Force definitely does not equal mass times velocity. You have $F=ma,$ and this holds up and down the slope, as well as normal to the slope (that is,...
Start with the red vector, which is $\begin{bmatrix}x_1-x_0 \\ y_1-y_0\end{bmatrix}$. If you rotate it "to the left" (in other words, anticlockwise)...