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
$$ \quad E(u|x)=0$$
However, I often see this condition written as:
$$ \quad E(ux)=0$$
Are these two things equivalent? I see that if  and $E(u)=0$ then we get ; however I don't understand why  would imply .