Question

A motor car running at he rate of 7ms-1 can be stopped by applying brakes in 10m. Show that total resistance to the motion,when the brakes are on, is one fourth of the weight of the car.

Answer 1

Vi=7 m/s
Vf=0 m/s[ sinc it applies brakes]
d=10 m
Vf^2= Vi^2 +2ad
a= Vf^2- Vi^2 / 2d= - 49/ 2*20 = - 2.45 m/s^2
Now we consider g = 9.8 m/s², then we get
a=-g/4= -2.45 m/s^2
Hence the resistive force , F= ma= mg/4 [ one fourth of the weight of the car]