Understanding the Dot Product Formula: An Explanation and Example

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Discussion Overview

The discussion revolves around understanding the dot product formula, particularly in the context of differentiating the expression involving velocity and its time derivative. Participants explore the relationship between the dot product and the product rule in calculus, seeking clarification on specific steps in the derivation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about the equation v ⋅ dv/dt equating to 1/2 d(v^2)/dt, referencing a book that states F=mv'=P'.
  • Another participant suggests differentiating the equation v^2 = v ⋅ v to clarify the relationship, indicating that the time derivative involves both terms of the product.
  • A participant confirms understanding of the differentiation rule d(x ⋅ y)/dt = x' ⋅ y + x ⋅ y', noting that this makes the left-to-right derivation easier but requires practice to recognize.
  • A later post inquires about the Cartesian component form of the expression ##\vec{v}\centerdot d\vec{v}##, indicating a shift towards a more specific mathematical inquiry.

Areas of Agreement / Disagreement

Participants generally agree on the application of the product rule for differentiation, but there remains some uncertainty regarding the specific steps and interpretations of the dot product in the context of the original equation.

Contextual Notes

The discussion does not resolve the initial confusion about the equivalence of the expressions, and assumptions regarding the definitions of the variables and their derivatives are not fully articulated.

Who May Find This Useful

Readers interested in mathematical physics, particularly those looking to deepen their understanding of vector calculus and the application of the dot product in dynamics.

Seung Ju Yoo
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In a book I was reading, it says
F=mv'=P'

so dot producting on both sides with v

Fv = mv ⋅ dv/dt = 1/2 m d(v2)/dt = d(1/2 m v^2)/dtI really don't get how v ⋅ dv/dt = 1/2 d(v2)/dt.
I have seen few threads and they say it's about product rule, but they don't really explain in detail.

Could anyone help me with this?
 
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Seung Ju Yoo said:
In a book I was reading, it says
F=mv'=P'

so dot producting on both sides with v

Fv = mv ⋅ dv/dt = 1/2 m d(v2)/dt = d(1/2 m v^2)/dtI really don't get how v ⋅ dv/dt = 1/2 d(v2)/dt.
I have seen few threads and they say it's about product rule, but they don't really explain in detail.

Could anyone help me with this?

Welcome to PF!

##v^2 = \textbf{v.v}##

Can you now differentiate that equation?
 
V^2 = V . V

And the time derivative of it is V' . V + V . V'
 
PeroK said:
Welcome to PF!

##v^2 = \textbf{v.v}##

Can you now differentiate that equation?
jedishrfu said:
V^2 = V . V

And the time derivative of it is V' . V + V . V'
Oh.. I see. I did not now that d(x ⋅ y)/dt = x' ⋅ y + x ⋅ y'

Knowing this, going right from left is easy, but I guess going left to right needs some practice to spot!

Thank you both peroK and Jedishrfu!
 
In Cartesian component form, what is ##\vec{v}\centerdot d\vec{v}##?

Chet
 

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