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Homework Help: Pulling Tablecloth Under Steel Ball, Finding Speed

  1. Nov 15, 2004 #1
    Ok so I'm guessing my TA made up this problem on his own so I don't know how to start, considering there are no numbers and lol I just love having numbers to work with. Anyways here goes again lol I tried all weekend to get the solution and it's not working. Got help a week ago from this board but didn't understand the help.

    A tablecloth covers a horizontal table and a steel ball lies on top of it. The tablecloth is pulled from under the ball (in a perfectly horizontal fashion), and friction causes the ball to move and roll. What is the ball's speed on the table when it reaches a state of rolling without slipping? Assume that the table is large enough that the ball does not fall off of it.

    Help! :cry:

    Our TA said the answer is 0 but I don't know at all how he got that.
    Last edited: Nov 15, 2004
  2. jcsd
  3. Nov 16, 2004 #2
    Thats because no force acts on the ball but on the cloth. So th ball is moved only by the act of friction by the movement of the cloth.Since it tell you to calculate the speed of the ball when it reaches a state of rolling without slipping(act of frictions) the answer is that it doesnt have any speed......... enough??
  4. Nov 16, 2004 #3


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    This is a neat example of conservation of angular momentum.
    You should be able to show that angular momentum WITH RESPECT TO A POINT COINCIDENT WITH THE CONTACT POINT OF THE SPHERE is always conserved THROUGHOUT THE ENTIRE MOTION.
    (that is time spent on cloth plus time spent off cloth up to the point of rolling on table)
    Last edited: Nov 16, 2004
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