Question

What will be the average braking force needed to bring a car of mass 800kg traveling 72km/hr to a stop at a distance of 100m

Answer 1

F = m*a

First, we find the acceleration.

Use the following equation of motion:
2*a*s = v^2 - u^2
s = 100m, v = 0, u = 72km/hr
= 20m/s

=> 2*a*100 = 0 - (20)^2
=> a = -400/200
= -2m/s^2

The sign is negative because the acceleration is retarding in nature. Similarly, the force will also have a negative sign:

F = m*a
= 800*(-2)
= -1600N

Hence, the magnitude of the force required will be 1600N

Answer 2

and, there is another method.
starting velocity=20m/s, ending velocity=0, average velocty=10m/s
time to go 100m, time=distance/avgV=10sec
Vf=vi+at
0=20)a*10
a=-2m/s then us F=ma to get force.