Question on partial derivative

In summary, the conversation is about a partial derivative that was used in a homework assignment. The person is unsure about the assumption made and is asking for someone to prove that it is zero. They mention that the partial derivative came up in their proof about fluid dynamics and that u is the velocity. The person also shares the term they tried to prove being positive and asks if the term ∂iuj ∂jui can be set equal to zero and why.
  • #1
Samoht
2
0
I just handed in a homework where I used the assumption below

iujjui=0 ?

but when I start thinking about it I'm not so sure, could someone prove to me that it is zero? Or is that assumption totally off?

Regards
 
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  • #2
Is there summation over i and j?
Is u something special, or just an arbitrary vector?
 
  • #3
This partial derivative came up in my proof that the rate of strain in fluid dynamics is always positive. u is the velocity and I believe that there is summation but I am very new at this.

The term I tried to prove being positive is:

iuj((∂iuj+∂jui)-2/3 ∂kuk δij)

When I multiply ∂iuj into the outer bracket then I get

(∂iuj(∂iuj+∂jui)-2/3 ∂kuk δijiuj)

which in my mind becomes

(∂iujiuj+∂iujjui)-2/3 ∂kuk δijiuj

the right side of the minus sign can be simplified to 2/3 (∂kuk)2

My question relates to the second term in the remaining bracket. Can the term ∂iujjui be set equal to 0? AND, if so why?

Regards
 

1. What is a partial derivative?

A partial derivative is a mathematical concept that measures the rate of change of a function with respect to one of its variables, holding all other variables constant. It is denoted by ∂ (pronounced "partial") and is commonly used in multivariable calculus and physics.

2. How is a partial derivative calculated?

A partial derivative is calculated by taking the derivative of a function with respect to the specific variable of interest, while treating all other variables as constants. This is done by using standard differentiation rules, such as the power rule or chain rule.

3. What is the difference between a partial derivative and a total derivative?

The main difference between a partial derivative and a total derivative is the number of variables involved. A partial derivative only considers the change in one variable, while holding all others constant. A total derivative takes into account the change in all variables at the same time.

4. How are partial derivatives used in real life?

Partial derivatives are used in many fields of science and engineering to model and analyze complex systems. They are particularly useful in physics and economics, where multiple variables are often involved. For example, in economics, partial derivatives can be used to calculate marginal cost and revenue.

5. Can a partial derivative be negative?

Yes, a partial derivative can be negative. This indicates that the function is decreasing in the direction of the variable of interest. However, it is important to note that a negative partial derivative does not necessarily mean that the overall function is decreasing. The function may still be increasing in the direction of other variables.

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