Directional derivatives and the gradient vector
Recall that the directional derivative of $f$ at $(a,b)$ in the direction of an arbitrary nonzero vector $\mathbf u =(u_1,u_2)$ is given by
\begin{eqnarray*}
D_u f(a,b)&=&f_{\mathbb u}(a,b)=\big( f_x(a,b),f_y(a,b)\big)\cdot \frac{\mathbb u}{||\mathbb u||}
\end{eqnarray*}
On the other hand, the gradient vector or simply gradient of $f$ is a vector with the partial derivatives as components
\begin{eqnarray*}
\text{grad}\, f&=& \nabla f =\big( f_x,f_y\big)=f_x\,\mathbf i+f_y\,\mathbf j.
\end{eqnarray*}
Simulation
This simulation shows the geometric interpretation of the directional derivative of $f$ in the direction of a unit vector $\mathbf u$ and the gradient vector of $f(x,y)$ at the point $\text{P}\in \mathbb R^2$.
Things to try:
- Change the function f(x,y). Example: f(x,y)= y^2-(x-1)^2
- Drag the point P or type specific values on the boxes.
- Activate box Dir. derivative to show the directional derivative. Change the angle $\theta$ by dragging the slider or type an specific value on the box.
- Use right click and drag the mouse to rotate the 3D view or click on View button.
Note: The blue arrow represents the gradient vector at point P.
Sorry, the simulation is not supported for small screens.
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