A block rests on a ramp whose angle you control. Gravity pulls the block straight
down, but only the component along the slope, mg·sinθ, tends to slide it.
Friction opposes that tendency. While the block is still, static friction
adjusts itself — up to a maximum of μₛ·N — to exactly cancel the sliding
component, so the block stays put. Once the required force exceeds that maximum, the
block breaks free and kinetic friction, a smaller, roughly constant
force of μₖ·N, takes over, letting the block accelerate down the slope.
θc = arctan(μₛ). Below this angle the block is held by static friction; above it, it slides.N = mg·cosθ, shrinking as the ramp steepens, which is why the maximum static friction also shrinks with angle.a = g(sinθ − μₖ·cosθ), always positive once motion starts because μₖ ≤ μₛ.Kinetic friction is almost always slightly lower than the maximum static friction for the same pair of surfaces — this is why a stuck object "breaks free" with a sudden jolt and then keeps moving more easily than it took to start.
A block sits on a ramp whose angle, and whose static and kinetic friction coefficients, you control — watch it stay locked in place by static friction until the incline gets steep enough, then suddenly break free and accelerate under kinetic friction.
Static friction self-adjusts up to μₛ·N to hold the block still; once gravity's along-slope component exceeds that maximum at the critical angle arctan(μₛ), the block slides under the smaller, roughly constant kinetic friction μₖ·N.
Drag the incline angle up until the block breaks loose, or pick a surface preset to load realistic μₛ/μₖ pairs. Live arrows show gravity, the normal force and friction; reset to put the block back at the top.
Because kinetic friction is normally lower than the maximum static friction, once an object breaks free it typically keeps accelerating rather than settling back into place — the classic "jolt and slide" you feel pushing a heavy box.