Newton's Third 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. What force is the counterforce of the normal force of the books on the table?
Figure 3. Collision of a ball with a bat The force exerted by the ball on the bat, according to Newton's 3rd law, is opposed by a force exerted by the bat on the ball, of the same magnitude but in the opposite direction. However, the ball is noticeably deformed. Couple of forces for deformation of a ball in a collision with a baseball bat We know from experience that a donkey can pull a cart. But Newton's 3rd law says that the cart acts on the donkey with the same force in the opposite direction.
Slika 3. The donkey and the cart The force with which the donkey pulls is, according to Newton's 3rd law, opposed by the force with which the car acts on the donkey, of the same amount and in the opposite direction. Nevertheless, the donkey manages to pull the cart. If there was ice under the donkey's hooves (low friction) and the wheels were in mud (high friction), the donkey would not be able to pull the cart.
Figure 4. A horse on an icy road Depending on the friction under the hooves, the horse can pull with a force always less than its own weight. The force of friction is generally the product of the pressure on the surface (i.e. the weight of the horse) and the coefficient of friction which depends on the surface (grass, asphalt, ice, gravel) and ranges between 0.2 and 0.4. So a shod horse of 700 kg can pull with a force of about 1500 N on dirt, but it cannot pull at all on ice. Of course, a horse or a donkey pulling a cart does not have "starting blocks" like athletes, which would enable them to "push off" the ground without slipping. To a certain extent, horseshoes help horses on dirt, but on asphalt they even reduce the friction between iron and smooth asphalt.
Figure 5. Increasing and decreasing friction Two devices for opposite purposes related to on-ground moving friction.
Figure 6. Cog railway The low friction of the railway wheels is not enough on the steep sections in the Alps. This is where rack and pinion traction is applied along the entire track. The drive is transmitted by a gear, whereby slippage cannot occur. Rolling friction (of the idle wheels) opposes the car's motion, while the friction between the drive wheels and the road is the traction force that allows the car to move without wheel slippage, provided that the engine force FEg is less than the wheel friction:
Figure 7. Car and road In normal car driving, the driving force of the engine is equal to the frictional force FEg = Ffr. On the graph of the dependence of the friction force vs. the engine force, this can be seen as proportionality. The friction force is adjusted to the engine force until it reaches the maximum possible value Ffr_max = µs·Fp, where Fp is the wheel pressure force and µs is the coefficient of static friction. When this happens, the wheel slips, and the friction force is reduced to the value Ffr = µd·Fp, where µd, is the coefficient of dynamic friction, which is usually called for short, the coefficient of friction µ. The wheel pushes the road, and since the road is attached to the Earth, it pushes the entire Earth. According to Newton's third law, the Earth pushes the wheel away with an equal force in the opposite direction. 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 demonstration 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 7. Pushing a wall If you push a wall, the wall exerts an equal force in the opposite direction. |