braking force

Example: When a driver saw a blind man was walking in front of his car with mass 200kg 50m. The driver tried to brake the car as soon as possible and finally the car did not hit the man but touched him. Assume that before braking the speed of the car was 120km/h, what is the braking force.

Solution:
Consider the braking acceleration,
by v^2 - u^2 = 2as,
0 - [(120)/(3.6))^2 = -2(50)a
a = 11.1 ms-2

Applied Newton's Second Law,
F = ma
Therefore, the force required = 200(11.1) = 2220

Remark: sometimes you may need to find the acceleration by the other methods such as the equation v = u + at , s = ut + 0.5at^2 , etc.

braking force通常可用 F(force)=m(mass)a(acceleration)計,如係係take forword係+,咁a應該係negative,因為係deceleration.....
braking force is not equal to friction,for example:一架車加速行緊/行緊個時同道路可以係有friction,但冇braking force~