打点计时器是一种使用____（填“交流”或“直流”）电源的计时仪器，电磁式打电计时器的工作电压为4V～6V．当电源的频率是50Hz时（我国工农业和生活用的交流电的频率都是50Hz），它每隔____ s打一个点，因此纸带上的点就表示了与纸带相连的物体在不同时刻的位置，研究纸带上点之间的间隔，就可以了解物体运动的情况．

    It is Saturday morning. The sky is blue. My father and I are going to the park. There are a lot of people in the park. Some children are running on the grass. A dog is running after them. Some men are sitting on the grass. They are talking. Two girls are flying a nice kite. Three women are standing under a tree, and they are watching the children playing. There is a boat in the lake and two women are boating. Everyone in the park is very happy.

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