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Active: 32804 users

Physics HW help plz

Blogs > BluzMan
Post a Reply
1 2 Next All
BluzMan
Profile Blog Joined April 2006
Russian Federation4235 Posts
October 28 2008 12:28 GMT
#1
My professor for physics is a real bitch, he gives rather complicated HW, although I'm not nearly a good student and want to drop physics altogether.

This time I need to find the general solution for this pair of equations:

[image loading]

[image loading]


It seems quite easy at first, but I'm having some trouble trying to find the exact form of the solution. Help please, I need to get it done tomorrow or I will have troubles graduating.

You want 20 good men, but you need a bad pussy.
Chill
Profile Blog Joined January 2005
Calgary25979 Posts
Last Edited: 2008-10-28 13:22:23
October 28 2008 12:35 GMT
#2
ㅡ_ㅡ;;

Edit: Please start the problem, or indicate the areas you are having trouble with. "SOLVE THIS OMG OLOLOL" is not an acceptable OP.
Moderator
MasterReY
Profile Blog Joined August 2007
Germany2708 Posts
October 28 2008 13:23 GMT
#3
LAWL
EPIC SMILEY !

to topic: a agree with chill
https://www.twitch.tv/MasterReY/ ~ Biggest Reach fan on TL.net (Don't even dare to mention LR now) ~ R.I.P Violet ~ Developer of SCRChart
TL+ Member
jgad
Profile Blog Joined March 2008
Canada899 Posts
October 28 2008 13:25 GMT
#4
Strange to write the divergence as V•∇ - it's usually written with the gradient first, but dot product is commutative so it should be zero in any case. That makes the whole first term zero, so you essentially have to solve

∂V/∂t = v∇V - 1/ρ∇P + ƒ

The second term is independent of V (so it seems), so you just have to solve the basic form

∂V/∂t = v∇V - C

Which should be pretty straightforward.
콩까지마
Cambium
Profile Blog Joined June 2004
United States16368 Posts
October 28 2008 14:04 GMT
#5
FUck, i miss the days when i used to know stuff (and cared about capitalization on tl -.-)
When you want something, all the universe conspires in helping you to achieve it.
Maenander
Profile Joined November 2002
Germany4926 Posts
Last Edited: 2008-10-28 14:06:58
October 28 2008 14:06 GMT
#6
On October 28 2008 22:25 jgad wrote:
Strange to write the divergence as V•∇ - it's usually written with the gradient first, but dot product is commutative so it should be zero in any case. That makes the whole first term zero, so you essentially have to solve

∂V/∂t = v∇V - 1/ρ∇P + ƒ

The second term is independent of V (so it seems), so you just have to solve the basic form

∂V/∂t = v∇V - C

Which should be pretty straightforward.

dot product is commutative for normal vectors, which contain (commutative) scalars and not derivatives, which are of course not commutative.

I wish I could help you Bluzman, but I did this stuff long time ago and I am not going to delve into it again for this one problem, it takes too much time.

Maybe you can simplify it by using the epsilon tensor (Levi-Civita symbol) and writing it out.
jgad
Profile Blog Joined March 2008
Canada899 Posts
October 28 2008 14:12 GMT
#7

dot product is commutative for normal vectors, which contain (commutative) scalars and not derivatives, which are of course not commutative.


Yeah, I thought of this too, but it would seem that the first term would end up being

-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)V

anyway, which would come out as :

-V(∇•V)

so I think -(V•∇)V = -V(∇•V) = 0 (given the second equation)

I could be wrong.
콩까지마
Maenander
Profile Joined November 2002
Germany4926 Posts
October 28 2008 14:18 GMT
#8
On October 28 2008 23:12 jgad wrote:
Show nested quote +

dot product is commutative for normal vectors, which contain (commutative) scalars and not derivatives, which are of course not commutative.


Yeah, I thought of this too, but it would seem that the first term would end up being

-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)V

anyway, which would come out as :

-V(∇•V)

so I think -(V•∇)V = -V(∇•V) = 0 (given the second equation)

I could be wrong.

You are wrong, it is

-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)Vx,
-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)Vy,
-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)Vz,

which is a vector in itself and that vector does not equal (0,0,0), atleast not because ∇•V=0 (scalar!)
Chill
Profile Blog Joined January 2005
Calgary25979 Posts
October 28 2008 14:24 GMT
#9
lol i got owned
Moderator
jgad
Profile Blog Joined March 2008
Canada899 Posts
Last Edited: 2008-10-28 14:40:36
October 28 2008 14:40 GMT
#10
On October 28 2008 23:18 Maenander wrote:
Show nested quote +
On October 28 2008 23:12 jgad wrote:

dot product is commutative for normal vectors, which contain (commutative) scalars and not derivatives, which are of course not commutative.


