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Push mass up a ramp

  1. Dec 26, 2011 #1
    if i were to push a mass up a ramp by giving a horizontal force, the normal force do no work since it is perpendicular to the displacement. But then, what makes the mass have an upward and vertical component in its displacement?
     
  2. jcsd
  3. Dec 26, 2011 #2
    An even more interesting thing to consider is how, when the only work being done moving an object up a ramp that is fricitonless, is down in LIFTING the object verticallycan a horizontal force which is at right angles to the direction of the work being done, move the object at all??
     
  4. Dec 27, 2011 #3

    Drakkith

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    I don't understand your question. If the force was at a right angle to the work being done, wouldn't that be like trying to push a brick into a vertical wall? A ramp requires that some of the work be done in moving the object horizontally.
     
  5. Dec 27, 2011 #4
    If you decompose the normal force vector into two vectors (Nx and Ny) you will find that Ny does positive work while Nx does the same amount of work, but negatively.
     
  6. Dec 28, 2011 #5
    It might help to think about the situation in terms of a coordinate system and the dot product.

    You could define your coordinate system in any way, and it wouldn't matter. Work is defined as a dot product of force and displacement: work = F*d*cos(theta), where theta is the angle between the force and displacement vectors. Regardless of whether you define your coordinate system so that discplacement has components, theta is always 90 degrees and the work done is always 0.
     
  7. Dec 28, 2011 #6
    In other words, as long as the force and dispacement vectors are perpendicular, it doesn't matter what their components are.
     
  8. Dec 28, 2011 #7
    An even more interesting thing to consider is how, when the only work being done moving an object up a ramp that is frictionless is done in LIFTING the object vertically, can a horizontal force which is at right angles to the direction of the work being done, move the object at all??

    OK that's corrected so think about this and explain how this is possible intuitively.
     
  9. Dec 28, 2011 #8

    Drakkith

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    What does "explain this intuitively" mean?
    Also, how can the only work being done be in the vertical direction if you are using a ramp? You have to push it horizontally at least a little bit or it isn't a ramp.
     
  10. Dec 29, 2011 #9
    It takes no work to move an object at constant speed horizontally on a frictionless surface so the only work done in moving an pbject up this ramp is mgh or the vertical height times the weight

    Explaining intuitively means without using mathematical definitions like a dot product

    Rather one uses basic physics principles applicable to this physical situation with cause and effect

    The horizontal push causes a certain effect

    What now does that effect
    cause and so on
     
  11. Jan 2, 2012 #10
    Ok guys and gals - how does an object move vertically when it's pushed horizontally.
     
  12. Jan 2, 2012 #11

    Drakkith

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    The ramp turns your horizontal force into a vertical one?
     
  13. Jan 2, 2012 #12
    And that's why it's called a simple machine. Actually it doesn't turn all the force into a vertical one but by producing a normal force perpendicular to the ramp it now has a component that is vertical so the object can indeed move vertically when a horizontal force is exerted.

    I remember my very first physics course which I confess I failed. (took calculus, took the physics over and got an A) I was so enthralled with things like this as well as astrophysics which remains my first love, but physics is not only fascinating but so useful. How many things in life are beautiful, fascinating and useful? SiGHHH
     
  14. Jan 3, 2012 #13

    Drakkith

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    I'm confused. Are you asking a question or lecturing?
     
  15. Jan 3, 2012 #14
    Neither - just telling you that you, as I'm sure you know, are pretty much right - that the incline changes the horizontal force into a force that has a vertical component which can move the object straight up (as well as sideways of course). Then just reminiscing.
     
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