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Homework Help: Calculating acceleration and brake force

  1. Dec 30, 2008 #1
    1. The problem statement, all variables and given/known data Hi all, the question for revision is; A car of mass 600kg travelling at 45m/s comes to rest in 12m when the brakes are applied. What is the average force exerted by the brakes?

    2. Relevant equations Force = mass x acceleration V^2=u^2 +2as

    3. The attempt at a solution I know I need to work out the acceleration but am having trouble correctly transposing that equation I think its this v^2-u^2 divided by s =2a, its what to do with the 2 to get a on its own, does it then become 1/2 s on the other side?
  2. jcsd
  3. Dec 30, 2008 #2


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    You're using the correct equation, but it looks like you need some help in "solving for a" to get the acceleration.

    Let's write out what you correctly have so far:


    Hopefully that helps you see better what to do ... if not, then post again.

    p.s. I'm not quite sure if
    means "(1/2)s" or 1/(2s).
  4. Dec 30, 2008 #3
    That is what i have so far, Im just not sure where the 2 goes so you just get a on its own, I was wondering the 2 then moves to the side of s to become1/2s.
  5. Dec 30, 2008 #4
    If I solve that equation with 2a. I did the following 45x45-0x0, divided by 12 = 168.75 x 2 =337.5. Is this the correct acceleration?
  6. Dec 30, 2008 #5

    [tex]\Rightarrow \frac{v^2-u^2}{2s}=a[/tex]
  7. Dec 30, 2008 #6
    Yes I have it, thank you, my transposition of equations knowledge is in there, but its 15 years or so since I used it! I often write the suvat list out, i find it very helpful.
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