Mathematica Series and Solve function - wolfram-mathematica

Question

This is my first mathmatica code,
I defined the functions:
\[Beta] := v/c
\[Gamma] := 1/Sqrt[1 - \[Beta]^2]
TotalE[\[Gamma][\[Beta]]] := \[Gamma]mc^2
KE := TotalE[\[Gamma][\[Beta]]] - mc^2
No i want to make a series expansion of KE at β → 0 up to order 2,
I tried:
Series[KE, {\[Beta], 1, 2}]
But i got the error massage:
General::ivar: v/c is not a valid variable.
I also wanted to define Ekin as function of β,
so i used Solve function to get the inverse function, β[Ekin]:
Solve[KE, \[Beta]]
The same errors arises again:
Solve::ivar: v/c is not a valid variable.

Answer 1

Try this
Clear[\[Gamma],\[Beta],mc,KE,s,v,c]
\[Gamma] = 1/Sqrt[1 - \[Beta]^2];
TotalE[\[Gamma]*\[Beta]] = \[Gamma]*mc^2;
KE = TotalE[\[Gamma]*\[Beta]] - mc^2;
s=Normal[Series[KE, {\[Beta], 1, 2}]]/.\[Beta]->v/c
Reduce[KE==0, \[Beta]]/.\[Beta]->v/c
which returns
O-mc^2 + mc^2/(Sqrt[2]*Sqrt[1 - v/c]) -
(mc^2*(-1 + v/c))/(4*Sqrt[2]*Sqrt[1 - v/c]) +
(3*mc^2*(-1 + v/c)^2)/(32*Sqrt[2]*Sqrt[1 - v/c])
and
(mc != 0 && v/c == 0)||(-1+v^2/c^2 !=0 && mc == 0)
What that is trying to do is do your calculations with the simple variable beta, before you turn that into v/c and after the calculations replace beta with v/c.
But there are still things about the way you have written that which worry me. You are kind of writing TotalE like it is a function, but that is not the way to define a Mathematica function and I am concerned this may be going to get you into trouble.
Please let me know if I have misunderstood some of what you are trying to do and explain what I've done wrong and I will try to find a way to fix that.