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Relationship between deceleration and friction

  1. Sep 15, 2007 #1
    The problem says:
    A box is given a push so that it slides across the floor. How far will it go, given that the coefficient of kinetic friction is 0.20 and the push imparts an initial speed of 4.0 m/s.

    I cant find the applied forced because i don't know the time or distance it took to get to the initial speed, and i am unaware of any direct relationships between acceleration and coefficient of kinetic friction. So any suggestion or help on how to do this problem?
  2. jcsd
  3. Sep 15, 2007 #2
    Why do you want to find the applied force? What equation do you plan on sticking it in?

    What equations do you know that involve kinetic friction? I know of 1. In this 1 equation that I know, there is a variable which is involved in another equation which does include acceleration. In other words, while you may not have learned the direct relation between kinetic friction and acceleration, you can combine 2 simple equations to relate the two.
  4. Sep 15, 2007 #3
    and can i know the equation?
  5. Sep 15, 2007 #4
    No, it's a secret given only to those who first take a blood oath never to reveal it.

    But seriously:

    1) I don't know if telling you the equation fits in with the policies or spirit of this homework helping forum

    2) Whatever book this problem came from has to have the equations. If it's not from a book, then whatever teacher/professor gave you the problem has to have told you the equation. And if all that fails, any online search engine can get you that equation in less than 30 seconds. Unless you want to come here for every single problem you ever have, you'll have to eventually learn how to find equations anyway.
  6. Sep 15, 2007 #5
    i understand but i spent my evening on this problem and reading the section 3 times. I need some help
  7. Sep 15, 2007 #6
    1. What is the equation which defines the linear coefficient of friction?

    2. What is the resultant frictional force according to question #1?

    3. How do you define energy in terms of velocity?

    4. How do you define energy in terms of force over a distance?

    5. What does conservation of energy have to say about questions #5 and #6?
    Last edited: Sep 15, 2007
  8. Sep 15, 2007 #7


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    The equation you need is "Force equals mass times acceleration"! You don't need to know the initial force that started the motion because that stops as soon as the box starts moving. The force after that is just the friction force- the weight of the box times the coefficient of friction. Since weight itself is "9.8*mass", acceleration will be 9.8 times the coefficient of friction.

    Once you know the acceration (negative, of course, it's really "deceleration"), calculate how long it will take for the velocity to drop to 0 and then calculate how far the box will move in that time.
  9. Sep 15, 2007 #8
    Its ok dont worry. Now first of all you have got to know the equations of kinematics.You also have to know the equations for dynamics. Now do you have even a faintest idea about these equations. Reply soon.Always ready to help.
  10. Sep 16, 2007 #9
    yea i got it...i forgot that the initial force didn't matter because after the motion started it wasn't being applied.
    thanks for helping
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