（10分） 从某一高度处以某一初速度水平抛出一个质量为2 kg的物体，经过时间
t=2s落地。落地时速度方向与竖直方向的夹角为37°。g取10 m/s2，已知sin37°=0.6，cos37°=0.8，求：
（1）物体抛出时的水平初速度；
（2）物体落地时重力的瞬时功率。
（1）15m/s（2）P="400" W
（1）落地时速度方向速度
（3分）
水平方向的速度
（2分）
（2）重力的瞬时功率P=mgvy （3分）
得重力的瞬时功率P="400" W （2分）
本题考查平抛运动规律的应用，竖直方向自由落体，水平方向匀速直线运动，把瞬时速度分解，根据两个方向的分运动求解，由功率P=Fv，速度v为F方向的分速度，由此可求得重力瞬时功率
Computers can injure (伤害) you. Most other injuries happen suddenly. For example, if you fall over a bike and break your arms, it happens very . But computer injuries happen slowly. You probably know how to ride a bike safely. Now learn to use a computer safely. Your eyes light can injure your eyes, never sit too close to a computer screen. Your eyes should be 50centimeters（厘米）on the screen. remember to look away from it. This gives your eyes a rest. When you use a computer, the window be on your left or your right. If it is behind you, the light will reflect on（反射）the screen. If the window is in front of you, the sun and the screen will shine into your eyes. Your hands and wrists(手腕) Hand and wrist injuries can happen because the hands and wrists are moved in the same way. Hundreds of times. If you use a keyboard for a long time, these three rules: 1）Rest your wrists on something. 2）Keep your elbows（肘）at the same height as the keyboard. 3）Stop sometimes and exercise your hands, wrists and fingers in a different way. Your back Some people sit for many hours in front of a computer. If you sit in the wrong way, you can injure your back or your neck. So you should sit with your back straight. The top part of the screen should be in front of your eyes. Your forearms, wrists, hands and the upper part of your legs should all be parallel（平行）to the floor. If you are sitting for a long time, get up every 30 minutes and your arms, legs and neck. your computer, but use it safely.
|