I have a question regarding basic econometrics. Consider the model $$y_i=\alpha +\beta x_i +u_i$$
I understand that assumption 4 of the linear regression model states
$$[1] \quad E(u|x)=0$$
However, I often see this condition written as:
$$[2] \quad E(ux)=0$$
Are these two things equivalent? I see that if [1] and $E(u)=0$ then we get [2]; however I don't understand why [2] would imply [1].