A block starts sliding when tan(θ) exceeds µ — so a 45° ramp needs µ ≥ 1 to hold.
Why it works
On an incline, gravity’s along-slope pull beats friction exactly at tan θ = µs. Tilting a surface until things slide is a legitimate way to measure friction.
Do it exactly
Estimate with the rule, then check it against the calculator that models it properly.