Yeah, I thought of this too, but it would seem that the first term would end up being

-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)V

anyway, which would come out as :

-V(∇•V)

so I think -(V•∇)V = -V(∇•V) = 0 (given the second equation)

I could be wrong.

You are wrong, it is

-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)Vx,
-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)Vy,
-(Vx∂/∂x + Vy∂/∂x + Vz∂/∂z)Vz,

which is a vector in itself and that vector does not equal (0,0,0), atleast not because ∇•V=0 (scalar!)


Yeah, you're right. So the first coordinate in the first term reduces to

-(Vx∂/∂x + Vy∂/∂y + Vz∂/∂z)Vx
= -Vx∂/∂x(Vx) - Vy∂/∂y(Vx) - Vz∂/∂z(Vx)
= -Vx(1) - Vy(0)- Vz(0)
= -Vx

and the same for the rest (y and z), so -(V•∇)V = -V, it would seem?

Anyway, I just noticed that the second term in the OP is ∆V and not ∇V - this seems confusing as it's either a typo or we need to know what ∆V is a delta of (V(t2) - V(t1)) or (V(x2) - V(x1)) - it could be anything.
콩까지마
Maenander
Profile Joined November 2002
Germany4926 Posts
Last Edited: 2008-10-28 14:52:54
October 28 2008 14:50 GMT
#11
∆ is the Laplace-Operator, I guess in the US you would write ∇ ^2.
And I don´t see why
-Vx∂/∂x(Vx) - Vy∂/∂y(Vx) - Vz∂/∂z(Vx) = -Vx(1) - Vy(0)- Vz(0)
should be correct.
edit: And I agree with chill here, Bluzman should show us his work, so we can point out errors. Again I am too rusty and too lazy to solve this (I should do other things anyway ^^).
micronesia
Profile Blog Joined July 2006
United States24660 Posts
October 28 2008 14:52 GMT
#12
On October 28 2008 23:24 Chill wrote:
lol i got owned

I don't see you getting pwned. How did you get pwned?
ModeratorThere are animal crackers for people and there are people crackers for animals.
BuGzlToOnl
Profile Blog Joined November 2006
United States5918 Posts
October 28 2008 15:02 GMT
#13
Just interested in knowing what level physics this is?
If you want to make God laugh, tell Him your plans.
jgad
Profile Blog Joined March 2008
Canada899 Posts
Last Edited: 2008-10-28 15:20:36
October 28 2008 15:10 GMT
#14
looks like ~third year electrodynamics. Incompressible vector fields (∇•V=0) define the magnetic field or the electric field in regions of no space charge, for example.

콩까지마
jgad
Profile Blog Joined March 2008
Canada899 Posts
Last Edited: 2008-10-28 15:17:42
October 28 2008 15:12 GMT
#15
On October 28 2008 23:50 Maenander wrote:
∆ is the Laplace-Operator, I guess in the US you would write ∇ ^2.


Weird, and yeah, I've only ever seen it as ∇^2.

Anyway, I agree that more information is needed about what these fields are supposed to be - if these are all general quantities which could be anything then I think the solution is going to be really messy.

And I don´t see why
-Vx∂/∂x(Vx) - Vy∂/∂y(Vx) - Vz∂/∂z(Vx) = -Vx(1) - Vy(0)- Vz(0)
should be correct.


Yeah, I'm trying to all of this too quickly. It occured to me just after I posted it, but yeah, there's no reason why V can't be Vx(x,y,z), Vy(x,y,z), Vz(x,y,z), in which case this all gets really horrible.
콩까지마
BluzMan
Profile Blog Joined April 2006
Russian Federation4235 Posts
October 28 2008 15:40 GMT
#16
Oh please help me, I'm so lost...
You want 20 good men, but you need a bad pussy.
BuGzlToOnl
Profile Blog Joined November 2006
United States5918 Posts
October 28 2008 16:07 GMT
#17
On October 29 2008 00:10 jgad wrote:
looks like ~third year electrodynamics. Incompressible vector fields (∇•V=0) define the magnetic field or the electric field in regions of no space charge, for example.



Fun, fun.
If you want to make God laugh, tell Him your plans.
cgrinker
Profile Blog Joined December 2007
United States3824 Posts
October 28 2008 16:21 GMT
#18
On October 28 2008 23:24 Chill wrote:
lol i got owned


I thought the smiley was quite nice actually. Aren't you only supposed to police the strat forums though?
cgrinker
Profile Blog Joined December 2007
United States3824 Posts
October 28 2008 16:25 GMT
#19
I've heard that in Russia you have to get the right answer to math problems. It must be nice eto have a mathematics conceptual system that isn't dumb as hell.
jgad
Profile Blog Joined March 2008
Canada899 Posts
October 28 2008 16:38 GMT
#20
I know what this equation is - this is fluid mechanics, the Navier-Stokes equation! Good luck finding solutions - there's a million dollar prize if you do.

Got $1m?
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