Resolve the weight. The weight mg points straight down. Along the ramp it has a component mg sin θ (pulling the box down the slope), and into the ramp a component mg cos θ.

The normal force. The ramp pushes back at right angles to the surface: N = mg cos θ (there is no acceleration into the ramp).

Friction. It acts up the slope. When the box is sliding, f = μN = μmg cos θ. When the box is at rest, friction is only as big as it needs to be, mg sin θ, and it has a maximum of μN.

F = mg sinθ − μmg cosθ, so a = g (sinθ − μ cosθ)

The mass cancels. Try a different mass: the forces change, but the acceleration does not. The box starts to slide when tan θ is greater than μ.

Energy. The box loses gravitational potential energy mgh. Some becomes kinetic energy and the rest is heat from friction: mgh = ½mv² + f d. The bars show this adding up.

The box slides from rest along 2.0 m of the ramp. Time and speed use the exact results t = √(2d/a) and v = at.