ruby operator overloading question

i've been messing around with ruby and opengl for entertainment purposes, and i decided to write some 3d vector/plane/etc classes to pretty up some of the math.

simplified example:

class Vec3
    attr_accessor :x,:y,:z

    def *(a)
        if a.is_a?(Numeric) #multiply by scalar
            return Vec3.new(@x*a, @y*a, @z*a)
        elsif a.is_a?(Vec3) #dot product
            return @x*a.x + @y*a.y + @z*a.z
        end
    end
end

v1 = Vec3.new(1,1,1)
v2 = v1*5 #produces [5,5,5]

which all fine and dandy, but i also want to be able to write

v2 = 5*v1

which requires adding functionality to Fixnum or Float or whatever, but i couldn't find a way to overload or extend fixnum's multiplication without replacing it entirely. is this possible in ruby? any tips?

(obviously i can just write all my multiplications in the correct order if i need to)