Question

A car starts from rest and moves along the x axis with constant acceleration 5 m per second square for 8 seconds if it then continuous with constant velocity what distance will the car cover in 12 seconds since it started from the rest

Answer 1

for the first 8 seconds 
u=0 a=5m/s^2 t=8sec 
distance covered in first 8 secs is given by s=ut+1/2 at^2
s=0+1/2*5*8^2
s=180m (for the first 8 secs)
velocity v in first 8 secs = v
v=u+at
v=0=40
v=40m/s
after 8 secs there is no acceleration and the velocity is constant ie 40m/s
and it goes further for 4 seconds
therefore 
s=vt
s=40*4
s=160 m
therefore total displacement
s=180+160
s=340m