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124_Physics ProblemsTechnical Physics

124_Physics ProblemsTechnical Physics - 126 P5.20 The Laws...

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126 The Laws of Motion P5.20 (a) An explanation proceeding from fundamental physical principles will be best for the parents and for you. Consider forces on the bit of string touching the weight hanger as shown in the free-body diagram: Horizontal Forces: F ma x x = : + = T T x cos θ 0 Vertical Forces: F ma y y = : + = F T g sin θ 0 FIG. P5.20 You need only the equation for the vertical forces to find that the tension in the string is given by T F g = sin θ . The force the child feels gets smaller, changing from T to T cos θ , while the counterweight hangs on the string. On the other hand, the kite does not notice what you are doing and the tension in the main part of the string stays constant. You do not need a level, since you learned in physics lab to sight to a horizontal line in a building. Share with the parents your estimate of the experimental uncertainty, which you make by thinking
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