126 The Laws of MotionP5.20(a)An explanation proceeding from fundamental physical principles willbe best for the parents and for you. Consider forces on the bit of stringtouching the weight hanger as shown in the free-body diagram:Horizontal Forces: Fmaxx∑=: −+=TTxcosθ0Vertical Forces: Fmayy∑=: −+=FTgsinθ0FIG. P5.20You need only the equation for the vertical forces to find that the tension in the string isgiven by TFg=sinθ. The force the child feels gets smaller, changing from Tto Tcosθ, whilethe counterweight hangs on the string. On the other hand, the kite does not notice what youare doing and the tension in the main part of the string stays constant. You do not need alevel, since you learned in physics lab to sight to a horizontal line in a building. Share withthe parents your estimate of the experimental uncertainty, which you make by thinking
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