Driving home from school one day, you spot a ball rolling out into the street (figure 5-21). you brake for 1.20 s, slowing your 950-kg car from 16.0 m/s to 9.50 m/s.
a) what was the average force exerted on your car during braking?
b) how far did you travel while braking?

A ) v = v o + a t  ( the acceleration will be negative )
9.50 = 16.0 + a * 1.2
a * 1.2 = -16.0 + 9.50
a * 1.2 = - 6.5 
a = - 6.5 : 1.2
a = - 5.4167 m/s²
F = m * a = 950 kg * 5.4167 m/s²
F = 5,145.8 N ( the average force exerted on a car during braking )
b ) d = v o - a t² / 2
d = 16.0 * 1.2 - ( 5.4167 * 1.2² / 2 ) =
= 19.20 - 3.90 = 15.30 m