【題目】判斷下列情況下物體的動能、重力勢能、機(jī)械能的變化情況。（選填“增大”或“減小”“不變”）

（l）在平直軌道上，列車從車站啟動，動能 ，勢能 ，機(jī)械能 。

（2）站在勻速下降的電梯中的乘客，動能 ，勢能 ，機(jī)械能 。

【答案】（1）增大；不變；增大．（2）不變；減小；減小

【解析】

試題分析：動能與物體的質(zhì)量和運動速度有關(guān)，質(zhì)量越大，速度越快，動能就越大．

重力勢能與物體的質(zhì)量和高度有關(guān)，質(zhì)量越大，高度越高，重力勢能就越大．

機(jī)械能是動能和勢能的總和．

列車從車站啟動，速度增大，故動能增大；列車在平直軌道上，高度不變，故重力勢能不變；動能增加，重力勢能不變，故其總和機(jī)械能也在增大．

乘客是勻速下降，速度不變，故動能不變；電梯下降，高度減小，故重力勢能減小；動能不變，重力勢能減小，故其總和機(jī)械能也在減小．

故答案為：（1）增大；不變；增大．（2）不變；減小；減小．

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