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Quick physics question

  • 07-05-2007 3:44pm
    #1
    sternn
    Registered Users, Registered Users 2 Posts: 929 ✭✭✭


    Bit stuck, its prob very easy but cant seem to be able to start it...

    A car of mass 1200kg which is travelling at 100km/h wants to stop in a distance of 100m to avoid a collision. If the maximum retarding force due to breaks is 2000N, will the car avoid the collision? What is the minimum force needed to avoid collision?

    Cheers!!


Comments

  • #2
    PurpleFistMixer
    Registered Users, Registered Users 2 Posts: 5,851 ✭✭✭PurpleFistMixer


    This is quite possibly wrong but I'll give it a go anyway...

    You want to find the acceleration (or deceleration as the case may be) of the car, so...
    v^2 = u^2 + 2as...
    100km/h = 1000/36 m/s = u
    v = 0
    a = ?
    s = 100

    So a comes at as.. -3.86 or so.

    Then, F=ma, want to see the force required to cause the deceleration, so
    a = 3.86
    m = 1200
    F therefore = 4629.6N
    therefore, retarding force due to breaks isn't sufficient, and the F there is the minimum force needed.

    I think!


  • #3
    JC 2K3
    Closed Accounts Posts: 7,794 ✭✭✭JC 2K3


    Wrong.

    The answer was "lol, retarding".


    You can also just say 2Fs/m must be greater than or equal to u^2 for the breaks to stop the car, but 2(2000)(100)/1200 < (1000/36)^2, therefore they won't, no need to get the accelleration. For the minimum force to stop the car, F = m(u^2)/2s


  • #4
    ron-burgandy
    Closed Accounts Posts: 51 ✭✭ron-burgandy


    JC 2K3 wrote:
    Wrong.

    The answer was "lol, retarding".


    You can also just say 2Fs/m must be greater than or equal to u^2 for the breaks to stop the car, but 2(2000)(100)/1200 < (1000/36)^2, therefore they won't, no need to get the acceleration. For the minimum force to stop the car, F = m(u^2)/2s

    Could you tell me what you derived that from?


  • #5
    JC 2K3
    Closed Accounts Posts: 7,794 ✭✭✭JC 2K3


    Well for the minimum force required to stop the car:
    v^2 = u^2 + 2as
    v = 0
    a = F/m
    0 = u^2 + 2Fs/m
    u^2 = -2Fs/m
    However, since the force is backwards you can write it as u^2 = 2Fs/m (you can leave it as it was above, but remember you'll have to take the 2000N as -2000N since it's in the opposite direction to the velocity vector)


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