本题目来源于试卷: gc physics textbook chapter 6 problems，类别为 IB物理学
[问答题]
Is the work done by k.a matq t)g7,j)j*x3o8zk rbl inetic ldqa np3:l4r; friction forces always negative? [Hint: Consider what qr; lp3 a:dnl4happens to the dishes when you pull a tablecloth out from under them.]
参考答案: The kinetic force of friction opposes the relative motion between two objects. As in the example suggested, as the tablecloth is pulled from under the dishes, the relative motion is for the dishes to be left behind as the tablecloth is pulled, and so the kinetic friction opposes that and moves the dishes in the same direction as the tablecloth. This is a force that is in the direction of displacement, and so positive work is done. Also note that the cloth is moving faster than the dishes in this case, so that the friction is kinetic, not static.
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