vật có khối lượng m ; gắn vào lò xo độ cứng k . vật m đặt trên tấm ván nằm ngang.ban đầu lò xo thẳng đứng và chưa biến dạng dài l0. kéo tấm ván từ từ ; do ma sát giữa vật và ván nên m di chuyển theo.đến khi m bắt đầu trượt tren ván thì lo xo hợp với phương thẳng đứng 1 góc alpha . hãy tính công của lực ma sát tác dụng lên vật từ khi cân bằng đến khi bắt đầu trượt

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