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Homework Help: FootBall Players

  1. Mar 4, 2008 #1
    1. The problem statement, all variables and given/known data
    Fred (mass 60.0kg) is running with the football at a speed of 6.0m/s when he is met head-on by Brutus (mass 120kg), who is moving at 4.0m/s. Brutus grabs Fred in a tight grip, and they fall to the ground. How far do they slide?
    Part A
    The coefficient of kinetic friction between football uniforms and Astroturf is 0.3.
    Give answer in cm

    2. Relevant equations

    MfVf + MbVb=(Mf+Mb)Vs

    Ax=-mew x g

    3. The attempt at a solution

    I did the problem and doubled check my work and even had a friend check it and we couldn't find a mistake but I still get it wrong

    To find the velocity after the collision I used the conservation of momentum equation above and calculated the "after impact) velocity to be -2/3 m/s. I then proceeded to find the acceleration and used Ax=-mew x g and got -2.94 m/s^2. From there I solved for time by plugging the velocity and acceleration into Vfx=Vix+Ax(delta T). I got the time for
    the slide to be .22 seconds. I then plugged everything into the distance kinematics equation Xf=...and I got .075 m which I then converted to 7.55 cm, but it still says its wrong. Do you see any mistakes I made?
  2. jcsd
  3. Mar 5, 2008 #2


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    Homework Helper

    Is "it" that says the answer wrong a computer problem-system? If so, what units does it ask for the answer in? (Possibly #1 reason why the result of a correct method is rejected...) BTW, I agree with your solution (I used the "velocity-squared" equation,
    (v_f)^2 = (v_i)^2 + 2a(delta_x), and found the same distance).
  4. Mar 5, 2008 #3
    It wants it in cm. I don't get why it says I am wrong.
  5. Mar 5, 2008 #4


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    Not being there myself to see what the computer is doing, I can only be mystified. If you're sure you've used the right quantities for the problem, your method should give the correct answer.

    It is conceivable that the computer system has a difficulty for this problem. The formula encoded for its calculation could be in error, or the tolerance for input answers might have been set to zero. (This happened once a ways back in a physics course here for one problem: good luck getting the computer's answer to sixteen decimal places...)

    I would bring this up with the instructor and see if there is something set up wrong for that problem. It's not like that's never happened before. (Besides, I'm feeling cocky: I helped a student last week to get the instructor in a 5000-level stats course to re-examine their solution to an exam problem and find the error in the problem statement... ;-) )
    Last edited: Mar 5, 2008
  6. Mar 5, 2008 #5
    I talked to my teacher about it and he came to the same answer when he did it. I did show answer on the website and they calculated 8.8 cm. I don't know how they got that :confused: He sent an e-mail to the website tech support to notify them of the error.

    Thanks for the help!
    Last edited: Mar 5, 2008
  7. Mar 5, 2008 #6


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    I'm glad to hear the teacher agreed with us also! :) These difficulties do crop up from time to time with the computer-based systems; they aren't going to replace living instructors anytime soon...
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