Centrifugal Force

• Centrifugal force is a phenomena observed from a point on a rotating object (a rotating system). It is an apparent force that does not exist when observed from a stationary point not located on the moving object (such as a stationary system or an inertial system in uniform motion).
• Let’s think about an object experiencing circular motion (it is rotating about a point). For example, an object in circular motion on a turntable, as shown in the diagram, is experiencing a force pointing toward the center of rotation (centripetal force) due to friction between the object and the turntable. This force consistently creates acceleration toward the center of rotation (centripetal acceleration) to maintain circular motion. No centrifugal force is involved.
• Next, we observe this phenomenon while riding on an object that is experiencing circular motion. Let’s assume we are on a very large turntable. A person riding on a turntable feels a force acting on him. This force is called the centrifugal force. If there is no external force acting on the object, it will continue to follow straight-line motion (inertia). Therefore, an object in circular motion will inherently try to move in the direction tangential to the circular motion. When you stand on a rotating turntable, you feel as though there is a force pulling you to the outside of the circular orbit. Therefore, when you make observations from a point on a rotating object, we assume that this apparent force, the centrifugal force, is acting. When the centripetal force acting on an object is in equilibrium with the centrifugal force, circular motion is maintained, and it looks as if nothing is happening.
• This explanation is also applicable to a person riding in a car as it goes around a curve. Inherently, an object will try to go along a straight line. Therefore, one would feel an apparent force that is pushing toward the outside of the circular orbit; this is the centrifugal force. I am sure a lot of you have experienced this force pushing you toward the outside of a curve. People riding in this car need to create a centripetal force with the seat or the handle to continue the circular motion. At this instant, one can assume that the apparent centrifugal force and centripetal force are balanced.
• Let’s assume a person outside of the car, standing on a sidewalk (an inertial system) is observing the person riding in the car as it is turning through a curve. If no force is acting, a person riding in the car will try to move along a straight line. However, the person will create forces pointing toward the center of the curve (centripetal forces) by holding onto the hand grip and receiving force from the seat, so the person turns toward the inside of the curve together with the car. There is no centrifugal force. The car and the person only receive centripetal forces that create an acceleration pointing toward the center of rotation, so they follow a circular orbit.
• In fluid dynamics, when considering a vortex or a fluid with any rotating motion, we often consider the motion of a particle riding on the fluid (a rotating system). The particle experiences an apparent force, the centrifugal force, pushing it to the outside of the rotation. When the centripetal force is in equilibrium with this, the fluid will continue its circular motion (vortex). In a rotating fluid, the outboard side becomes a high-pressure region, while the inboard side becomes a low-pressure region. The difference in these pressures generates an inward force acting on fluid particles; this becomes the centripetal force.
• Therefore, when considering a vortex or a fluid in rotating motion, we often think in terms of rotating systems.