documentclass{article}
usepackage{amsmath}

begin{document}

begin{eqnarray}
  vec{F} &=& mvec{a}\
          && + frac{dm}{dt} vec{v}
end{eqnarray}

begin{align}
  vec{F} =& mvec{a}\
          & + frac{dm}{dt} vec{v}
end{align}

end{document}

documentclass{article}
usepackage{amsmath}

begin{document}

begin{eqnarray}
  vec{F} &=& mvec{a}\
          && + frac{dm}{dt} vec{v}
end{eqnarray}

begin{align}
  vec{F} &= mvec{a} \
          & phantom{=:} + frac{dm}{dt} vec{v}
end{align}

end{document}

documentclass{article}
usepackage{amsmath}

begin{document}

begin{eqnarray}
  vec{F} &=& mvec{a}\
          && + frac{dm}{dt} vec{v}
end{eqnarray}

begin{align}
  vec{F} &= mvec{a} \
         & qquad + frac{dm}{dt} vec{v}
end{align}

end{document}

$$eqalign{%
  (x+y)(x-y) &= x^2 -xy +yx -y^2cr
             &= x^2 -y^2cr
  (x+y)^2    &= x^2 +2xy +y^2cr
}$$
%
now, with ampersands and equal signs interchanged:
%
$$eqalign{%
  (x+y)(x-y) =& x^2 -xy +yx -y^2cr
             =& x^2 -y^2cr
  (x+y)^2    =& x^2 +2xy +y^2cr
}$$

bye

documentclass{article}
usepackage{amsmath}

begin{document}

begin{eqnarray}
  vec{F} &=& mvec{a}\
          && + frac{dm}{dt} vec{v}
end{eqnarray}

end{document}

documentclass{article}
usepackage{amsmath}

begin{document}

begin{align}
  vec{F} ={}& mvec{a}\
          & + frac{dm}{dt} vec{v}
end{align}

end{document}