Speed Converter

The braking distance formula v²/(2μg) has a critically important implication: doubling speed quadruples braking distance. At 30 mph on dry asphalt (μ = 0.7), braking distance ≈ 13 meters. At 60 mph, braking distance ≈ 53 meters — four times longer for twice the speed. Add the reaction distance (speed × 1.5 seconds), and total stopping distance at 60 mph exceeds 80 meters on dry road and exceeds 110 meters on wet road. This is why speed limit reductions from 60 mph to 50 mph on wet motorways are not merely symbolic — the reduction in total stopping distance is approximately 30%. For pedestrians, the consequences are even more stark: collision at 40 mph is fatal in roughly 85% of cases; at 30 mph the fatality rate drops to approximately 45%; at 20 mph, to below 10%.

Speed is measured in different units depending on application, and the conversions between them arise from fundamental definitions. The meter per second is the SI base unit, derived from the definition of the meter and the second. Miles per hour dominates US and UK road transport because road distance has historically been measured in miles. Kilometers per hour is used in most of the world where the metric system is standard. Knots (nautical miles per hour) are used in maritime and aviation contexts because nautical miles are defined as 1/60 of a degree of latitude, making position and distance calculations at sea directly integrable with geographic coordinates. Mach number is dimensionless and context-specific — Mach 1 is 343 m/s at sea level and 20°C but only 295 m/s at cruising altitude where the air is colder. Understanding which unit to use in which context prevents errors in navigation, safety calculations, and engineering specifications.

When a moving object stops suddenly — in a collision — its kinetic energy must be dissipated into the surrounding materials: the crumple zone of a car, the deformation of a barrier, or the body of a pedestrian. The kinetic energy formula KE = ½mv² has two important implications. First, mass matters: a fully loaded semi-truck at highway speed carries roughly 30× the kinetic energy of a passenger car at the same speed, which is why collision severity between light and heavy vehicles is so asymmetric. Second, speed matters far more than mass: doubling speed (while keeping mass constant) quadruples kinetic energy, while doubling mass only doubles it. This is the physical basis for why speed is the primary factor in collision fatality rates, not vehicle size. Modern vehicle safety systems — crumple zones, airbags, seatbelts — work by extending the deceleration time (Δt in the impact force formula F = mv/Δt), spreading the same energy absorption over a longer period and thereby reducing peak force on occupants.