题目

   Alice was now in a long dark room with doors all round the walls, and she could not see the White Rabbit anywhere.    She tried to open the doors, but they were all locked (锁上的). “How will I ever get out again?” she thought sadly. Then she saw a little glass table with three legs, and on the top of it was a very small gold key. Alice quickly took the key and tried it in all the doors, but she couldn’t open any.    Then she saw another door, a door that was only forty centimetres high. The little gold key unlocked this door easily, but Alice couldn’t get through it人类的祖先钻木取火，为人类文明揭开了新的一页。钻木取火的一种方法如图所示，将削尖的木棒伸到木板的洞里，用力压住木棒来回拉动钻弓。木棒在木板的洞里转动时，板与棒互相摩擦，机械能转化为内能，而热集中在洞内，不易散发，提高了木棒尖端的温度，当达到约260℃时木棒便开始燃烧。因木头是热的不良导体，故受热厚度很薄，木棒受热部分的质量只有0.25克。已知：来回拉一次钻弓需1.0秒，弓长为s=0.25米，人拉弓的力为16牛顿，木头比热c=2×103 焦/（千克·℃），室温为20℃，问：（1）人来回拉一次钻弓克服摩擦力所做的功为多少？（2）人克服摩擦力做功使机械能转化为内能，若其中有25%被木棒尖端吸收，则1秒内可使木棒尖端温度提高多少℃？（3）请你估算用多长时间才能使木棒燃烧起来？