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Homework Help: Basic Forces Question (Piano Slides Across Floor)

  1. Sep 24, 2012 #1
    1. The problem statement, all variables and given/known data
    A piano slides across a floor with a mass of 500kg.

    A) solve for the normal force
    B) A horizontal force 1300 is necessary to keep it in motion at constant speed find the coefficient of friction

    2. Relevant equations

    Fnet = MA

    3. The attempt at a solution

    Given: m= 500kg , fnetx=1300N

    fnety= FN - FG = 0
    FN=4900 N upward

    b) fnetx = max = 1300N = -FFR
    (i made my +x opposite the force of friction)

    1300N = -FFR
    1300 = mu*FN
    mu = 1300/FN
    mu = 1300/4900
    mu= 0.265

    Now a question arises...

    I understand that ax must be 0 because it is at a constant velocity...

    But i'm just curious because FFR = Fnetx

    That means FFR= 1300N

    So doesn't this mean that this unbalanced force will slow down the piano and not keep it at a constant motion? Anyways Yea i would appreciate some help please.
  2. jcsd
  3. Sep 24, 2012 #2
    1300N is not Fnetx. A force of 1300N is applied in the x-direction, and a friction force of 1300N is applied in the negative x-direction. Fnetx is the resultant of these two forces, and since they're equal/opposite forces, the resultant force is 0.
  4. Sep 24, 2012 #3
    So how can that be right if the net force is 0, that means that the forces are balanced, therefore the object must be at rest.

    IE: FNETY=FN-FG=0 this proves an object stationary in the vertical direction

    So if Fnetx= Fa - FFr = 0

    Therefore the object cannot moving horizontally because there is no imbalanced force? It would be at rest..

    Also to note.

    If FA = FFR

    Therefore 1300 = Mu*FN
    Mu = 1300/FN = 0.265

    This yields also, the same answer.

    Please further elaborate your answer?

    *Also i don't think you can say because they are equal and opposite forces it is equal to zero*

    The only thing that implies Fnetx=0 is the fact it says it is uniform velocity.

    So therefore Fnetx=m*ax if ax=0 Fnetx = 0
    Last edited: Sep 24, 2012
  5. Sep 24, 2012 #4
    :( some one else help me plz?
  6. Sep 24, 2012 #5
    If the net force is 0, that means there is no acceleration (no change in velocity). Whatever initial velocity the object had will be maintained. Presumably, at some point there were imbalanced forces which accelerated the piano to some velocity before the start of the problem.

    Your answer should be unaffected by this, and it seems to me that you did everything right otherwise.
    Last edited: Sep 24, 2012
  7. Sep 25, 2012 #6
    So is it similiar to the property of Inertia, in the sense that you are trying to apply a force strong enough to to compensate for the friction, so that it is almost as if no forces act horizontally? This way in the sense of inertia, the object is still moving at the same velocity, because no "External" force is able to act, because the external force is compensated for?

    And also thank you.
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