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Energy question about kinetic energy and potential

  1. Mar 24, 2015 #1
    1. The problem statement, all variables and given/known data

    Objects A and B, of mass M and 2M respectively, are each pushed a distance d straight up an inclined plane by a force F parallel to the plane. The coefficient of kinetic friction between each mass and the plane has the same value [PLAIN]https://www.physicsforums.com/x-apple-ql-id://3C529BAE-7096-42AC-A3AD-BBB311BE09A3/x-apple-ql-magic/2A7E0075-C083-4913-99A2-A5DE41E321DE.pdf [Broken] [Broken]. At the highest point,

    a.https://www.physicsforums.com/x-apple-ql-id://F3828464-FA8A-4215-8BE7-4A023DC5CD41/x-apple-ql-magic/5507A7ED-921A-42CF-9951-B813D9EAF084.pdf [Broken].

    b.https://www.physicsforums.com/x-apple-ql-id://2E7FAF13-D7EF-40BF-AC42-B48E3047EE43/x-apple-ql-magic/C52E68C5-7BFE-4BCC-8299-858DDE6FFF1F.pdf [Broken].

    c.https://www.physicsforums.com/x-apple-ql-id://FC2B1409-9B06-476D-A519-19DF4D82235F/x-apple-ql-magic/C54A8274-657D-49B6-A032-6347DA0F9DBB.pdf [Broken].

    d.The work done by F on A is greater than the work done by F on B.

    e.The work done by F on A is less than the work done by F on B.

    2. Relevant equations
    K= 0.5mv^2
    PE= mgh
    Ffriction = Fn * [PLAIN]https://www.physicsforums.com/x-apple-ql-id://3C529BAE-7096-42AC-A3AD-BBB311BE09A3/x-apple-ql-magic/2A7E0075-C083-4913-99A2-A5DE41E321DE.pdf [Broken] [Broken]

    3. The attempt at a solution

    The answer is A.
    I don't even know how to approach this question.. I'm studying by myself so I don't have any teachers to ask :( Can anyone help me out??
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Mar 25, 2015 #2

    haruspex

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    Five images in your post (including the whole of a, b, c) show as broken links on my screen. I'm guessing the only reason they're not plain text is that they contain a character such as ##\mu##. Please try posting using LaTeX, or by copying and pasting the mu in my post, or just use the word "mu".
     
  4. Mar 25, 2015 #3
    I'm sorry I didn't know post show as broken link. I'll write it down again here!

    Objects A and B, of mass M and 2M respectively, are each pushed a distance dstraight up an inclined plane by a force F parallel to the plane. The coefficient of kinetic friction between each mass and the plane has the same value Uk. At the highest point,

    a. Ka > Kb.

    b. Ka = Kb.

    c. Ka < Kb.

    d.The work done by F on A is greater than the work done by F on B.

    e.The work done by F on A is less than the work done by F on B.

    the friction equation is Fn * Uk
     
  5. Mar 25, 2015 #4

    PhanthomJay

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    are you familiar with work and work-energy equations? Or kinematic equations and Newton's laws if not? Please list them under your 'relevant equations', and show some attempt or at least state how you think you would approach this problem using these equations.
     
  6. Mar 25, 2015 #5
    I kind of get a concept of this question. By using F=ma, it is obvious that mass M will have higher velocity than 2M when they move same distance because 2M will have lower acceleration. And since M has higher velocity, it might have higher K? I want to know how exactly Ka is bigger than Kb with solution, not just by a concept.
     
  7. Mar 25, 2015 #6
    I attached my work. It's kinda messy, but can you check if my idea is right?? What I basically did was to put Mass as 1 and 2, distance as 10m, F as 20N, theta as 30 degrees, and kinetic friction as 0.2 and just solved the velocity of both and get the kinetic Energy of both...
     

    Attached Files:

  8. Mar 25, 2015 #7

    PhanthomJay

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    Loo
    looks good using that approach! Work energy approach might be simpler if you use it properly, but your method is fine. Note that you chose some good numbers, getting a positive acceleration up plane. Other number choices might result on no motion or motion down the plane, so be careful if your result came out negative. Did you check to see if choice d or e might also be correct?
     
  9. Mar 25, 2015 #8
    I haven't checked it yet!! I will do that now! Thanks
     
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