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Hand pushes block

  1. Jul 29, 2007 #1
    1. The problem statement, all variables and given/known data
    Not an actual HW problem, but related to alot of probs i'm doing.
    Lets say a hand pushes a block across a surface (with friction).
    You take pic of 2 timepoints
    1) when hand pushes block
    2) After hand has let go and block is slowing due to friction

    Now when I draw the FBD for situation #2, I am confused about the horizontal forces. (i know that vertical forces cancel)

    I know that 1 vector (kinetic friction) will point opposite the motion.



    2. Relevant equations
    BUT: Is there a forces that goes in the direction of motion???
    If there isn't, then how is the block moving??
    If there is, then what force could that be, seeing as the hand is no longer in contact with the block??


    Thanks in advance for any explanation!
    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jul 29, 2007 #2

    PhanthomJay

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    yes, correct.



    What does your FBD show you?
    The hand sure helped. What would happen to the object after the hand was released if there were no friction? Why?
    Right, good point.
     
  4. Jul 29, 2007 #3
    My problem is in drawing the correct FBD, so at moment im not relying on FBD for info
    But this is how i think it should look: (W and N are sposed to be equal lengths)

    http://www.imagination3.com/LaunchP...l=mujadeo@yahoo.com&to_name=&_lscid=184982417

    Yes I know hand got it moving, but if FBD is sposed to show forces on it, then there should be no force in pos x direction (right), because the hand is no longer in contact with block, right?
    But if no force in in pos x, then according to diagram, Fnet is to the left, and object is moving to the left (according to the diagram)???

    If there was no friction, then block would move at constant velocity in pos x direction. (and accel would be zero)

    tanks for your help
     
    Last edited: Jul 29, 2007
  5. Jul 30, 2007 #4

    PhanthomJay

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    Yes, your FBD is OK. You have correctly noted that there is only one horizontal force acting, the friction force, which is the net force, acting left. But per Newton 2, if the net force is left, then the acceleration is left, not necessarily the motion, which in this case is still to the right until the block stops. As you also have correctly noted, you don't need forces to keep a body moving. Forces retard or accelerate the motion; they do not keep it in motion, as noted in Newton 1.
     
  6. Jul 30, 2007 #5
    SO am i corerect in saying that A FBD tells us nothing about which direction an object is moving (the velocity vector) , only its direction (and mag) of acceleration.
    If thats true then Fnet says nothing about velocity, and a system with Fnet=0 could be stationary or going any speed or direction? ( velocity is completely independent of Fnet at all right?)
     
  7. Jul 30, 2007 #6

    PhanthomJay

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    That is correct. You get velocity from the kinematics or energy equations, or given data, etc. A system with F_net = 0 is either at rest or moving at constant speed in a certain direction, and will remain at rest, or moving in a straight line at a constant speed in that direction, as the case may be, unless acted on by a net unbalanced force (Newton 1).
     
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