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Homework Help: Simple HW Junior Psychics Q

  1. Sep 12, 2005 #1
    Ok here it is: A sports car is advertised to be able to stop in a distance of 50m from a speed of 90 km/h. What is its acceleration in m/s^2?

    I turned 90km/hr into 25m/s but where do I go from there... I'm pretty bad at physics...

    Ok I need to know what formula to plug these numbers into. I can't use the acceleration formula because there's no time that can be plugged in. Help please :)

    A walkthrough on acceleration would be nice too :)
  2. jcsd
  3. Sep 12, 2005 #2

    Tom Mattson

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    You're going to have to figure that out for yourself, but we will help you.

    Go through the problem and list things that you know. For instance you know that the distance traveled by the car is 50 m. Since you can define the "zero" of position to be anywhere you want, you can say:

    xi=0 m
    xf=50 m

    Do that for all of the given information. Then determine what is being asked for ("the unknown").

    Then select the equation that relates what you've been given to what is being asked for.
  4. Sep 12, 2005 #3
    I've been looking at this problem for 45 minutes now and its not coming to me

    Is it 1 of these 4 formulas?

    v = v0 + at

    x = x0 +v0t +0m5 at^2

    v^2 = v0^2 +2a(x-x0)

    v = (v +v0)/2

    All the "0" are supposed to be in subscripts. I can't figure out which formula to use in this occasion. You're starting at 0 ok. you go to 50. The car is going at 90 m/km. The car on average, needs to be slowing 0.5 km/m of the 50 meters to stop from 25 m/s to 0 m/s... So what next?
  5. Sep 12, 2005 #4
    Wait would the answer be

    Acceleration = -1/2m/s^2?
  6. Sep 12, 2005 #5

    Tom Mattson

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    Yes, it's one of those. But you're putting the cart before the horse here. You shouldn't be scanning the list of equations until after you've identified the given information and the unknown.

    OK, so in that case let x0=0 m and x=50 m.

    Try assigning these numbers to the letters that appear in the equations. You've got x0 and x already. Now try to complete this table:


    Only then should you attempt to find the equation.
  7. Sep 12, 2005 #6
    List your known quantities and then list your unknowns. After that, look at the equations you have to see if you can solve for the unknown you want.
  8. Sep 12, 2005 #7
    So would I be right in saying v0 = 90000 m/s and v = 0m/s?

    so what does a and t stand for? My textbook really doesn't say anything about it.

    t is time I think but how would i find that?
  9. Sep 12, 2005 #8
    a is acceleration, you should probably know that.

    What I meant in my post was to list the quantities from your problem that you know and figure out what you are solving for.
  10. Sep 12, 2005 #9

    Tom Mattson

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    v0 is not 90000 m/s. It is 25 m/s, as you yourself stated in the opening post. You are correct about v.


    Here's the real question: Do you even need it?

    The question is asking for acceleration, right? So you have:

    x0=0 m
    x=50 m
    v0=25 m/s
    v=0 m/s

    You weren't given t, and you weren't asked for it. So, choose the one equation that has all the symbols in the list above and that doesn't have t. Then plug in all the known quantities and solve for the one unknown.
  11. Sep 12, 2005 #10
    v^2 = v0^2 +2a(x-x0)

    ^That equation then?

    so it would be:

    0^2 = 25^2 + 2a(0-50)
    -625 = -100a

    ACCELERATION = 6.25 m/s???
  12. Sep 12, 2005 #11

    Tom Mattson

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    Looks good to me. :cool:
  13. Sep 12, 2005 #12

    it's 12:33 eastern time right now and I can finally move on. Well this is been a monumental effort.

    Having SAT prep class from 6 PM - 9PM really cuts out homework time and after 11 PM you really can't think. Thank you all that helped.
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