A point mass particle $m$ will slide down on incline with acceleration along the incline
$$\tag{1} a = g \sin\theta - \mu g \cos\theta, $$
where $\mu = \mu_k$ is kinetic friction between point mass and the incline surface, and $\theta$ is angle of the incline with horizontal direction. Previous equation becomes
$$\tag{2} a = g \sin\theta $$
for a frictionless incline.
- Carl Rod Nave, “Mass on Incline With Friction”, HyperPhysics, 2017, url http://hyperphysics.phy-astr.gsu.edu/hbase/mincl.html [20221129].