When teaching physics begins to explain Newton’s mechanics, the famous three laws of dynamics, and the nature of forces, everything usually starts off well. Forces are abstract entities (anything capable of changing the state of rest or movement of a body), but they are logical, because they are experienced daily.
The examples are innumerable. Lifting a box, which implies changing its state of rest to make it rise, involves making a force, which will entail a greater effort the more mass the box has. When you receive a push, the force exerted is clearly noticeable.
Sooner or later, in the syllabi the term ‘fictitious forces’ or ‘inertial forces’ appears , whose definition is simple: forces that ‘appear’ when a movement is described from a non-inertial frame of reference (in essence, one that is subject to to accelerations). It is very easy to give an example of one such system. A person traveling in a car will unconsciously take the car itself as a reference system, since it is still with respect to it. However, the car is subjected to accelerations all the time: when braking, accelerating or cornering. Therefore, a running car is a non-inertial frame of reference.
The concept of fictitious forces starts out as picturesque, but when the physics teacher asserts that the force experienced when a car brakes, pushes forward, or pulls the trunk sideways when cornering are fictitious, there are those who lose faith in what they are telling you. How is it possible that the force that pushes you forward when braking is fictitious, if the seat belt actually hurts you when you hold on? How can the force that wants to hit me against the door be fictitious when the vehicle turns?
The explanation is simple. When a car brakes, the slowing force is exerted on the car , not on the people inside. The passenger will tend to continue moving at the same speed as the vehicle was driving before braking. As physics is described from the point of view of the car, the force it has received only has the effect of reducing the speed of the reference frame itself. The passenger feels that something is pushing him forward, but that is not a force. What is a force is the one that the seat belt, which is attached to the vehicle, exerts on the passenger to match its speed to that of the car.
It can be understood if the same event is described from a reference system that can be considered inertial, such as that of an observer who is standing still on the shoulder. When that observer sees the car braking, he notices a force that slows the car. You will not see any force acting on the passenger that pushes him forward. In fact, if the seat belts and the windshield did not exist, the passenger would be thrown out of the vehicle because nothing would slow him down.
The same origin has the feeling of being pushed out of a curve. The one who receives the force that makes it turn is the car. The passenger tends to continue traveling in a straight line and it must be the seat belt or another element that exerts a force towards the inside of the curve so that the passenger turns with the car.
Related to rotations, there are two well-known forces: the centrifugal and the centripetal. Since the equations that give its intensity (modulus) are identical, they tend to be confused and, furthermore, as the centrifuge is the one felt when traveling in a vehicle, it is common to forget the centripetal which, with a nuance, is a force real.
As mentioned before, to change the movement of a body you have to apply an acceleration. Sometimes you don’t want to reduce the speed modulus (km / h), but change the direction of movement. To do this, you have to exert a force directed towards the center of rotation (for example, to rotate in a circumference, towards the center of it). The force necessary to keep a body rotating under such conditions is the centripetal force, and it is greater the greater the mass and velocity and less the greater the radius of gyration.
The centrifugal force, the one that the passenger of a vehicle thinks he experiences, has the same intensity, but the opposite sense. And it does not exist. An observer who contemplates everything from the shoulder only sees that a force deflects the car, but none that tries to remove the passenger from the vehicle. Therefore, when solving a physics problem, the teacher will not want to see it solved by calculating the centrifugal force.
The nuance that was mentioned about the centripetal force is that it is not a force that appears when a body rotates, but the force that is necessary to impress a body in order for it to rotate. Therefore, some real element must exercise it or the turn will not be possible. When taking a curve without cant, it will be the friction between the tire and the asphalt. In the case of a sling or ball tied to a string, the centripetal force will be supplied by the tension of the holding material. Therefore, it is a “theoretical” force: someone has to do it and if the elements that must supply it cannot generate enough, the turn will be impossible. In the case of a car, it will involve the vehicle leaving the road.
That’s why cornering slowly, particularly if the road is wet or icy, is one of the lessons that comes from learning a little more about physics.
