A car driver applied his brakes for 6 sec and slowed his 800kg car from 25m/s to 10m/s...?

Question

What is the average force appied on the brakes?

Answer 1

mass = 800kg , initial velocity(u) = 25m/sec ,

final velocity(v) = 10m/sec.

time taken for retardation(t) = 6 sec.

retardation = (v - u)/t

                    = (10 - 25)/6

                    =  -5/2

Force = mass * retardation

           = 800 * (-5/2)

           =  -2000 Newtons

(the minus (-) sign denotes the force acting in the direction opposite to the movement of the car)

Answer 2

F=m*a

a=dv/dt=(10-25)/6=-15/6m/s^2

so

F=800kg*(-15/6m/s^2)=-2000N

(The minus sign denotes that F has the opposite direction to the initial velocity)