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Force and Momentum HELP

  1. Nov 7, 2007 #1
    Force and Momentum... HELP!!!

    1. A graph is given (Force/body weight vs time). The peak on the graph gives a maximum force of 2.2x the body weight, Suppose a 70kg runner with a stride of speed 4.5 m/s. The impact speed of a 70kg runners foot is measured with 1.5 m/s. The effective mass of the foot and lower leg is 4.2 kg. The shoe and heel compress the ground a distance of 1.cm upon impact. Estimate the peak impact force exerted by the food on the ground.

    2.Favg= delta P/impact time, impact time= [2(impact distance)/(impact velocity)]

    3. I plugged in 0.012m for the impact distance, 2.5 m/s for the impact velocity and got an impact time of 0.016 seconds, which im pretty sure is right. For the change in momentum i wasnt too sure on what to use (the mass of the person or the foot). I assumed the foot slammed into the ground and bounced back up so I did (4.2 kg)(1.5)(2). I got 12.6. I divided 12.6 by 0.016 since delta P/impact time is average force and got 787.5 N.

    I dont think I did this right since it should be (2.2)(70)(9.8) which is around 1000 N. Is my work wrong? Thanks!!!!
  2. jcsd
  3. Nov 8, 2007 #2
    i was thinking.... they wouldnt give the mass and velocity of the person for no reason right?
  4. Nov 9, 2007 #3
    can anyone helllp please?
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