1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rolling and sliding ring

  1. Mar 27, 2017 #1
    1. The problem statement, all variables and given/known data

    A uniform ring of mass m and radius r is projected horizontally with velocity v on a rough horizontal floor,so that it starts off with a pure sliding motion and it acquires a pure rolling motion after moving a distance d.If the coeffcient of friction between ground and ring is a,the work done by the friction in this process is,




    2. Relevant equations
    w = fd



    3. The attempt at a solution

    Since the ring starts with a pure sliding motion,the point of contact should move,since it is moving f =aN,N
    =mg
    so, f=amg
    and it begins to roll,after rolling motion starts,friction won't do any work so w=fd.
    so work done by friction is -amgd,but given answer is -0.5mv2

    Please tell me where have I gone wrong.
     
  2. jcsd
  3. Mar 27, 2017 #2

    Baluncore

    User Avatar
    Science Advisor

    What is the initial kinetic energy of the ring as it is projected?
     
  4. Mar 27, 2017 #3
    Sir,since initially no rotation motion would be present so it must be 1/2mv2
     
  5. Mar 27, 2017 #4

    Baluncore

    User Avatar
    Science Advisor

    Where will that energy go?
     
  6. Mar 27, 2017 #5

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    How did you convince yourself your answer is really wrong ? It may well be that it is in fact the same answer, only expressed in different variables ... :rolleyes:
     
  7. Mar 28, 2017 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, if they wanted the answer in terms of m and v they should have said so, or not provided the variable names d and a.
    But surely there is something wrong with the given answer. The initial energy is mv2/2. If friction does work -mv2/2 there should be none left, yet it is rolling.
     
  8. Mar 28, 2017 #7

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    From the problem statement you could read that v is kept constant. But it takes some liberty...
     
  9. Mar 28, 2017 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Too much liberty for me. It says projected, not dragged.
    I think the answer is supposed to be -mv2/4.
     
  10. Mar 28, 2017 #9

    Baluncore

    User Avatar
    Science Advisor

    Yes, I agree, the correct answer should be – 0.25 m v2
    Half the initial energy is converted to rotational energy.
     
  11. Mar 28, 2017 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    No, half is lost to friction and a quarter converted to rotational.
     
  12. Mar 28, 2017 #11

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Agree. How about interpreting ##v## as the final velocity :rolleyes:
     
  13. Mar 28, 2017 #12
    Thank you very much sir for replying but it actually happened that both options were there in the question when my answer matched with one of those,I didn't check the others.
     
  14. Mar 28, 2017 #13
  15. Mar 28, 2017 #14

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Are you saying it was multiple choice, with -mv2/2 being only one of the possible answers?
     
  16. Mar 29, 2017 #15
    yes sir,the others being none of the above and insufficient information
     
  17. Mar 29, 2017 #16

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Strange.
    Leaving that aside, what answer can you get expressed only in terms of m and v?
     
  18. Mar 29, 2017 #17
    I got -0.25mv2
     
  19. Mar 29, 2017 #18

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Right.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Rolling and sliding ring
  1. Rolling ring! (Replies: 1)

  2. Rings of slide (Replies: 18)

  3. Roll or slide? (Replies: 14)

  4. Rings sliding on rods (Replies: 12)

Loading...