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Physics problem to fine average force

  1. Nov 22, 2011 #1
    A set of crash tests consists of running a test car moving at a speed of 11 m/s (24.2 m/h) into a solid wall. Strapped securely in an advanced seat belt system, a 57 kg (125.4 lbs) dummy is found to move a distance of 0.81 m from the moment the car touches the wall to the time the car is stopped. Calculate the size of the average force which acts on the dummy during that time.

    I do not get this question, i have been working on it for days but i am extremely frustrated

    I used the equation
    v^2 + u^2 = 2as

    where v = final velocity, u = initial velocity, a = acceleration, s = displacement.
    so i said
    0^2 + 11^2=2(a)(0.81)
    It gave me the wrong answer.
    but i think that i have to use mass, and my teacher has not shown me a formula in which mass is included
     
    Last edited: Nov 22, 2011
  2. jcsd
  3. Nov 22, 2011 #2

    Doc Al

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    Staff: Mentor

    Show what you've done and where you are stuck.

    Hints: You can solve this using energy methods (consider the work done by the wall) or kinematics (find the average acceleration).
     
  4. Nov 22, 2011 #3
    can you answer it now .
     
  5. Nov 22, 2011 #4

    Doc Al

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    Staff: Mentor

    That should be v^2 = u^2 + 2as.

    That will give you the acceleration. Then use Newton's 2nd law to find the net force. (Yes, you'll need the mass.)
     
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