匀加速行驶的汽车，经路旁两根相距50m的电杆共用5s时间，它经过第二根电线杆时的速度是15m／S，则经第一根电线杆的速度为( )
A．2m／s
B．10m／S
C．2．5m／S
D．5m/s
答案：D
考点：
专题：直线运动规律专题．
分析：在匀变速直线运动中，物体的平均速度为
，据此可正确解答本题．
解答：解：物体经过两根电线杆的平均速度为：
①
由于物体做匀加速直线运动，所以有：
②
联立①②两式代入数据得：v1=5m/s，即经过第一根电线杆的速度为5m/s，故ABC错误，D正确．
故选D．
点评：本题考查了运动学推论公式的应用，对于这些推论要会正确推导，明确其适用条件．
Joe Simpson and Simon Yates were the first people to climb the West Face of the Siula Grande in the Andes mountains. They reached the top _______, but on their way back conditions were very _______. Joe fell and broke his leg. They both knew that if Simon _______ alone, he would probably get back _______. But Simon decided to risk his _______ and try to lower Joe down the mountain on a rope（绳）. As they _______ down, the weather got worse. Then another _______ occurred. They couldn’t see or hear each other and, _______, Simon lowered his friend over the edge of a precipice（峭壁）. It was _______ for Joe to climb back or for Simon to pull him up. Joe’s _______ was pulling Simon slowly towards the precipice. _______, after more than an hour in the dark and the icy cold, Simon had to _______. In tears, he cut the rope. Joe _______ into a large crevasse（裂缝）in the ice below. He had no food or water and he was in terrible pain. He couldn’t walk, but he _______ to get out of the crevasse and started to _______ towards their camp, nearly ten kilometers _______. Simon had _______ the camp at the foot of the mountain. He thought that Joe must be _______, but he didn’t want to leave _______. Three days later, in the middle of the night, he heard Joe’s voice. He couldn’t _______ it. Joe was there, a few meters from their tent, still alive.
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