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can you provide a shortcut for the following sum?

  1. on a 2-lane road, car A is travelling with a speed of 36kmph. Two cars B & C approach A in opposite directon with speed of both being 54kmph. At a certain instant when distance AB=AC,both beng 1 km,B decides to overtake A before C does.The minimum required acceleration of car B to avoid an accident is?

Asked by imtiyazmulla68 4th March 2018, 9:32 PM
Answered by Expert
Answer:
Speed of A, 36 km/hr = 10 m/s
Speed of B and C , 54 km/hr = 15 m/s
 
Let time taken by B to cross A before C with minimum acceleration is t seconds.
 
A travel 10×t m in t seconds.
If B crosses A with v m/s then the distance travelled by B in t seconds is ( (v+15)/2 )×t ; we have assumed average velocity of B is (v+15)/2 m/s.
 
Since the total distance travelled by A and B is 1 km, we write
 
10×t + ( (v+15)/2 )×t = 1000
 
let us eliminat t using t = (v-15)/a, where a is the minimum acceleration of B;
 
begin mathsize 12px style 10 open parentheses fraction numerator v minus 15 over denominator a end fraction close parentheses plus open parentheses fraction numerator 15 plus v over denominator 2 end fraction close parentheses open parentheses fraction numerator v minus 15 over denominator a end fraction close parentheses space equals space 1000 20 open parentheses v minus 15 close parentheses plus open parentheses v squared space minus space 225 close parentheses space equals space 2000 cross times a b u t space open parentheses v squared space minus space 225 close parentheses space equals space 2 a left parenthesis 1000 minus 10 t right parenthesis space space equals space 2000 cross times a space minus space 20 cross times t semicolon space left parenthesis space w e space h a v e space u s e d space v squared equals u squared plus 2 space a space S space f o r m u l a right parenthesis 20 left parenthesis v minus 15 right parenthesis space equals space 20 cross times t semicolon left parenthesis v minus 15 right parenthesis space equals t semicolon b ut space using space the space equation space relating space straight v space and space acceleration comma space we space have space left parenthesis straight v minus 15 right parenthesis space equals space a cross times t semicolon hence space at space equals space straight t space comma space or space straight a equals 1 space m divided by s squared end style
Answered by Expert 7th March 2018, 12:07 AM
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