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Homework Help: Magnitude of the force

  1. Sep 5, 2006 #1
    A car that weighs 1.5×104 N is initially moving at a speed of 45 km/h when the brakes are applied and the car is brought to a stop in 12 m. Assuming that the force that stops the car is constant, find the magnitude of that force.

    I found the mass to be 1.53e3 kg then I figured now I need to find the acceleration using the information that I have so I used the equation
    Vf^2 = Vi^2 + 2a(Xf-Xi)
    Vi = 12.5 m/s
    Vf = 0 m/s
    Xf = 12m
    Xi= 0 m

    Then plug the acceleration value into the equation F= ma but apparently that's wrong. Is this the correct method?
  2. jcsd
  3. Sep 5, 2006 #2

    Andrew Mason

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    Method is ok. [itex]a = \Delta v/\Delta t \text{ where } \Delta t = 2d/v_i[/itex] Since final velocity is 0, you have: a = v^2/2d

    You are in effect using energy: [itex]KE = F*d = \frac{1}{2}mv^2[/tex] which is a little more direct:

    [tex]F = mv^2/2d[/tex]

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