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Unformatted text preview: Physics 212 Classical and Modern Physics Spring 2008 Solutions for Hand-In Set #1 Problem A3 Attractive balloons Each balloon has three forces on it: gravity, tension, and the electric attraction. Heres the free body dia- gram for the balloon on the left F e T mg x y We are trying to get an expression for q in terms of m and , so well need to find the tension in terms of these quantities. Consider the y-component of the net force: X F y = T cos - mg = ma y = 0 T = mg/ cos Now we consider the horizontal components. The mag- nitude of the electric force is F e = k | q 1 || q 2 | r 2 = kq 2 x 2 Newtons 2nd law gives X F x = F e- T sin = ma x = 0 Now solve for q : F e = T sin kq 2 x 2 = mg cos sin q 2 = x 2 mg tan k q = x r mg tan k Wolfson Chapter 20-10 + + + + + + + + + + + + + + The positive charges on the balloon all mutually repel. This works against the elastic force, so it makes the bal- loon expand. If we used negative charges instead, theyloon expand....
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