Lesson 7 of 9 · 9 min
Stopping distance and reaction time
NCERT § "Kinematic Equations for Uniformly Accelerated Motion"
A child steps onto the zebra crossing ahead. How far Meera travels before stopping depends on two things, and one of them she cannot brake away.
The lesson in notes
In short
A vehicle moving at speed v₀ that brakes with constant retardation a stops in a distance d = v₀²/2a.
Because d varies as v₀², doubling the initial speed makes the stopping distance four times larger for the same braking.
Reaction time is the delay between seeing a signal and acting on it; during this time the vehicle keeps moving at its original speed, adding v₀ × (reaction time) to the stopping distance.
A simple way to estimate reaction time is to catch a falling ruler: if it falls a distance d before being caught, the reaction time is √(2d/g).
These are applications of the same constant-acceleration equations, not new laws.