1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Kinetic Friction. Did I do it right? Help please

  1. Mar 1, 2014 #1
    A 4700 kg truck carrying a 900 kg crate is traveling at 25 m/s to the right along a straight, level road, as shown below.

    Suppose the brakes are applied for 3.0s. During this 3.0s the truck travels 55 m.

    1. Calculate the acceleration of the truck during the 3.0s interval. (Assume it is constant.)

    2. Sketch a force diagram of the crate during the 3.0s interval described above.

    3. Calculate the minimum coefficient of friction required to keep the crate from sliding in the back of the truck.

    4. Calculate the minimum coefficient of friction required to stop the truck by the end of the 3.0s interval

    My attempt:
    1. Using X = Xo + Vot - 1/2at^2 a = -4.4 m/s^2
    2. I got it.
    3. F = ma ; F = uN
    uN = ma ==> umg = ma ==> u = a/g = -4.4/9.8 = 0.449
    4. This is where I need help. Since it was never mentioned that the truck came to a complete rest with -4.4 m/s^2 in 3 seconds. But now we do want it to stop in 3 secs. So I am guessing negative acceleration has to be greater.
    a = (0-25 m/s)/3 sec = -8.33 m/s^2
    u = a/g ==> -8.33/9.8 = 0.850
    Somebody please review my answers. Did I go about them the right way?

    Thanks!!
     
  2. jcsd
  3. Mar 1, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Looks good.
    The problem statement in (4) is a bit unclear - for example, truck and crate could have different coefficients of friction, or the crate could have the coefficient from (3) and the truck has to compensate for that, or whatever. Well, I think your interpretation is the way the question is meant.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Kinetic Friction. Did I do it right? Help please
  1. Did I do this right? (Replies: 3)

  2. Did I do these right? (Replies: 15)

  3. Did I do this right? (Replies: 2)

  4. Did I do this right? (Replies: 15)

Loading...