下面列举的情况中所做的功不为零的是（ ）
A．举重运动员，举着杠铃在头上方停留3s，运动员对杠铃做的功
B．木块在粗糙的水平面上滑动，支持力对木块做的功
C．一个人用力推一个笨重的物体，但没推动，人的推力对物体做的功
D．自由落体运动中，重力对物体做的功
A、举重运动员，举着杠铃在头上方停留3s的过程中，没有位移，所以运动员对杠铃不做功，故A错误；
B、木块在粗糙的水平面上滑动，支持力方向向上，位移水平方向，力的方向和位移方向垂直，支持力不做功，故B错误；
C、一个人用力推一个笨重的物体，但没推动，位移为零，所以人的推力对物体不做功，故C错误；
D、自由落体运动的物体，重力方向向下，位移向下，所以重力做正功，不为零，故D正确；
故选：D．
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village. It was so hot that 1 felt very 1 . Then I began to think of the sweet watermelons and other fruits in Grandma's house. I could hardly wait to arrive there and 2 my grandma earlier. It took us one more hour to get there. When we walked into the house, Grandma was busy preparing 3 in the kitchen. I knew from her smile face that she was very glad to see us. She brought out a big watermelon and cut it. It was so nice to eat the watermelon on such a hot day. After lunch my mother felt tired and went to bed to have a rest. I asked my grandma to tell me a fair tale. We sat on the bed and she began to tell it. I listened to her 4 , It was not long before I fell 5 . I was dreaming when a terrible sound woke me up. I saw Grandma was lying on the ground. I couldn't believe my 6 . How fearful a sight was! I hurried to wake my mother up. We took Grandma to the 7 in a hurry. But it was too late. My grandma's heart stopped beating (跳动) when we got there. You'll never imagine how 8 we felt then. Grandma was very well all the time. I had never imagine she would 9 us so soon. That evening I lay in bed alone. Tears ran down my face. I prayed (祈祷) and prayed, wishing my grandma would be able to 10 back. Now several years have passed. My dear grandma, where are you? I 11 you so much. |