How far will the car skid with locked brakes at 80 km/h ?

Question

This question is typical on some driver's license exams: A car moving at 50 km/h skids 16 m with locked brakes.

Answer 1

Use the kinematic relation

 

v_f² - v_i² = 2ax

 

v_f = final speed = 0 if it comes to a full stop

 

v_i = initial speed

 

a = acceleration

 

x = stopping distance

 

Get acceleration knowing that a car moving at 50 km/h skids 16 m with locked brakes.

 

50 km/hr = 50,000 m / (3600 s) = 13.89 m/s

 

a = (v_f² - v_i²) / (2x) = (0² - 13.89²) / (2*16) m/s² = -6.029 m/s²

 

Assume the brakes produce the same acceleration when starting from 80 km/hr.

 

80 km/hr = 80,000 m / (3600 s) = 22.22 m/s

 

x = (v_f² - v_i²) / (2a) = (0² - 22.22²) [2(-6.029)] m = 40.95 m