如圖所示OA是起重機(jī)的吊臂，可繞O點(diǎn)轉(zhuǎn)動(dòng)．在距O點(diǎn)6m遠(yuǎn)的B處吊有重3000 N的物體．為保證吊臂在水平位置平衡，則繞過(guò)定滑輪斜向下的拉力F為_____N.將吊臂緩慢拉起，使用A點(diǎn)升高2m的過(guò)程中，拉力變______（選填變大、變小或不變）．（繩重、吊臂重、摩擦均不計(jì)）
3600，變小

試題分析：如圖可知，阻力是3000N，阻力臂是6m，動(dòng)力臂是5m，根據(jù)杠桿的平衡條件，，，∴；
將吊臂緩慢拉起，使用A點(diǎn)升高2m的過(guò)程中，阻力臂減小，所以阻力×阻力臂變小，動(dòng)力臂不變，所以動(dòng)力變小。
點(diǎn)評(píng)：解決本題的關(guān)鍵是找到動(dòng)力臂和阻力臂的大小，利用杠桿的平衡條件做題。
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