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$($a$)$ Evaluate the gravitational potential energy between two $6$ kg spherical steel balls separated by a center$-$to$-$center distance of $15$ cm $.$

$P$

$($b$)$ Assume the balls are both initially at rest relative to each other in deep space $($when separated at the distance in part A$),$ and they then drift towards one other until they collide. Use conservation of energy to find how fast will they be traveling upon impact. Each sphere has a radius of $4\mathrm{.}9$ cm $.$

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