Q.

A driver of a car A travelling at 52 km/h applies the brakes and accelerates uniformly in the opposite direction. The car stops in 5 s. Another driver B going at 30 km/h in another car applies his brakes slowly and stops in 10s. Which car travelled farther and at what distance?


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a

Car B, 41.5 m

b

Car B, 52 m

c

Car A, 46.2 m

d

Car A, 44.8 m 

answer is A.

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Detailed Solution

Car B will travel farthest at 41.5 m distance.
Given:
For the first car A,
Initial speed, u1 = 52 km/h
                     =52×10060×60
                     = 14.4 m/s
The final velocity, v1=0 in time = 5 sec  .
For second car B,
Initial speed, u2 =30 km/h
                             =30×100060×60
                             =8.3 m/s
The final velocity, v2=0 in time = 10 sec  .
For car A
we know
            v=u+at
         o=14.4+a×5 
              a= -14.45 m/s2 
We know
            s=ut+12at2
       s=14.4×5+12(-14.45)×52
           s=72-36
                   = 36 m
For car B
we know
            v=u+at
         =8.3+a×10 
    -8.310m/s2=
We know
              s=ut+12at2
         s=8.3×10+12(-8.310)×102
         s=83-41.5
             s= 41.5 m
 
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