8.4.1 Centrifugal Force?
If the car turns a corner quickly, you feel “thrown” against the door. But there’s really no such force because there is no agent exerting it. FIGURE 8.12 shows a bird’s-eye view of you riding in a car as it makes a left turn. You try to continue moving in a straight line, obeying Newton’s first law, when—without having been provoked—the door suddenly turns in front of you and runs into you! You do, indeed, then feel the force of the door because it is now the normal force of the door, pointing inward toward the center of the curve, causing you to turn the corner. But you were not “thrown” into the door; the door ran into you.
FIGURE 8.12 Bird’s-eye view of a passenger as a car turns a corner.
The “force” that seems to push an object to the outside of a circle is commonly known as the centrifugal force. Despite having a name, the centrifugal force is fictitious. It describes your experience relative to a noninertial reference frame, but there really is no such force. You must always use Newton’s laws in an inertial reference frame. There are no centrifugal forces in an inertial reference frame.
NOTE You might wonder if the rtz-coordinate system is an inertial reference frame. It is not! The rtz-coordinate system rotates as it moves with the particle, so it is not an inertial reference frame moving with a constant velocity. The important point is that we never make measurements in the rtz-system. That is, velocities and accelerations are measured in the laboratory reference frame. The particle would always be at rest if we measured velocities in a reference frame attached to the particle. We can use the rtz-coordinate system, as we do, to find the directions of vectors as long as we do all our calculations in a laboratory reference frame.