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Homework Help: How close can you get to a stop sign before you start braking?

  1. Sep 20, 2007 #1
    Please help! This is urgent!

    1. The problem statement, all variables and given/known data
    1) A typical car's brakes can create a maximum acceleration of less than 5 m/s^2 . How
    close can you get to a stop sign before you start braking?


    2. Relevant equations
    no idea at all since there isn't even enough information given to solve this.

    3. The attempt at a solution

    I don't think it's even doable, since there's no initial velocity. Or am I wrong?
     
  2. jcsd
  3. Sep 20, 2007 #2

    tony873004

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    I'm stumped too. You need an initial velocity and a more exact acceleration than > 5m/s^2. You could do the problem with 5 m/s^2 and phrase your answer as "it needs at least x meters". But you still need an initial velocity. As your initial velocity approaches 0, your stopping distance also approaches 0. What if you were already at a stop? Then your stopping distance would be 0, regardless of how good your brakes were.

    Is this a trick question and we just don't get the trick? Are you sure you worded it right?
     
  4. Sep 20, 2007 #3
    I am sure I worded it right.

    Actually, I just copied it straight out of the textbook.

    I tried to put the Vi as 0, but that doesn't make sense. And what am I suppose to put the acceleration as? I can't really put it as <5m/s^2. :S
     
  5. Sep 20, 2007 #4
    Well, here are my thoughts about this problem.
    You must find a distance, measured in units of length (m). You are given acceleration, which has units of (m/s^2). Thus, you MUST have some other physical quantity, which involves units of time (s), or you won't be able to transform acceleration in such a way so as to get distance. I'd suggest you assume a typical acceleration as allowed in the city streets, such as 50 km/h and then the problem becomes fairly easy.
     
  6. Sep 20, 2007 #5

    J77

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    Yeah -- it could be a question to sort of those with a bit of common sense from those who read/do questions like robots.

    If I came across a question in a test, the first thing I would do is write something like, "As no velocity is given, I will assume the car is travelling along a flat road at 50 km/hr."

    If I was setting school tests, I'd always include a question which needed some common sense assumptions :smile:
     
  7. Sep 20, 2007 #6
    Isn't it just asking you to find x in terms of vi?

    No need to be sticking assumed numbers in.
     
  8. Sep 20, 2007 #7

    tony873004

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    That makes sense. Perhaps the answer is expected to be a formula. You'd still need to preface your answer with "no further than" and then use 5m/s/s as your acceleration.
     
  9. Sep 20, 2007 #8
    okay given a certain speed you can calculate the stopping distance
    x= [m]
    v= [m/s]
    a= 5 [m/s^2]
    t= braking time
    for a certain speed you calculate braking time:
    t=v/a
    x=(1/2)*a*t^2
    as stated above t=v/a so -> t^2=(v/a)^2.
    x=(1/2)*5*(v/5)^2
    x= 2.5*(v^2)/(5^2)
    x=0.1*v^2
    praise me!
     
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