题目

质量为500 t的机车以恒定的功率由静止出发.经5 min行驶2.25 km,速度达到最大值54 km/h,设阻力恒定且取g=10 m/s2,问:(1)机车的功率P是多大?(2)机车的速度为36 km/h时机车的加速度a是多大? 答案：解析：(1)以机车为研究对象,由静止开始出发至达到速度最大值的过程,外力有牵引力做正功,阻力做负功,动能由零增加到最大值,由动能定理得:P·t-Ff·s=mvm2-0                                                           ①且机车速度达最大时有F牵=FfP=F·vm=Ff·vm              In our daily life, many of us feel stressed more or less. Some like this pressure and work better because of it. Others are not comfortable with any stress at all; they soon become unhappy if they feel stressed. Sometimes stress can lead people to do things they wouldn’t usually do, such as overeat, smoke, drink, or use drugs. Stress, however, is a very normal part of life.It is important to understand that stress doesn’t come from an event itself, that is, from the things that are happening in our lives. It comes from the meaning we give to what has happened. For example, a crying baby may be stressful to one person, but it may not bother another person at all; a traffic jam may be stressful to one person while another person may be able to stay calm.We can experience stress any time we feel we don’t have control. It can come from a feeling that we can’t do anything about a situation. Basically, it is the body’s way of showing anxiety or worry.Stress is not just caused by our mental or emotional condition. It is also influenced by how tired we are, whether we have a balanced diet with enough vitamins and minerals, whether we get enough physical exercise, and whether we can relax.The key point of relieving the stress is that we need to find what is causing the stress in our lives. Once we have found it, we should try to change that part of our lives. If we believe that we can control stress, we can begin to control our lives. Then we can start to use stress in a positive way.【1】According to the text, ________ can cause stress.A.overeatB.smokeC.a traffic jamD.a hard test【2】The author mentions the example of a crying baby in order to ________.A.explain that stress is everywhereB.tell different people react to stress differentlyC.help people learn that stress comes from an eventD.give the definition of stress【3】What does the author suggest dealing with stress?A.Finding the cause and making changes.B.Controlling the emotions.C.Having enough sleep.D.Taking some drugs.【4】From the text, we can conclude that ________.A.if someone has stress he will do something wrong unavoidablyB.anyone can experience stress when he is out of controlC.one will feel no stress at all if he gets enough physical exerciseD.one is sure to work better because of the stress