某校课外活动小组自制了一枚质量为3.0kg的实验用火箭。设火箭发射后，始终沿竖直方向运动。火箭在地面点火后升至火箭燃料耗尽之前可认为做初速度为零的匀加速运动，经过4.0s到达离地面40m高处燃料恰好耗尽。忽略火箭受到的空气阻力，g取10m/s2。求：
（1）燃料恰好耗尽时火箭的速度大小；
（2）火箭上升离地面的最大高度；
（3）火箭加速上升时受到的最大推力的大小。
解：（1）设燃料恰好耗尽时火箭的速度为v，根据运动学公式
解得
m/s=20m/s
（2）火箭燃料耗尽后能够继续上升的高度
m=20m
火箭离地的最大高度：H=h+h1=40+20=60m
（3）火箭在飞行中质量不断减小，所以在点火起飞的最初，其推力最大。根据加速度定义及牛顿第二定律
=5m/s2
F－mg=ma
F=m（g+a）=3×（10+5）=45N
Have you had a headache during a test Have you ever been so worried about something that you have a headache or even can’t sleep at night If so, then you know what stress is. Stress is what you feel when you are worried about something. This worry in your mind can make your body feel bad. You may feel angry, sad, scared, or afraid—all of which can give you a stomachache or a headache.
However, there are different kinds of stress. Some kinds of stress are good and others are bad. Good stress might happen when you’re called to answer questions in class or when you have to give a speech. This kind of stress can help you to get things better done. For example, you may do a better job on your test if the stress pushes you to prepare better before the test.
On the other hand, bad stress can happen if stress lasts too long. You may not feel well if your parents are fighting, if a family member is sick, if you’re having problems at school, or if anything else makes you unhappy every day. That kind of stress isn’t going to help you. And it can actually make you sick.
The best way to fight stress is to have a balanced life. Make sure you keep your SELF in mind: Sleep, Exercise, Leisure(休闲), and Food. If you get enough sleep and eat properly, and if you exercise and leave time for fun, you’ll probably feel less stressed.
| Title: Face the Stress | ||
| Meaning of stress | Stress is a (68) when you are worried about something. | |
| Possible effect | You may feel uncomfortable in your (69) and body. | |
| Good stress | · It might happen when you have to (70) in public. · It will be more (71) for you to do the job successfully. · It makes you (72) better before doing the job. | |
| Different kinds | Bad stress | · It might happen if your stress (73) too long. · You may not feel well with the (74) of a family member, problems at school or something else that worries you every day. · It might stop you from getting your work well done. |
| How to keep stress (75) | Have a balanced life. · Have enough sleep and have (76) food. · Take exercise and have time to (77) yourself. | |