Braking: finding deceleration
A car traveling at 20 m/s brakes to a stop over 40 m at constant deceleration. What is the magnitude of its deceleration?
Answer C: explanation for every choice
A
This divides speed by distance (v/d), which gives units of s⁻¹.
B
This divides by 4d instead of 2d.
C
Correct. v² = v₀² − 2ad, so a = v₀²/(2d) = 400/80 = 5 m/s².
D
This leaves out the factor of 2: v₀²/d.
TakeawayWith no time given, use v² = v₀² + 2ad.
Equation usedVelocity–displacement relation
See your set score →Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID cp56.