If a car moving at 80mph takes 400ft to stop with uniform acceleration after its brakes are applied, ?

Question

how far will it take to stop under the same conditions if its initial velocity is 40mph?

a) 20ft

b) 50ft

c) 100ft

d) 200ft

Answer 1

vf^2 = vi^2 + 2ax

-vi^2/2*a = x

vi = 80 mi/h * (5280 ft/1mi) * (1 h/3600 s)  = 117.33 ft/s

x = 400 ft

a = -17.2 ft/s^2

x = -(vi^2)/(2*a)  ... 40mph = 58.66 ft/s

x = 100 ft