Q: Why do some scientists claim that centrifugal force is "fictitious"?
A: When the car in which you are riding suddenly brakes, you feel as if
you are thrown to the front of the car. What mysterious force grabs you
and pulls you toward the front of the car? No force at all. To slow the
car the braking force pushes backwards on it resulting in its negative acceleration,
or deceleration. Hopefully the seat belt and friction from the seat slows
you along with it. From within the decelerating reference frame of the car,
your senses fail to distinguish between the car's deceleration and your
acceleration. The information they receive is ambiguous because you and
the car are exerting equal and opposite forces on each other. They invent
a fictitious force to explain the cause and it feels like you are thrown
forward when you are really tugged backwards. Likewise, when the car accelerates
away from the stop light, you feel pressed back into the seat. In reality
the seat is pushing forward on you to accelerate you along with the car.
As before, your inertia resists being accelerated and pushes back on the
seat in the opposite direction. Your senses, expecting nonaccelerating circumstances,
are confused about the forces and misinterpret them. As in all cases involving
force and acceleration, there are a pair of equal and opposite forces acting.
In this case the car seat exerts a forward force on you while you exert
and equal but opposite backwards force on it. Now do you see why we get
confused about centrifugal force? When pressed against the door on a curve
you are experiencing a similar pair of forces as the door pushes inward
on you through the curve. From the general reference frame outside the car
it is clear that there is a pair of equal and opposite forces acting between
the passenger and the car. From the specific reference frame inside the
car, centrifugal force exists as a fictitious force which appears to try
to eject you from the car as the door pushes on you to counteract your inertial
tendency to keep moving in a straight line at a constant speed. When our
state of motion changes our senses confuse the relationship between forces
and inertia. As a result we feel forces acting on us in the opposite direction
from the actual forces that are causing us to accelerate. This is because
our perception of forces and motion evolved in a nonaccelerating reference
frame and because there are always at least two objects involved which exert
equal and opposite forces on each other.
Richard Brill is assistant professor of science at Honolulu Community
College . Send questions to Honolulu Community College, 874 Dillingham Blvd.,
Honolulu, HI 96817 or email
to rickb@hcc.hawaii.edu
Fictitious Force ©1996 Richard C. Brill