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Average Net Force

  1. Sep 26, 2006 #1
    An 810-kg car accelerates from rest to 27m/s in a distance of 120m. What is the magnitude of the average net force acting on the car?

    120m / 27m/s= 4.4 s

    F=ma
    =810kg * 4.4s
    = 3600 N

    This is not one of my choices. What did I do wrong?
     
  2. jcsd
  3. Sep 26, 2006 #2

    radou

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    Homework Helper

    You can not apply t = s / v, because the car is accelerating. The velocity of the car is given with 27 = a * t. The second equation is the displacement of the car, 120 = 1/2*a*t^2. Now, plug in the first equation into the second one, and get the time and acceleration. Then you can calculate the average force acting on the car.
     
  4. Sep 26, 2006 #3
    So, using your equations I get:

    120m = 1/2 (27m/s)^2
    120m = 1/2 (729m^2/s^2)
    120m = 364m^2/s^2
    rearrange--> s^2 = 364m^2/120m
    s^2 = 3.03m

    This isn't right....
     
  5. Sep 26, 2006 #4
    Anyone have any info on this?
     
  6. Sep 26, 2006 #5

    radou

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    Homework Helper

    You have two equations:

    (1)... 27 = a*t
    (2)... 120 = 1/2*a*t^2

    From (1) you obtain a = 27/t. Putting that into (2) gives t = 8.89 seconds, which implies (from (1) ) a = 3.04 m/s^2. Hence, F = m*a implies F = 810 * 3.04 = 2462.4 [N]. Do not mix units with variables. You do not have tu put units (such as m/s, for example) into your equations. In other words, you wrote
    which reads: 'Seconds squared equals 3.03 meters.' Does not make any sense, right?
     
    Last edited: Sep 26, 2006
  7. Sep 26, 2006 #6
    WOW!!! I brain farted that one. For some reason I was thinking to replace the "a*t" with 27. Don't know why... Thank you for your help, I really appreciate it.
     
  8. Jan 13, 2008 #7
    solve the problem

    a = ?
    a = v/t
    solve t using d=.5(v-V)t
    t= 8.8sec

    sub in the equation
    a = (27m/s)/8.8sec
    a = 3.06m/s^2

    F= mass*acceleration
    810kg*3.06m/s^2 = 2485N = 2500N
     
  9. Dec 18, 2009 #8
    KE = 1/2 * m v^2
    KE = (1/2) (810) (27^2)
    W= deltaKE
    W = (1/2) (810) (27^2)
    W = force * distance
    (1/2)(810)(27^2) = 120*F
    F= [(1/2)(810)(27^2)] / (120)
    F= 2460.375 N
     
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