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Find the average force of friction

  1. Jan 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A 5kg box is tossed across the floor at 4 m/s and slides to a stop in 3 seconds. What is the average force of friction?

    2. Relevant equations
    I believe the answer is 6.66N, I found that because the problem was pulled from the book. But I am trying to understand how and why. What formula is used, so if I come across it again, I have a better understanding to figure it out.

    3. The attempt at a solution
     
  2. jcsd
  3. Jan 6, 2015 #2

    Stephen Tashi

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    You can calculate the initial momentum of the box. The momentum of the box is zero when it stops moving. What formula relates force to a change in momentum?
     
  4. Jan 6, 2015 #3
    That would be impulse, f*t
    and f=m*a
    so the impulse would be, (m*a)*t
    impulse=(5*(4/3))*3=20
     
    Last edited: Jan 6, 2015
  5. Jan 6, 2015 #4

    Stephen Tashi

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    Yes, if you use 4/3 as average acceleration. However, to solve for average F in equation for impulse, you'd do better to keep F as an unknown instead of writting it as m*a. To solve for F using F = m*a , you could claim that average force = mass * average acceleration.
     
  6. Jan 6, 2015 #5
    OK, so then I know that t=3, and I(impulse)=20, If F is unknown then F=I/T= 6.66N
     
  7. Jan 6, 2015 #6

    Stephen Tashi

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    Yes
     
  8. Jan 10, 2015 #7
    Thank you for the help. I greatly appreciate it :)
     
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