If a falling hammer hits your foot, it will hurt you because at the moment of contact it exerts a force on your foot. This is easy to understand and accept.
It turns out that your foot also applies to the hammer a force of equal intensity to the force it receives from the hammer. This is already harder to understand and to accept.
Let us then choose a more convincing example. Imagine an egg falling to the floor. At the moment of contact, the egg applies a vertical downward force on the floor and the ground applies an upward vertical force of the same intensity. It is this force that makes the egg break!
When you kick a ball, you apply a force to it that makes it move. At the same time it applies a force to your foot that you can feel. By striking a wall with your hand, you are applying a force to it. At the same time, your hand will receive a force of the same intensity from the wall, which may even hurt it.
Newton expressed ideas like these through the so-called Newton's Third Law, or principle of action and reaction, which can be stated as follows.
|For any force that body A applies to body B, there will be a force of the same intensity, of the same direction, but in the opposite direction, applied by body B to body A. One of these two forces, no matter which, may be called action. and the other reaction.|
Action and reaction act on distinct bodies
Newton's third law reveals an important feature of forces: they always occur in pairs. In other words, forces are the result of interaction between bodies. It is the hammer interacting with the foot, the egg interacting with the floor, the hand interacting with the wall, etc.
A very important feature of every pair of action-reaction forces is that they act on distinct bodies, never on the same body. When someone tries to push the wall (the word "push" here indicates an application of a force and not a movement), the wall pushes that person with the same intensity and direction but the opposite direction. One of these forces, that applied by the wall, acts on the person.
Since action and reaction act on different bodies, they often have different effects. When a soccer ball hits a window pane, both bodies interact; The force the glass applies to the ball slows it down, while the force the ball applies to the glass can break it.