The δ Notation in Calculus of Variations

2 replies · 2K views · last reply
sams
Gold Member
Messages
84
Reaction score
2
On page 224 of the 5th edition of Classical Dynamics of Particles and Systems by Stephen T. Thornton and Jerry B. Marion, the authors introduced the ##δ## notation (in section 6.7). This notation is given by Equations (6.88) which are as follows:

$$\delta J = \frac{\partial J}{\partial \alpha}d\alpha$$ $$\delta y = \frac{\partial y}{\partial \alpha}d\alpha$$

I know that the δδ notation stands for the variation from the actual path, but I cannot relate the geometrical interpretation to the above equation. Can anyone please explain the above terms and provide an explanation on why do the right-hand sides of these relations represent the variation (varied path) from the actual path?

Any help is much appreciated. Thank you so much.
 
Maybe this image I have found on Wikipedia helps a bit:
3D surface f(x, y) with a small green infinitesimal element at point (x, y, f) to (x+dx, y+dy, f+df), with dx and dy shown along the x and y axes; an inset shows the element with df = (∂f/∂x)dx + (∂f/∂y)dy.


(by Muhammet Cakir - Eigenes Werk ; https://de.wikipedia.org/wiki/Totales_Differential)
 
  • Like
Likes   Reactions: jedishrfu