如图所示，在倾角为θ的光滑斜面上有两个用轻质弹簧相连接的物块A、B，它们的质量分别为mA、mB，弹簧的劲度系数为k，C为一固定挡板．系统处于静止状态，现开始用一大小为F的恒力沿斜面方向拉物块A使之向上运动，重力加速度为g，求物块B刚要离开C时物块A的加速度a和从开始到此时A、B及弹簧组成的系统增加的机械能．
设未加F时弹簧的压缩星为x1，由胡克定律得
mAgsinθ=kx1
设B刚要离开C时弹簧的伸长量为x2，此时A的加速度为a，由胡克定律和牛顿定律
kx2=mBgsinθ
F-mAsinθ-kx2=mAa
联立得a=F-(mA+mB)gsinθ mA
由题意得物体A上滑的距离为 d=x1+x2
则d=(mA+mB)gsinθ k
故系统增加的机械能：W=Fd=F(mA+mB)gsinθ k
答：物块B刚要离开C时物块A的加速度a是
．从开始到此时A、B及弹簧组成的系统增加的机械能为F-(mA+mB)gsinθ mA
．F(mA+mB)gsinθ k
Meat and vegetables are measured in grams and kilograms. Milk and other liquid foods are measured in liters or milliliters. These units only measure quantity: they do not measure the value of the food to the body. The unit which measures the quality or value of food is the calorie the amount of heat given off by food when it brurns. This measurement tells how much energy a certain food has when it is completely used by the body. Our bodies use varying（不同程度的） amount of calories. The more exercise we take, the more calories we burn. If we eat food which contains more calories than we use up, then it is possible that we would increase in weight. In order to avoid becoming overweight, it is advisable to eat a balanced diet and not eat too many foods that have a high calorie rating. The table below gives you some idea of the number of calories in food.
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