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ACCELERATION AND FORCE IN CIRCULAR MOTION by Peter Signell
Both of the above can be easily observed in action for a passenger riding in a car. If a car swerves around a corner, a passenger's body seems to move towards the outer edge of the car and then pushes against the door. In the reference frame that is rotating together with the car (a model which those inside the car will often find natural), it looks as if a force is pushing the passenger away from the center of the bend. This is a Fictitious Force --not an actual force exerted by any other object. The illusion occurs when the reference frame is the car, because that ignores the car's Acceleration . A number of physicists treat it much as if it were a real force, as they find that it makes calculations simpler and gives correct results. However, the force with which the passenger pushes against the door is real. That force is called a ''reaction force'' because it results from passive interaction with the car which actively pushes against the body. As it is directed outward, it is a centrifugal force. Note that this real centrifugal force does not appear until the person touches the body of the car (ignoring any force exerted by the seat on the person's body, etc). The car also exerts an equal but Opposite Force on the person, called " Centripetal Force ". 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 path which the car follows; instead, the car's path curves to meet the passenger. Once the car contacts the passenger, it then applies a sideways 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''.http://members.tripod.com/~gravitee/booki2.htm It is critical to realize that this centrifugal force acts upon the car, not the passenger. The ''centrifugal reaction force'' with which the passenger pushes back against the door of the car is given by: where is the mass of the rotating object (kg), the rotational speed (rad/s) and the radius of the rotation (m). The reactive centrifugal forcehttp://www.infoplease.com/ce6/sci/A0811114.html is a real force, but the term is rarely used in modern discussions.http://www.cc.utah.edu/~u0324774/pdf/Comparison_of_Bearings.pdfhttp://www.cartercopters.com/high_technology_gyroplane.html ROTATING REFERENCE FRAMES In the classical approach, the inertial frame remains the true reference for the laws of mechanics and analysis. 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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