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Homework Help: Forces with universal gravity

  1. Mar 25, 2008 #1
    1. The problem statement, all variables and given/known data

    A bowling ball (mass = 7.2 kg, radius = 0.10 m) and a billiard ball (mass = 0.48 kg, radius = 0.028 m) may each be treated as uniform spheres. What is the magnitude of the maximum gravitational force that each can exert on the other?

    2. Relevant equations


    g being gravitaional constant

    3. The attempt at a solution

  2. jcsd
  3. Mar 25, 2008 #2
    Where is r^2?
  4. Mar 25, 2008 #3
    i have solved it by


    the only thing i didnt understand was where the objects relative to each other so i presumed they were touching.
  5. Sep 23, 2008 #4


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    Gold Member

    Take a look at the formula [tex]F=\frac{Gm_1m_2}{r^2}[/tex]. [tex]G[/tex] is a constant and so are [tex]m_1[/tex] and [tex]m_2[/tex]. Now we ask ourselves a question : " when will [tex]F[/tex] reach a maximum when [tex]r[/tex] varies?". It's obvious that more [tex]r[/tex] is little more [tex]F[/tex] is greater. So [tex]F[/tex] reaches a maximum when [tex]r[/tex] reaches its minimum. And what is the minimum of [tex]r[/tex]? It's simply the sum of the radius of the 2 balls, as you did. So it will be [tex]0.128\text{ m}[/tex].
    Take care about what you wrote in the denominator of
    , I think you forgot that [tex]r[/tex] was squared and not multiplied by [tex]2[/tex]. Otherwise everything's good.
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