1. Limited time only! Sign up for a free 30min personal 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!

Relative speed problem

  1. May 5, 2014 #1
    1. The problem statement, all variables and given/known data
    Mary sits on the perimeter of a merry-go-round undergoing uniform circular motion. Matt stands on the ground 12 m from the centre of the merry-go-round. Matt observes Mary coming towards him at a speed of 1 m/s. What is Matt's speed from the point of view of Mary?

    (Ans: 2m/s)

    2. Relevant equations

    3. The attempt at a solution
    I made a sketch according to the given problem. A denotes Mary and B denotes Matt.

    Shouldn't the answer be 1 m/s? If Matt observes Mary coming towards him with 1m/s, then Mary should also observe Matt coming towards her with 1 m/s. I don't see why this is wrong.

    Any help is appreciated. Thanks!

    Attached Files:

  2. jcsd
  3. May 5, 2014 #2
    Neither do I.

  4. May 5, 2014 #3
    I myself wanted to dismiss the answer as wrong but considering that the problem is from a very trusted source, I couldn't. Here is the source: http://www.komal.hu/verseny/feladat.cgi?a=feladat&f=P4619&l=en

    Is my interpretation of problem correct? Would it be right to say that 1m/s is along the tangent of merry go round?
  5. May 5, 2014 #4
    Maybe it loses something in the translation. They are talking about speeds, not velocities. Who know's what they really meant?

  6. May 5, 2014 #5
    I think you are right. Usually the translation is ok but looks like they missed it this time.

    I tried to look at the Magyar version of the website. This is the Magyar version of the problem: http://www.komal.hu/verseny/feladat.cgi?a=feladat&f=P4619&l=hu . From Google Translate, I see that they mention the radius of merry-go-round which is absent from the English version. Maybe they missed something more?
  7. May 5, 2014 #6
    I think they meant velocity by sebesség.

    (Szerintem a sebesség velocity lenne angolul)

    Also, I don't think that sugár is a brilliant replacement word for radius, but that's just my opinion.
  8. May 5, 2014 #7
    Do you know Hungarian? If so, can you please translate the problem? That would be greatly appreciated. Thanks!
  9. May 5, 2014 #8
    Yes I speak it fluently.

    Mari egy 6 méter sugarú, egyenletesen forgó körhinta szélén ül. Máté a körhinta középpontjától 12 méterre a földön áll. Máté úgy látja, hogy Mari éppen feléje mozog 1 m/s sebességgel. Mekkora sebességgel mozgónak látja Mari Mátét?

    Mary is on the edge of a uniformly rotating carousel with radius of 6 meters. Mate stands 12 meters away from the axis of the carousel on the ground. Mate sees as Mari approaches her with a velocity of 1 m/s. At what velocity does the motion of Máté appear to Mari?
  10. May 5, 2014 #9
    Thank you once again!

    So looks like the missing piece of info was the radius of merry-go-round and the velocity part. But still, I don't see why the answer should differ from 1m/s.
  11. May 5, 2014 #10
    I suspect that there is a Chance that the English to Hungarian translation went wrong somewhere. (the Question on the Hungarian website is taken from an American paper)
  12. May 5, 2014 #11
    Ah, looks like I should accept that the problem is erroneous. :(
  13. May 5, 2014 #12
    There are many ways for a question to go wrong. Possibly the answer of 2 on the website is a typo. Who knows?
  14. May 5, 2014 #13
    The problem statement is probably erroneous because I doubt they would ask such an easy problem (with answer 1m/s) for 4 points. :tongue:
  15. May 5, 2014 #14


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    There's a way to get an answer of 2 m/s. Note first that Mary sits on the edge of the merry-go-round.

    What if she sat at the center of the merry-go-round? Would Mary see Matt moving at the same speed that Matt sees Mary moving?
  16. May 5, 2014 #15
    Why should it be different? :confused:
  17. May 5, 2014 #16


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    If Mary sits at the center, then she sees Matt (and trees, houses, etc.) whirling around her.
  18. May 5, 2014 #17
    Ah yes, I think I see how you reach the answer of 2m/s. If I consider the angular velocity to be 6 rad/s, then Matt has a speed of 2 m/s in frame of Mary. This is probably what the question meant. Thanks TSny! :smile:
  19. May 5, 2014 #18


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    6 rad/s?
  20. May 5, 2014 #19
    Woops, I meant 1/6 rad/s. :redface:
  21. May 5, 2014 #20


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    OK. Sounds good.
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: Relative speed problem
  1. Relative speed (Replies: 2)

  2. Relative Speed (Replies: 1)

  3. Relative Speed (Replies: 1)

  4. Relative Speed (Replies: 0)