牛顿运动第二定律定义

牛顿第二运动定律指出，当合力作用在物体上时，物体因合力而产生的加速度与力成正比。

作为一个等式，我们写道，

The summation sign,  , indicates that one needs to add all the forces using vector addition and find the resultant (or the net) force. According to Newton’s second law of motion, the resultant force is proportional to acceleration. This means that if the resultant force acting on a body is doubled, then the body’s acceleration would also double. If the resultant force is halved, the acceleration will also be halved and so on.

另一种表达牛顿第二运动定律的方法是使用动量。在这个定义中，物体所受的合力等于物体动量的变化率。

If we take the case of a body whose mass stays c***tant, since , this expression becomes:

现在，让我们看一个牛顿第二运动定律的简单例子。

牛顿第二运动定律举例

两名海盗拉着一个重55公斤的宝箱。一个海盗用18N的力把它拉向大海，而另一个海盗用30N的力把它拉向相反的方向。找到宝箱的加速度。

The two forces given by the two pirates are in the opposite directi***, so the resultant force is (30-18) = 12 N away from the Sea. Now, using Newton’s second law, we have   away from the Sea.

如何求解牛顿第二定律问题

电梯问题

To conclude this article, we will look at a classic physics problem involving the reaction force on a person in a lift. Suppose a person with mass   is standing inside a lift. Forces acting on the person are the weight   acting downwards and the reaction force  from the floor of the lift acting upwards.

 First, let’s take the case when the lift is still. The forces on the person are balanced. i.e.  .

Now, suppose the lift is accelerating downwards. In this case, there is a resultant force acting downwards on the person. The resultant force gives an acceleration  . Then, taking downwards direction to be positive, we have

.

假设升力现在以相同的加速度向上移动。在这种情况下，

.

因此，当电梯向上加速时，人会感受到更大的反作用力。这是很直观的：当电梯的地板向上冲去迎接这个人时，他们应该感觉到比地板试图从他们身上“脱落”时更大的力量。当电梯向下加速时，所经历的较低的反作用力通常会使你在乘坐电梯时感觉更轻。