Formula Info
\begin{aligned}F_\parallel&=mg\sin\theta\\F_\perp&=mg\cos\theta\\N&=mg\cos\theta\\f_k&=\mu N\end{aligned}
\begin{aligned}a_{ideal}&=g\sin\theta\\a_{sliding}&=g(\sin\theta-\mu\cos\theta)\\|f_s|&\le\mu N\end{aligned}
Ideal / frictionless model: friction is zero and a = g sin θ.
Downhill is positive. Negative sliding acceleration means slowing down. Once velocity reaches zero, the block stays at rest if friction can hold it; it never reverses uphill.
