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Homework Help: Confused about rolling sphere problem

  1. Dec 15, 2015 #1
    1. The problem statement, all variables and given/known data
    A sphere of radius .06 m and mass .5 kg rolls down a ramp that is angled 30 degrees down the incline. It starts rolling from a height of 7 feet and does not slip

    What is its final linear velocity?
    Now, I used mgh=translational +rotational KE and found that the final velocity was 9.9, but I didn't want to do that.

    I wanted to use the formula acm=(g*sinθ)/(1+β) where β is the coefficient in front of mr^2 (for a sphere, .4 or 2/5).
    So, Vf^2=vo^2+2*a*Δx.

    My question is, which Δx do I use? I initially used the height and my answer was wrong...I was supposed to use the ramp, which by trig is 14 meters.

    2. Relevant equations
    KE and PE equations.

    3. The attempt at a solution

    I already know the answer. I just want to know why the length of the ramp is used for v^2=vo^2+2adelta x equation.

    Thanks, this should only take a moment. Appreciate any input guys! :) :)
  2. jcsd
  3. Dec 15, 2015 #2


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    You should use the length of the ramp because the force you used to obtain the acceleration is in the downramp direction (g*sinθ), not vertical.
  4. Dec 16, 2015 #3
    Aha, that is what I suspected. By the way Haruspex, you have commented on all of the posts that I have made on physics forums so far. Appreciate the help!!!​
  5. Dec 16, 2015 #4


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    Maybe our timezones match.
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