Distância de Frenagem

Braking distance from physics: kinetic energy meets friction

The formula is d = v² / (2 · μ · g), where μ is the friction coefficient and g = 9.81 m/s². Take a car doing 100 km/h (27.8 m/s) on dry asphalt (μ = 0.7): it needs roughly 55 m to come to rest. Make the road wet (μ = 0.4) and that jumps to 97 m. On ice (μ = 0.1) you're looking at 390 m. ABS helps by modulating brake pressure so the tires stay near peak grip, and the difference between a new tire and a bald one can easily double the figure. One thing the formula leaves out is reaction distance, which is your reaction time (somewhere around 0.7-1.5 s) multiplied by the speed. Add that in and you get the real total stopping distance behind the wheel.

Applications

Driver education and ANTT/CETRAN training rely on it, as does traffic-accident forensics. It also backs up the two-second following-distance rule, defensive driving courses, and the kind of kinematics problem you get for physics homework.

FAQ

Why does the distance scale with v²? Kinetic energy is ½mv², and the brakes have to dissipate every joule of it. Double the speed and you quadruple the distance.

What μ should I use in real life? Rough guides: dry asphalt 0.6-0.8, wet 0.3-0.5, gravel 0.4, packed snow 0.2, ice 0.05-0.15. Your own tires are the final word, so test when you can.

Does ABS shorten the distance? On dry pavement the gain is small, maybe 5-10%, though it does keep you in control of the steering. On wet or loose surfaces the payoff is bigger and it can cut the stop by 30%.