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【题目】两个完全相同的条形磁铁,放在平板AB上,磁铁的NS极如图所示,开始时平板及磁铁皆处于水平位置,且静止不动。

a现将AB突然竖直向上平移平板与磁铁之间始终接触,并使之停在A″B″处,结果发现两个条形磁铁吸在了一起。

b如果将AB从原来位置突然竖直向下平移,并使之停在位置A′B′处,结果发现两条形磁铁也吸在了一起,请回答下列问题:

1开始时两磁铁静止不动,磁铁间的相互作用力是________力;右侧的磁铁受到的静摩擦力的方向向________

2a过程中磁铁开始滑动时,平板正在向上加速还是减速呢?答:________

3b过程中磁铁开始滑动时,平板正在向下加速还是减速呢?答:________

【答案】1吸引 2减速 3加速

【解析】1磁铁静止时,二者间的相互吸引力与静摩擦力大小相等,对右侧的磁铁来说,吸引力向左,静摩擦力向右。2当平板向上运动而磁铁吸在一起时,说明摩擦力减小,磁铁所受支持力减小,磁铁应处于失重状态,所以平板应向上减速运动,加速度方向向下。3当平板向下运动而两磁铁吸在一起时,磁铁应处于失重状态,所以加速度仍向下,故是向下加速运动。

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