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A car at 100 km/h carries 463 kJ. At 50 km/h — 116 kJ, a quarter. The squared velocity is why small speed differences decide crash outcomes.
KE = ½mv² looks gentle until you notice the square: energy grows four times faster than speed, which is why 80 vs 100 km/h is 56% more crash energy, not "a bit faster." This calculator shows the energy, the half-speed comparison, and ideal braking distances — the physics that road-safety campaigns are really about.
Drivers and physics students learn the same law: ½mv² — a car at 100 km/h carries 463 kJ, at 50 km/h a quarter of that. Speed kills by squaring.
Because both the momentum carried AND the distance over which work is done grow with speed. Deriving it: work = force × distance, and stopping from speed v with constant force takes a distance proportional to v² (since d = v²/2a) — multiply the two and energy inherits the square. The road-safety consequences are stark: 100 km/h carries 4× the energy of 50, so doubling speed quadruples both braking distance and crash severity; 130 vs 120 km/h is 17% more energy for 8% more speed. This is why urban 30 km/h zones save pedestrians — struck at 30 km/h a pedestrian survives ~90% of the time; at 50 km/h survival drops below 50%. The numbers are not linear because physics never agreed to be.
Stopping distance divides kinetic energy by braking force: d = v²/(2μg), where μ is tire-road friction. A car at 100 km/h (27.8 m/s) needs ~56 m on dry pavement (μ=0.7) and ~98 m on wet (μ=0.4) — before adding reaction distance (~21 m at 1.5 s), which grows only linearly. Both terms punish speed: at 120 km/h the dry total is ~106 m versus ~77 m at 100 — that extra 29 m is longer than a basketball court, decided in "just 20 over." Motorcycles and loaded trucks sit lower on friction or higher on mass-per-brake; ice (μ≈0.15) stretches 100 km/h stops to ~260 m. The calculator shows the ideal physics floor — real stopping always runs longer.
Scale anchors make joules intuitive. A 145 g baseball at 40 m/s (90 mph pitch): 116 J — hurts, rarely kills. A 9 g bullet at 400 m/s: 720 J — a quarter the baseball's momentum but 6× the energy, because velocity is squared; that asymmetry is why projectiles wound. A 70 kg runner at 5 m/s: 875 J. A 1,200 kg car: 116 kJ at 50 km/h, 463 kJ at 100, 720 kJ at 130. A 400 kg motorcycle-plus-rider at 100 km/h: 154 kJ — a third of the car, which is why bikes stop shorter but riders have no crumple zone. Lightning aside, the biggest KE most people meet is the family car at highway speed — roughly the energy of 640 baseball pitches, all in one bumper.