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Physics

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I really don't get why I am getting this question wrong? Please can you explain to me how to do this question step by step so that I can understand it please? :( i have been stuck on this question for quite a while now.

Determine the stopping distances for an automobile with an initial speed of 89 km/h and human reaction time of 2.0 s for the following:
(a) an acceleration a = -4.0 m/s2.
(b) a = -8.0 m/s2.

  • Physics - ,

    Vo=85000m/h = 85000m/3600s = 23.61 m/s.

    a. V = Vo + a*t
    T = (V-Vo)/a + Tr
    T = (0-23.61)/-4 + 2s = 7.9 s = Stopping time.
    d = Vo*t + 0.5a*t^2
    d = 23.61*7.9 + 0.5*-4*7.9^2 = 61.7m

    b. T = (0-23.61)/-8 + 2s = 4.95 s.
    d=23.61*4.95 + 0.5*(-8)*4.95^2=18.86 m.

  • Physics - ,

    CORRECTION:
    Vo=89,000m/1h = 89000/3600s = 24.72 m/s

    a. V = Vo + va*t
    T = (V-V0)/a + Tr
    T = (0-24.72)/-4 + 2s.=8.18 s.=Stopping
    time.
    d = Vo*t + 0.5a*t^2
    d=24.72*8.18 + 0.5*(-40*8.18*2=68.38 m.

    b. T = (0-24.72)/-8 + 2s = 5.09 s.
    d=24.72*5.09 + 0.5*(-8)*5.09^2= 22.19 m.

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