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Since the reaction time of the driver is t1, therefore the automobile, during this time, will continue to move with uniform velocity v1.
Distance covered during t1 is
s1=v1•t1.
After this the automobile begins to decelerate. Relationship between initial velocity v1 and final velocity v2=0, acceleration a and covered distance s2
s2=(v2^2-v1^2)/2a.
From Newton’s Second Law m•a=Ffr=k•N=k•m•g.
a=k•g,
where k is coefficient of friction, g is acceleration due to gravity.
Total distance travelled is
s = s1+ s2= v1•t1+ (v2^2-v1^2)/2•k•g