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Homework Help: Kinetic Friction, Undefined Pushing Force, Find Normal Force.

  1. Dec 30, 2011 #1
    1. The problem statement, all variables and given/known data
    A crate is pushed at constant velocity across a rough, horizontal surface by a push force P. The crate has a weight of 73N. Coefficient of kinetic friction between the crate and the floor is 0.41. Vector force P points at an angle of 37 degrees below the horizontal. What is the magnitude of the normal force?

    2. Relevant equations
    fk = μkN
    constant velocity indicates v0=vfinal and therefore a=0
    no movement in the y direction, therefore ƩFy=0

    These are random things that I have found online but I'm not sure if they're true:
    Fcosθ=f (The horizontal component of the push force is equivalent to to friction)

    3. The attempt at a solution
    The problem is, I don't have the magnitude of the push force. I don't know where to go from here, but I have tried many things.

    To find the kinetic friction:
    fk = μkN

    Using Fcosθ=f, I assume that 29.9N is the horizontal component of the push force. I then find the vertical component of the push force:

    I then add this to the weight to find the Normal Force:

    The Normal Force is not 92.6N. This problem is from an exam I failed last semester, and I have the correct answer now, but I can't get to it.
    Thank you in advance, for anyone who can help me out.
    Last edited: Dec 30, 2011
  2. jcsd
  3. Dec 30, 2011 #2


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    Welcome to Physics Forums.

    You'll need to draw a force (free-body) diagram for the crate, and then look at x- and y-components of the net force.
  4. Dec 30, 2011 #3
    Thank you.
    I have drawn a free body diagram.
    There is a Push force which is exerting some negative vertical force, some positive horizontal force (this is at a 37 degree angle).
    There is a frictional force pointing in the negative x direction.
    There is mg (73N as specified in the question). The Normal force is what is unknown. I can upload a picture of this free body diagram if this helps more.
  5. Dec 30, 2011 #4
    Alright, after spending about 8 hours trying to figure this question out, googling, flipping thru my textbook, asking all my friends, I've finally got it (with some help from the online version of my textbook-- I had to piece together several different problems to get this).

    Recognizing that there is a=0,

    ƩF_x = Pcosθ-f_k = 0
    Therefore, Pcosθ-(f_k)(μ_k)(N)=0

    ƩF_y = Psinθ+N-mg = 0
    Therefore, N=mg-Psinθ

    ** Recognize at this point that N (normal force) is not mg. So we are missing the magnitude of the force P, and N (which will be the final answer).
    -- Start by finding P.
    Substitute (mg-Psinθ) for (N) in the first formula [Pcosθ-(f_k)(μ_k)(N)=0].
    -- From here you should be able to isolate and solve for P. This is the magnitude of the Push force.
    -- Next, using the angle, you can extract the vertical component of the Push force (P).
    -- You should be able to solve for the normal force from this.

    // Side note, signing up and asking a question here was honestly my last resort. It was just ironic that I happened to give in 1 hour before I solved this problem. I will definitely be back for more help during the next semester of Physics, though. Just gotta get thru 5 more months of it.
  6. Dec 30, 2011 #5


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    Good job, but note that it should be -P sinθ for the y-forces, since the crate is being pushed with a downward angle. That will change things, but your method is sound.

    Good job recognizing that constant velocity → a=0

    In general, listing the forces as you have done, along with having drawn the figure for yourself, will usually be good enough.

    Good luck with the rest of your course!
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