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Both of the above can be observed in action on a passenger riding in a car. If the car swerves around a corner, the passenger's body pushes against the outer edge of the car. This is the reactive centrifugal force, which is called a reaction force because it results from passive interaction with the car which actively pushes against the body. Using a reference frame which is fixed relative to the car (a model which those inside the car will often find natural) and while ignoring its rotation, it looks like an external force is pulling the passenger out of the car. This is the fictitious centrifugal force, so called because it is not an actual force exerted by some other object. REACTIVE CENTRIFUGAL FORCE When viewed from an Inertial Frame Of Reference , the application of Newton's Laws Of Motion is simple. The passenger's inertia resists Acceleration , keeping the passenger moving with constant speed and direction as the car begins to turn. From this point of view, the passenger does not gravitate toward the outside of the car; instead, the car curves to meet the passenger. Once the car contacts the passenger, it then applies a sidewise force to accelerate him or her around the turn with the car. This force is called a centri''petal'' ("center seeking") force because its vector changes direction to continue to point toward the center of the car's arc as the car traverses it. If the car is acting upon the passenger, then the passenger must be acting upon the car with an equal and opposite force. Being opposite, this reaction force is directed ''away'' from the center, therefore centri''fugal''. It is critical to realize that this centrifugal force acts upon the car, not the passenger. Basing the physics on an inertial reference frame, there is no mystery force pulling the passenger outward. The ''centrifugal reaction force'' with which the passenger pushes back against the door of the car is trivial and is simply given by: where is the mass of the rotating object. ROTATING REFERENCE FRAMES In the classical approach, the inertial frame remains the true reference for the laws of mechanics. When using a Rotating Reference Frame , the laws of physics are mapped from the most convenient inertial frame to that rotating frame. Assuming a constant rotation speed, this is achieved by adding to every object two ''coordinate accelerations'' which correct for the rotation of the coordinate axes.
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