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Homework Help: New Problem I dont get!

  1. Aug 28, 2005 #1
    A sports car is advertised to be able to stop in a distance of 55 m from a speed of 85 km/h.
    (a) What is its acceleration in m/s2?

    (b) How many g's is this (g = 9.80 m/s2)?

    I am confused as to how you would find the acceleration.Iwould be able to put it in to g's but i dont understand how to get the acceleration!
    Last edited: Aug 28, 2005
  2. jcsd
  3. Aug 28, 2005 #2


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    Homework Helper

    Do you know the equation:
    [tex]v_f ^ 2 = v_i ^ 2 + 2ad[/tex]?
    It's used to solve for a when v_i, v_f, and d is known.
    Viet Dao,
  4. Aug 28, 2005 #3
    would v_i =23.6 m/s,v_f=55m,and d=0
  5. Aug 28, 2005 #4
    No. Where do you get that 23.6 ms^-1 from? v_f is also a speed, so it isn't a distance. Also, why did you put d = 0?
  6. Aug 28, 2005 #5
    I have no idea what to do in this problem.I divided 85km/h to get it into m/s.
  7. Aug 28, 2005 #6
    [tex] v_f = 0[/tex] m/s
    [tex] v_i = 23.611[/tex] m/s
    [tex]d = 55[/tex] m
  8. Aug 28, 2005 #7
    Use the basic kinematics equation
    [itex]v^2 - u^2 = 2as[/itex]

    For your second part , divide your answer by g , that will give you the reqd. number.

    Donot forget to convert km/hr to m/sec.

  9. Aug 28, 2005 #8
    Thank you!I understand what I did!Is the answer a negative number?
  10. Aug 28, 2005 #9
    Yes, indeed. The velocity has decreased down to zero which means that the acceleration must be negative. A negative acceleration is also called deceleration/retardation.
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