Motion and Its Causes

Force and Newton’s Laws of Motion

Why do you feel an outward force on you when you are in circular motion?

When we introduced Newton’s Laws we neglected to state an important warning. These laws work only in an “inertial reference frame,” that is a reference frame where there is no acceleration. A car when it is speeding up, slowing down, or changing direction is accelerating. Therefore, a person in the car experiences other “forces.” When the car is speeding up, you feel pushed back into the seat. When the car is slowing down you feel yourself being pushed forward. When rounding a curve, you feel pushed outward. These are not real forces, but are often called “inertial,” “fictitious,” or “pseudo” forces. For objects in circular motion there are two such forces, the centrifugal and Coriolis forces. The centrifugal force is the fictitious outward force you feel when your car is rounding a curve.


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