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Homework Help: Speed of a Bowling Ball

  1. Oct 20, 2008 #1
    1. The problem statement, all variables and given/known data
    One version: A bowling ball is thrown down the alley with speed v(0). Initially, it slides without rolling, but due to friction, it begins to roll. Show that it's speed when it rolls without sliding is 5/7*v(0) .
    Another version: Given a bowling ball thrown down the lane with speed v(0) , find the speed v(f) in terms of v(0) at which the ball rolls without slipping.

    2. Relevant equations
    KE = 1/2*m*v^2
    F = ma
    v(f)^2 = v(0)^2 + 2ax
    T = Ia

    3. The attempt at a solution
    My second idea was that the torque of the ball must equal the work due to friction. Following this line worked out, and I got that the final speed is 5/7 of the initial speed. Cool beans.
    My first idea was to use conservation of energy. So...

    1/2 * m * v(0)^2 = 1/2 *m * v(f)^2 + 1/2 * I * omega(f)^2 + F(fr) * D

    Except... what is D? According to various methods I tried, it's NOT R*theta, or the x in the third equation, or even R*theta - x. I say it's not because I didn't get 5/7 for the answer. (I would admit to doing math wrong, except I sat down with my professor and watched him do it and get the same wrong answers that I did. So, either we both suck at math, or the answers were "right".)
    Knowing what v-final is *supposed* to be, I went back and substituted it into my energy equation. I also substituted in F(fr)= ma = m * ((v(f)^2 - v(0)^2) / 2d )
    I ultimately got D being 7/12 * d, d = distance the ball is translated.
    (I assume this value will work if I plug it into my energy equation and try solving for v(f), but I did NOT double check.)

    So, if my value for D is correct... what does it mean? It seems like a rather random number.
    Or even if my value for D isn't correct... what am I supposed to use? And how am I supposed to find it?
  2. jcsd
  3. Oct 20, 2008 #2
    First, semantics: "torque" can never equal "work".

    Second, the work done by friction is NOT equal to F*D, or even \int F(x) dx. The reason is that the point at which the frictional force acts (the bottom of the bowling ball) is moving at a different velocity than the center-of-mass of the ball. The work friction does per unit time is \vec{F}\cdot\vec{v} where \vec{v} is the velocity of the bottom of the ball. For instance, when the ball is rolling without slipping, friction does zero work (even though the frictional force is not zero)!
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