Third Newton's Law of Mechanics
Newton's third law states that when two bodies interact, they always exert equal and opposite forces on each other. This law is commonly known as the law of action and reaction. For example, a book lying on a table produces a downward force equal to its weight on the table. The third law states that the table exerts equal and opposite force on the book. The opposite force occurs because the weight of the book causes an invisible deformation of the table, which pushes the book upwards like a compressed spring. (Figure 2.) Figure 1. Driving a nail When a nail is driven, the force of the hammer and the counterforce of the nail are equal, from initial contact to the end of motion. It should be noted that Newton's first law also describes the action of equal and opposite forces, but on the same object, and as a result of the balance of forces, the object rests or moves uniformly in a straight line. This is fundamentally different from Newton's third law, which deals with equal and opposite forces, where each force acts upon the other object. As a result, these forces can have varying effects on the objects. Much like the aforementioned nailing. Depending on the vector sum of all forces, the total force on each of these objects could be non-zero. Figure 2. Balance of forces The weight of the book causes a deformation of the table, invisible to the naked eye. Convincing students of the ideas contained in Newton's third law — that there is an equal and opposite reaction to every force — can be difficult at times. If object A exerts a force on object B, object B responds with an equal and opposite force. As an auxiliary explanation, it is commonly stated that: if you push the wall with some force, the wall will push you back in the opposite direction with equal force. To overcome this difficulty, the following simple experiment can help. All of the students in the class should follow the instructions demonstrated with hands, and they repeat them for themselves. All you need for accessories is a wall and, of course, a hand to push it! Figure 3. Pushing a wall If you push a wall, the wall exerts an equal force in the opposite direction. |