = x+5
Our goal is to transform the above to the interval [0,6].
Let w = x-1.
Transform the whole problem to the interval w E [0,6]. (write in terms of w)
2. Homework Equations /concepts
No knowledge of partial differential equation is needed. Anyone with a solid background in mutlivariable calculus and change of variables should be able to answer this question. (I think)
The Attempt at a Solution
ux=uw dw/dx = (uw) (1) = uw
uxx = ...(apply chain rule again) = uww
[On the left side, think of u as u(x,t). On the right side, think of u as u(w,t)]
BC:
x=1 <=> w=0
x=7 <=> w=6
So the boundary conditions get transformed to u(0,t)=u(6,t)=0 [here think of u as u(w,t)]
IV:
We know u(x,0) = x+5
=> u(w,0) = w+5
[I believe the logic in this step cannot be wrong, consider e.g. f(4z)=cos(4z), now how do we find f(4z-y)? Of course, f(4z-y)=cos(4z-y). How do we find f(z)? Surely, f(z)=cos(z). Right??]
So my final answer is: [here think of u as u(w,t)]
ut - k uww = w+1+2t, 0<w<6, t>0
BC: u(0,t) = u(6,t) = 0
IV: u(w,0) = w+5
However, I really have some bad feeling that the result u(w,0) = w+5 is wrong, but I don't know where the mistake is.
I tried to calculate it in a different way and the answer is the same.
u(w,0)
= u(x-1,0) = (x-1)+5
=> u(w,0) = w+5
Can someone please kindly explain why and where my mistake is? What is the correct answer?
Any help is greatly appreciated!