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Homework Help: Determine the uniform deceleration of car A

  1. Jan 2, 2006 #1
    cars A and B are d=60m apart and travelling at u(A)=8.89 and u(B)=6.67.knowing that 45s after driver A apply his brake to avoid overtaking B,the two cars collide,determine the uniform deceleration of car A.

    i have tried to form eqns using s=ut + 1/2a(t^2) for both cars and substitute into [car A-car B=60] and can't find the answer.
    the answer is 0.0395.

    pls help........thanx.....
  2. jcsd
  3. Jan 2, 2006 #2


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    Homework Helper

    If the units of velocities of the cars is m/s, I am geting 0.1965 m/s/s.

  4. Jan 2, 2006 #3


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    Take the initial position of B to be x= 0. Its position after t seconds is given by xB= 6.67t m. (assuming that by "u(B)= 6.67" you mean "the speed of B is 6.67 m/s. Please be complete.) A's position when t= 0 is x= -60 m. 45 s later, A's position is xA(45)= 8.89(45)= 400.05 m. If we let "a" be A's decceleration, then A's position at time t, t> 45, is given by
    xA(t)= 400.05+ 8.89(t-45)- (a/2)(t-45)2.

    Just given the information that "the two cars collide" you can't determine A's decceleration. You can calculate the "greatest decceleration so that they collide"- that is, the largest value for a so that xA(t) and xB(t) are equal for some value of t.
    Last edited by a moderator: Jan 2, 2006
  5. Jan 2, 2006 #4
    i also got the same answer as urs in threat 2
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