Question

This question is typical on some driver’s license exams: A car moving at 43 km/h skids

14 m with locked brakes.
How far will the car skid with locked brakes
at 129 km/h? Assume that energy loss is due
only to sliding friction.
Answer in units of m

Answer 1

Braking distance 126 meters

Step-by-step explanation:

Given:

V₁ = 43 km/h = 43 000 m / 3 600 s ≈ 12 m/s

D₁ =14 m

V₂ = 129 km/h = 129 000 m / 3 600 s ≈ 36 m/s

____________________

D₂ - ?

D₁ = V₁² / (2·a)

2·a·D₁ = V₁²

a = V₁² / (2·D₁)

D₂ = V₂² / (2·a) = V₂²·2·D₁ / (2·V₁²) =

= V₂²·D₁ / V₁² = D₁·(V₂ / V₁)²

D₂ = 14·(36 / 12)² = 14·3² = 126 m

The braking distance has increased 9 times!