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Is pushing considered a force?

  1. Sep 18, 2009 #1
    1. The problem statement, all variables and given/known data
    A man pushes on a piano of mass 180kg so that it slides at a constant V of 12.0 cm/s down a ramp that is inclined at 11.0 degrees above the horizontal. No appreciable friction is cting on the piano. calculate the magnitude and direction of this push if the man pushes the piano up the plane instead, also at 12 cm/s parallel to the incline. if the man pushes the piano horizontally, but still with the speed of 12.0 cm/s?


    2. Relevant equations

    F=ma

    3. The attempt at a solution
    I've already drawn the free body diagram, but i can't find anywhere what to do if the object is pushed. does the vector point toward the object, or away from the object in the direction it is being pushed?
    and if V is constant, then that means no acceleration, so doesn't that mean no force in the direcion of the push? regardless of what direction the man pushes (parallel to ramp or horizontal)?
     
  2. jcsd
  3. Sep 18, 2009 #2
    You need to take gravity into account. If v is constant, that means there's no acceleration; that implies that gravity is negated. Since gravity is a force, it would take a contrary force of equal magnitude to negate it.
     
  4. Sep 18, 2009 #3
    when i drew the free body diagram, i drew the axes so that they were parallel and perpendicular to the ramp. i have the normal force going along the positive y, and then gravity having components in both the negative x and negative y directions.

    is that it? if there is no acceleration from the push since V is constant, and the only force that needs to be considered is gravity, then the magnitude and direction is the same going up the ramp, down the ramp, and regardless of angle of push?
     
  5. Sep 18, 2009 #4

    rl.bhat

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    When the net force acting on the piano is zero, it will move the uniform velocity.
    Resolve the weight and the push into the vertical and horizontal components on the inclined plane.
     
  6. Sep 18, 2009 #5
    Well, not completely regardless of angle of push. If there's a different angle of push, there needs to be a different magnitude to negate the acceleration due to gravity.
     
  7. Sep 18, 2009 #6
    so push is a force acting on the object? again, on the FBD do i draw it tail at object, tip poinging away, or pointing toward object?
     
  8. Sep 18, 2009 #7
    Pushing is a force, yes. Sorry, I never really answered that. :'D It should be pointing at the object.
     
  9. Sep 18, 2009 #8
    ok... push is a force. it points toward the object in the direction of the push. so when i solve for the net force i get...

    sin(11)/mg = (x component of gravity) where mg = 1764N
    that times the mass plus the push so...

    Fx = (sin(11)/1764N) + push

    what is push? V isn't a force so I can't plug that in, and a = 0.

    EDIT: is it really as easy as rearranging to solve for push? i think i lost sight of the point of this problem...
     
  10. Sep 18, 2009 #9
    Once you find the components of gravity, it's pretty simple to find the magnitude of the push. Its components will simply be the opposite of gravity's.

    So, for instance, if gravity's component in a given direction is a, the force needed to reconcile that component to zero is negative a. Do this for both components and you can find your resultant magnitude.
     
  11. Sep 18, 2009 #10
    awesome, i think i got it. tyvm ^.^
    you're very patient, lol.
     
  12. Sep 18, 2009 #11

    Hurkyl

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    Or, at least, this would be true if gravity and the push were the only forces acting on the piano.
     
  13. Sep 18, 2009 #12
    i know there's also normal force, and in this case no friction.
     
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