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Frequency of a Car's Vibrations?

  1. Jul 24, 2009 #1
    Frequency of a Car's Vibrations????

    1. The problem statement, all variables and given/known data
    When four people with a combined mass of 297 kg sit down in a car, they find that the car drops 2.8 cm lower on its springs. Then they get out of the car and bounce it up and down. What is the frequency of the car's vibration if its empty mass is 2210 kg


    2. Relevant equations
    F = kx
    x=k/F
    f=(1/2)(pi)sqrt(m/k)
    F=mg

    3. The attempt at a solution
    I tried using F=mg=(297)(9.8) =2910.6 N
    k=103,950 N/m
    f=(1/2)(pi)sqrt(2210/103950)=0.229 which was the wrong answer. Not really sure what i'm doing wrong.
    Please provide solution and approach to problem. Thanks!!
     
  2. jcsd
  3. Jul 24, 2009 #2

    berkeman

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    Staff: Mentor

  4. Jul 24, 2009 #3
    Re: Frequency of a Car's Vibrations????

    ok. I found the equation for the frequency to be f=w/2(pi) and w=sqrt(k/m)
    so, w=sqrt(103950)/297 =18.7 and f=18.7/(2)(pi) =2.97 which is also wrong. ??
     
  5. Jul 24, 2009 #4

    berkeman

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    Staff: Mentor

    Re: Frequency of a Car's Vibrations????

    There are two different "m" quantities here. The first m is the mass of the people who sit in the car so that the k value can be calculated for the springs. The second m is the mass of what? What is resonating when they get out and start pushing on the car to acheive resonance?

    BTW, you also had a typo in your Relevant equations when you manipulated the F=kx equation. You did the calculation correctly when you calculated k later, but x is not equal to k/F. Just a typo right there, not affecting your final math.

     
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