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质量为m的钢板与直立轻弹簧的上端连接,弹簧下端固定在地上.平衡时,弹簧的压缩量为X0,如图所示.一物块从钢板正上方距离为3X0的A处自由落下,打在钢板上并立刻与钢板一起向下运动,但不粘连,它们到达最低点后又向上运动.已知物体质量也为m时,它们恰能回到O点;若物块质量为2m,仍从A处自由落下,则物块与钢板回到O点时,还具有向上的速度v,物块向上运动到达的最高点与O点的距离为h,求:

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    There are some injuries that are common to all types of extreme sports. The following are some of the important things to do to prevent extreme sports injuries. ____ But they can definitely minimize the risk of injuries.

    Warm up.

    ____ They include a combination of cardiovascular(心脏血管的)exercises, strength drills and stretching. Cardiovascular exercises increase body temperature, heart rate and blood circulation, which is required for further movement of the body. Strength drills increase the stamina of the body, while stretching warms up the muscles preparing them for sudden forceful movements. ____ It also plays an important role in boosting the team spirit for a team sport. A warm-up session should last for at least 20 minutes and it can extend up to half an hour.

    Avoid overworking.

    There are some people who do not exercise their body for the sport on a regular basis and perform a week's task in a day or two. ____ They do not realize the effect of overworking during the performance and keep doing it until they feel extremely tired. Apart from the actual sport, one should also avoid overdoing the warm-up exercises before the activity.

    ____

    Just as warming up is essential before any rigorous(充满活力的) physical activity, cooling down the body after playing a rigorous sport is also a significant part of physical conditioning. When you are done with your sports session, you can conclude with some walking, jogging or light running. Slight stretching exercises, focusing on specific muscles, will also be useful for cooling down the body.

A. Cool yourself down.

B. Look for a coach to train you.

C. These people are likely to suffer from extreme sport injuries.

D. However, there are no perfect methods which can assure you 100% safety.

E. Along with physical preparation, warming up prepares you mentally to begin a sport.

F. You should also be careful about your clothing selection for warmth and protection.

G. Warm-up exercises are very important before you begin any sports practice.