# Vector Field

• March 18th 2011, 05:05 PM
heatly
Vector Field
If

F(x,y,z) = sinyi + (xcosy + z )j + yk

find a function f (x,y,z) such that

F = grad f

how would you go about solving this one?
• March 18th 2011, 06:24 PM
alexmahone
Quote:

Originally Posted by heatly
If

F(x,y,z) = sinyi + (xcosy + z )j + yk

find a function f (x,y,z) such that

F = grad f

how would you go about solving this one?

$\frac{\partial f}{\partial x}=sin y$

$f=xsin y+A(y,z)$

$\frac{\partial f}{\partial y}=xcos y+z$

$f=xsin y+yz+B(x,z)$

$\frac{\partial f}{\partial z}=y$

$f=yz+C(x,y)$

By inspection, we get $f=xsin y+yz$
• March 19th 2011, 05:29 AM
HallsofIvy
Perhaps a touch simpler: from $f_x= sin(y)$ we get $f= xsin(y)+ A(y,z)$

Now differentiate that with respect to y: $f_y= xcos(y)+ A_y= x cos(y)+ z$ so that $A_y= z$. That tells us that $A(y,z)= yz+ B(z)$. f(x,y,z)= x sin(y)+ A(y,z)= x sin(y)+ yz+ B(z).

Differentiating with respect to z, $f_z= y+ B'(z)= y$ so B'= 0 and B is a constant.

f(x,y,z)= x sin(y)+ yz+ B

Alex Mahone left off the constant because the problem only asked for "a" function having that gradient.
• March 19th 2011, 06:42 PM
xxp9
Let me add one:
let $\omega=f_xdx+f_ydy+f_zdz$. It's easy to verify that $d\omega=0$. So F is really conservative and the line integral of $\omega$ doesn't depend on the choice of path.
For any point $p=(x_0, y_0, z_0)$, Choose a path C with the following 3 segments:
I: from (0,0,0) to $(0, y_0, 0)$, along the y-axis
II: from $(0,y_0, 0)$ to $(0, y_0, z_0)$, along the direction of z-axis
III: from $(0, y_0, z_0)$ to $(x_0,y_0,z_0)$, along the direction of x-axis
The $f(p)=f(0)+\int_C \omega = f(0)+\int_I 0 dy + \int_{0}^{z_0} y_0 dz + \int_{0}^{x_0} sin(y_0)dx$
$=y_0 z_0 + sin(y_0) x_0 + f(0)$
• March 20th 2011, 02:51 AM
heatly
Thanks everybody for your help.
• March 21st 2012, 11:49 PM
JohnMboston
Re: Vector Field
I'm new to the forum.
Question: Can you please explain yow are you pasting in these nice italicized vector function equations?
I've fooled around with typing in Word and doing print screen paste or upload a jpeg but these don't seem to work.
Thanks.
• March 22nd 2012, 01:21 AM
alexmahone
Re: Vector Field
Quote:

Originally Posted by JohnMboston
I'm new to the forum.
Question: Can you please explain yow are you pasting in these nice italicized vector function equations?
I've fooled around with typing in Word and doing print screen paste or upload a jpeg but these don't seem to work.
Thanks.

Click "Reply With Quote" to see the source code of any particular post.
• March 22nd 2012, 02:58 AM
bling
Re: Vector Field
Quote:

Originally Posted by JohnMboston
I'm new to the forum.
Question: Can you please explain yow are you pasting in these nice italicized vector function equations?
I've fooled around with typing in Word and doing print screen paste or upload a jpeg but these don't seem to work.
Thanks.

there is also a application in start menu>>accessories>>math
input panel. use tht.
• March 23rd 2012, 12:50 AM
JohnMboston
Re: Vector Field
Thanks for the tip. The Math Input Panel works for me in, say, MS Word but I cannot make entries into a Forum message box. If I copy and paste from Word I lose things like the right-pointing vector line symbol above capital F or the hats above i+j in a vector function. Suggestions?
(Sorry, I'm an email guy, not a whiz at online forums, but I'm learning.)
• March 24th 2012, 02:50 AM
bling
Re: Vector Field
correct even i tried it but doesnt work. sorry mate if i find something ill post it