Manipulate[
Animate[
Show[{
Plot[{},{x,-1,1},AspectRatio->1,AxesLabel->{"x","y"}],
Graphics[{
{Dashed,Gray,Circle[{0,0},0.5]},
{Red,Arrow[{{0,0},0.5{Cos[t+Δϕ],Sin[t+Δϕ]}}]},
{Blue,Arrow[{{0,0},0.5{Cos[t],-Sin[t]}}]},
{Darker[Green],Thick,Arrow[{{0,0},{Cos[((Δϕ)/2)],Sin[((Δϕ)/2)]}Cos[t+((Δϕ)/2)]}]}
}]
}],
{t,0.,2π}],
{{Δϕ,0,Dynamic["Δϕ = "<>ToString@Round[((Δϕ)/(2π))360,1.0]]},
0,2π},ControlPlacement->Top]