a = (vf-vo)/t
re-arranging t = (vf-vo)/a = (0-20.8333)/a = (-20.8333)/a 75 km / hr = 20.8333 m/s
xf = xo + vo t + 1/2 a t^2 Substitute the value of t above
135 = 0 + (20.8333)(-20.8333/a) + 1/2 a (-20.8333/a)^2
135 = -434/a + 1/2 a (434/a^2)
135 = -434/a + 217/a
135 = -217/a a = -1.607 m/s^2
F= ma
F = 750(-1.607) = -1205 N
B. I find a = -108.49 m/s^2 Use this in F=ma F = -81370 N
C. ratio 81370/1205 = 67.5 times higher
(NOTE : this is the same as the ratio of the stopping distances 135/2 = 67.5)