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Going Up a ramp

  1. Jun 4, 2013 #1

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    1. The problem statement, all variables and given/known data
    A ramp is shown in the picture.Which ramp is easier to rise an object of mass 2kg up?and why?


    2. Relevant equations
    W=FS



    3. The attempt at a solution
    The second ramp.The work done in rising the mass against gravity is same in the two ramps.That is
    W=200N*1 = 200J
    But rising in the second ramp,the mass has to move more distance.
    200J=5*F So less force is needed F=40N
    The first ramp
    200J=2*F F=100N
    In the second ramp one has to use less force to move it up so it is easier.
    My question is:If he uses 40N against gravity(200N),Why doesn't the mass accelerate down?
     

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  2. jcsd
  3. Jun 4, 2013 #2
    What does 'easier' mean exactly?

    If the mass is at rest before and after its displacement, the work done by the vertical component of your applied force, call it Wv, is the same in each case (and is equal in magnitude to the work done by gravity).

    Can you show the calculation that gave you a weight of 200 N?

    Here your applied force acts on the mass in a direction parallel to the incline of the ramp. The vertical component of this force does work that's less in magnitude than Wv in moving the mass up the ramp. If no other force besides your applied force and gravity act on the mass, then you have described a situation that violates the law of conservation of energy.
     
    Last edited: Jun 4, 2013
  4. Jun 4, 2013 #3

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    Simple F=ma
    F=2*10(-Acceleration due to gravity):smile:

    Why?
     
  5. Jun 4, 2013 #4

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    Simple F=ma
    F=2*10(-Acceleration due to gravity):smile:

    Why?Can you explain your answer.
    Sorry.I am not able to delete the first one!
     
  6. Jun 4, 2013 #5
    And what is 2*10 equal to?

    The work done on the mass by the force that acts in a direction parallel to the incline of the ramp is the sum of the work done by its horizontal and vertical components. You're saying this sum is equal to 200 J. If you know the work done by the vertical component has to be 200 J for the mass to move to its final position at the top of the ramp, then the work done by the horizontal component has to be 0 J. Since your applied force has a nonzero horizontal component, the mass can't move to the top of the ramp as described in your post without violating the law of conservation of energy.
     
  7. Jun 4, 2013 #6

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    Actually it was a spelling mistake,It should be 20kg:uhh:

    I was talking about the work done by gravity(That is vertical component)
     
  8. Jun 4, 2013 #7
    I don't follow. Could you be more specific?
     
  9. Jun 4, 2013 #8

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    This question was based on a question on my text book which is as follows:
    (In the picture)
    That answer did not refer to horizontal works done.It merely relied on vertical work.This is similar to my question><

    Can you give your answer in the way you were giving it before completely?
     

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  10. Jun 4, 2013 #9

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    What's the component of gravity acting down the incline?
     
  11. Jun 4, 2013 #10

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    200Newton
     
  12. Jun 4, 2013 #11

    Doc Al

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    No. 200 N is the total gravitational force, which acts downward. You need the component acting down the incline (parallel to its surface).
     
  13. Jun 4, 2013 #12

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    What is a component?
     
  14. Jun 4, 2013 #13

    Doc Al

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    Forces, like weight, are vectors. Have you studied vectors?

    Read this about Inclined Planes.
     
  15. Jun 4, 2013 #14

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    I know about vectors but not those about inclined planes.However,the question does not give any information about any components
     
  16. Jun 4, 2013 #15

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    Not directly, but you can figure it out. In fact, that's what you did, in effect.

    Without the benefit of a ramp, you must exert the full weight of 200 N upward to lift the mass 1 meter.

    But with a ramp, the force you have to exert needs only to balance the component of the weight acting down the ramp. (The normal force of the ramp surface does the rest.) For the first ramp, that component is 100 N. For the second, it's 40 N. Of course, the lower the force you have to exert, the longer a distance you need to push the mass to get it to the same height. The work done is the same.
     
  17. Jun 4, 2013 #16

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    Then is the answer to my question is the normal force of the ramp supplies the rest of 160N force for the second ramp?
     
  18. Jun 4, 2013 #17

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    The normal force "cancels" the component of gravity perpendicular to the surface, but that component is not 160 N (it's about 196 N). All that's left is the component parallel to the surface, which is what you have to exert to push it up the ramp. That parallel component is 40 N.
     
  19. Jun 4, 2013 #18

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    Can I assume the ramp supplies the rest of the force the I does not have to(That is I have to exert a force of 40N,If the ramp does not exert the rest of the (160)newton of force against weight,it would accelerate down?
     
  20. Jun 4, 2013 #19

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    Not exactly sure what you're asking. Since we presume that the mass is being raised at some constant speed, the net force on it must be zero. You are exerting a force of 40 N at an angle; the vertical component of your force is only about 8 N. The ramp surface exerts the other 192 N of vertical force needed to support the weight.
     
  21. Jun 4, 2013 #20

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    Does that mean we are exerting a force of about 8N against gravity?
     
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