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Velocity and acceleration

  1. Sep 30, 2008 #1
    1. The problem statement, all variables and given/known data
    Flash Maddie is running at a whopping velocity of 30 mph. She then sees Mr. T driving down the road straight towards her so she accellerates -5 mph/s for 10 seconds. what Is Maddie's velocity now. (In mph)

    2. Relevant equations
    V = Change in distance / change in time
    a = change in velocity / change in time


    3. The attempt at a solution
    I'm just not sure how to approach the problem in general and I'm not sure how to approach the negative acceleration.
     
  2. jcsd
  3. Sep 30, 2008 #2

    Hootenanny

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    Whilst you could in principle use those equations, this question is best solved using kinematics equations (of uniform acceleration).
     
  4. Sep 30, 2008 #3

    Redbelly98

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    Use this equation:
    Plug in the numbers you know, including any negative signs, and solve the equation. That will give you a key piece of information for solving the problem.
     
  5. Sep 30, 2008 #4
    Hootenanny: What are these equations and how would I use them?
    Red Belly: Should I multiply -5 MPH/s by 10 and then subtract it from 30 mph?
     
    Last edited: Sep 30, 2008
  6. Sep 30, 2008 #5

    Hootenanny

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    Yes, that's correct (with one minor correction, you should add the -50 to 30). In actual fact, both methods are identical:

    [tex]a=\frac{\Delta v}{\Delta t} = \frac{v_f-v_i}{\Delta t}[/tex]

    [tex]\Rightarrow v_f = v_i + a\Delta t[/tex]

    Which is one of the kinematic equations I was refering to.
     
  7. Sep 30, 2008 #6

    Redbelly98

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    It may be that swede's class has not quite gotten to the full set of kinematic equations ... at any rate, Hootenanny is entirely right.
     
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