Section outline

  • Boys and girls competing in a tug of war, pulling a rope in opposite directions.

    It’s boys against girls in this friendly tug of war. The two teams are pulling the rope in opposite directions. Which team do you think will win? It depends on which side pulls on the rope with the greatest force. As this example shows, more than one force may act on an object at the same time. Would it surprise you to learn that at least two different forces are acting on you as you read this article? Can you guess what they are?
    ::在这场友好的战争中,男孩与女孩对立。两支部队正向相反的方向拉绳子。 你认为哪个队会赢?这取决于哪一边用最强大的力量拉绳子。正如这个例子所示,不止一支部队可以同时对一个物体采取行动。你是否惊讶地获悉,在你阅读这篇文章时,至少有两支不同的部队在对你采取行动?你能猜猜他们是什么吗?

    Pulling Down and Pushing Up
    ::推拉拉拉拉拉

    One force acting on you—and all the other objects on Earth—is gravity. Look at the physics book in the Figure . Gravity pulls the book downward with a force of 20 Newtons. Why doesn’t the book fall to the ground? The table pushes upward on the book with the same amount of force. The combined force, or net force , acting on the book is 0 Newtons. That’s because upward and downward forces are balanced, so they cancel out. 
    ::一种力量在你身上,而地球上所有其他物体都是引力。看看图中的物理书。重力以20牛顿的力量拉下书本。为什么书不倒在地上?表格以同样的力度向上推动书本。 联合力量或净力量在书上的动作是0牛顿。这是因为向上和向下的力量是平衡的,所以他们取消了。

    A physics book on a table with equal forces of gravity and support shown.

    Forces Acting in Opposite Directions
    ::对面指挥部队

    In general, whenever forces act on an object in opposite directions—like the book on the table—the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal, or balanced, the net force is zero, as it is for the book. That’s why the book doesn’t fall to the ground but instead remains resting on the table. However, if the opposing forces are unbalanced, the net force is greater than zero, although it will be less than either of the individual forces. In this case, the object will move in the same direction as the net force.
    ::一般来说,每当部队以相反的方向对一个物体采取行动时——就像在桌上的书一样——净力等于两个部队之间的差别。 换句话说,一股力量从另一股力量中减去来计算净力。 如果对立力量是相等的或平衡的,那么净力是零的,就像书中的一样。 这就是为什么书不会落到地上,而是继续停留在桌面上。 但是,如果对立力量不平衡,净力就大于零,尽管将少于单个部队中的任何一个。 在这种情况下,目标将朝着与净力相同的方向移动。

    Look at the dogs playing tug-of-war in the Figure . The dogs are pulling the rope in opposite directions, but one dog is pulling with more force than the other. The net force acting on the rope is 2 Newtons to the right, so the rope will move to the right.
    ::看看图中玩拖战的狗。这些狗把绳子拉向相反的方向,但一只狗的拉力比另一只大。绳子上的网力在右边是2牛顿,所以绳子会向右移动。

    Two dogs pulling a rope in opposite directions with net force indicated.

    Q : The boys in the Figure are about to kick the soccer ball in opposite directions. What will be the net force on the ball? In which direction will the ball move?
    ::问题:图中的男孩们正要向相反的方向踢足球。球上的网力是什么?球往哪个方向移动?

    Two boys kicking a soccer ball in opposite directions with forces of 75 N and 125 N.

    A : The net force on the ball will be 50 N to the left (125 N – 75 N = 50 N), so the ball will move to the left.
    ::A:球的净力向左为50牛顿(125牛顿-75牛顿=50牛顿),所以球向左移动。

     

     

    Forces Acting in the Same Direction
    ::在同一方向行动的部队

    If two forces act on an object in the same direction, the net force is equal to the sum of the two forces. This always results in a stronger force than either of the individual forces alone. In the Figure , after the man on the left picks up the couch, he will push the couch to the right with a force of 25 Newtons, and the man on the right will pull the couch to the right with a force of 20 Newtons. The net force on the couch is 45 Newtons to the right, so that’s the way the couch will move.
    ::如果两支部队在同一方向对一个物体采取行动,则净力等于两支部队的总和。 这总比单方部队中的任何一个部队都更强大。 图中,左翼男子在拿起沙发后,会用25牛顿的力量将沙发推向右侧,右翼男子用20牛顿的力量将沙发拉向右侧。沙发上的净力是45牛顿向右,所以沙发的移动方式就是这样。

    Two men pushing a couch to the right with combined forces of 25 N and 20 N.

    Summary
    ::摘要

    • The net force acting on an object is the combination of all of the individual forces acting on it.
      ::在物体上行动的净力量是所有在物体上行动的个别力量的结合。
    • If two forces act on an object in opposite directions, the net force is the difference between the two forces. In this case, the net force is always greater than or equal to zero but less than either of the individual forces.
      ::如果两支部队对一个物体采取相反的行动,净兵力是两支部队之间的差别,在这种情况下,净兵力总是大于或等于零,但小于两支部队中的一支。
    • If two forces act on an object in the same direction, the net force is the sum of the two forces. In this case, the net force is always greater than either of the individual forces.
      ::如果两支部队朝同一方向对一个物体采取行动,净部队就是两支部队的总和,在这种情况下,净部队总是大于两支部队中的任何一支部队。

    Review
    ::回顾

    1. What is the net force acting on an object?
      ::在物体上行动的净力量是什么?
    2. If an object has two forces acting on it, how can the net force equal 0?
      ::如果一个物体有两股力量在它上行动,其净力量如何等于0?
    3. Under what conditions does the net force acting on an object equal the sum of the individual forces?
      ::在什么条件下,在物体上行动的净额部队等于个别部队的总和?
    4. What is the net force on the book in the Figure ? If the book moves, in which direction will it move?
      ::图中书上的净力是什么?如果书移动,它会朝哪个方向移动?

    Forces acting on a book indicate potential motion direction based on their interaction.