Need help understanding an equation

Taylor's theorem.

[tex]\delta V = V \left( t + \delta t \right) - V \left( t \right) = \dot{V} \left( t \right) \delta t + \frac{1}{2!} \ddot{V} \left( t \right) \left (\delta t \right)^2 + \dots[/tex]

Neglect higher order terms, take [itex]t = 0[/itex], and use [itex]V \left( 0 \right) = 0[/itex].

 

In other words, that is expanding V(x) in a Taylor's series about x= t, then replacing x by [itex]t+ \delta t[/itex].

Actually, I don't believe they are taking "V(0)= 0". Since they are taking the Taylor's series about x= t, the constant term would be V(t) and they are subtracting off V(t).

[tex]V(x)= V(t)+ \dot{V}(t)(x- t)+ \frac{\ddot{V}(x- t)^2+ \cdot\cdot\cdot[/itex]
[tex]V(t+ \delta t)= V(t)+ \dot{V}(t)(t+ \delta t- t)+ \frac{\ddot{V}(t)}{2}(t+ \delta t- t)^2+ \cdot\cdot\cdot[/tex]
[tex]V(t+\delta t)= V(t)+ \dot{V}(t)\delta t+ \frac{\ddot{V}(t)}{2}(\delta t)^2+ \cdot\cdot\cdot[/tex]
so
[tex]V(t+ \delta t)- V(t)= \dot{V}(t)\delta t+ \frac{\ddot{V}(t)}{2}(\delta t)^2+ \cdot\cdot\cdot[/tex]

 

Oops, I meant to write

[tex]\dot{V} \left( 0 \right) = 0.[/tex]

From the link given in the original post:

"Fill the interior of the sphere with test particles, all of which are initially at rest relative to the planet. ... Thus, after a short time [itex]\delta t[/itex] has elapsed, ..."

 

Oh, I see. I hadn't looked at the original attachment.