Facts of the Matter
The feeling of being pushed backward during acceleration is erroneous. This is known as a fictitious force.
Forces of nature no great secret
AS ADULTS we have an inherent concept of force that we have formed through experiences of voluntary muscular actions such as moving our limbs or feeling the resistance in lifting and carrying things.
It is likely that "force," "strength," "effort," "power," and "work" were synonymous to our ancestors from ancient times through the scientific revolution. They often are synonymous even now in everyday language.
The common expression "forces of nature" is a vestige of a primitive concept of force, which in mythology is usually connected to some supernatural agent and destiny.
Early Greek philosophers sought a compromise between force and destiny in a pantheon of gods, ruled by Zeus, representing the consummate icon of might and power as he wielded the dreaded thunderbolt.
Empedocles (circa 490-430 B.C.) reasoned that Love (harmony) is the uniting force that attracts all things and creates something new. Opposing Love was Strife (discord), the dividing force that separates and destroys things. This view is not so distant from the principles of Yin and Yang that developed in China around the same time.
Plato (428-348 B.C.) interpreted Empedocles' principles of love and strife as "attraction" and "repulsion."
Motion in Greek cosmology was seen as a type of change, like the rusting of iron. It was a view quite different from ours.
For our modern scientific description and perceptions of motion we can thank Galileo (1564-1642). He defined motion in terms of the ratio of measurable distances and times.
Sir Isaac Newton linked motion and force as stated in his three laws of motion, published in 1687.
The first law is the law of inertia: An object remains at rest or maintains a constant state of motion (speed and direction) unless acted upon by unbalanced outside forces.
The second law relates force to acceleration (change in motion) and mass (inertia, the resistance to change in motion).
The third law, often inaccurately called the law of action and reaction, states that forces must occur in pairs: one object to actively exert the force and another object for it to exert force upon.
The pair of forces act on two different objects. They are mutually exerted, equally and in opposite directions, and from within our own minds one is active and the other is passive.
This has given rise to a great deal of confusion about forces, especially when rotation is a factor.
In 1632 Galileo described what we call today "inertial reference frames."
Galileo recognized that when a ball is dropped while it is moving at a constant speed, the path that it takes as it falls appears different to different observers.
To a person inside a bus moving at a constant speed who drops a ball, it appears to fall straight downward just as it would if the bus were not moving.
An observer outside the bus (imagine a transparent bus) will see the ball follow a parabolic trajectory, as it would if it rolled off a level table.
Inertia and motion are the reasons for the discrepancy in perceptions, and both observers are correct in their respective frames of reference.
A state of constant motion is known as an inertial reference frame because there is no acceleration and therefore there are no outside forces involved.
When a change of speed (acceleration) is involved, it is a different ball game. Newton's second law guarantees that if there is acceleration then outside forces are involved.
Picture yourself sitting in the seat of a car at rest. When the driver pushes the accelerator pedal, the car begins to move, and gains speed as it accelerates.
You feel as if you are being pushed backward against the seat by some "invisible" force. To an observer outside the car it is obvious that the force you feel is the car seat pushing you forward.
You perceive the force to be in the opposite direction from the actual force because your senses cannot resolve the balanced pair of forces.
According to Newton's third law the force pair consists of you and the seat. The seat pushes you forward, you push the seat backward.
We naturally assume that we are being pushed backward: "I am not actively pushing yet I feel the force, therefore something must be pushing me."
The subconscious and erroneous conclusion that you are being pushed backward on the seat leads to the invention of a "fictitious force" to explain the backward push.
In the case of linear acceleration, the nature and origin of the fictitious force is straightforward.
When the acceleration involves a change in direction, the situation becomes immensely more complex. We did not evolve in a rapidly rotating environment, and our senses are not geared for it.
Curvilinear motion can be represented by connecting arcs of circles of various radii, so circular motion is a good model for motion that is not in a straight line. Circular motion is simple, but within a force and motion framework it confounds our senses.
Circular motion is a continual and uninterrupted change in direction, which according to Newton's second law requires a corresponding continual and uninterrupted force.
When we are in circular motion, such as when turning a corner in the car, we feel like we are being thrown against the passenger door. This is the same situation as when being "pushed" backward in the seat, and, likewise, necessitates the invention of a fictitious force.
This particular fictitious force is called centrifugal force.
Your push on the door is the reaction force to the action of the door that pushes you and your inertia around the corner. If the door was not there, your inertia would keep you going straight while the car turned and you'd fall out of the door, as if thrown by that fictitious force.
There is a real force pair, but there is no real fictitious force.
To an outside observer, the force-pair relationship is clear. Door pushes person, person pushes door. But our innate sense of inertia perceives us to be in an inertial state.
We did not evolve inside accelerating boxes and our brains are not programmed to perceive in a rotating reference frame. Our senses are geared for an inertial frame, either in inertial motion or activated muscle-driven accelerations.
Our ability to reason often exceeds our ability to perceive and in this case we invent, subconsciously or otherwise, fictitious forces to explain perceptions about forces and motion that we feel but cannot understand intuitively.
Thanks to Sir Isaac we can use physics based upon his laws of motion to understand it intellectually.
Richard Brill, professor of science at Honolulu Community College, teaches earth and physical science and investigates life and the universe. E-mail questions and comments to firstname.lastname@example.org