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  • Discuss the inertial frame of reference.
  • Discuss the non-inertial frame of reference.
  • Describe the effects of the Coriolis force.

What do taking off in a jet airplane, turning a corner in a car, riding a merry-go-round, and the circular motion of a tropical cyclone have in common? Each exhibits fictitious forces—unreal forces that arise from motion and may seem real, because the observer’s frame of reference is accelerating or rotating.

When taking off in a jet, most people would agree it feels as if you are being pushed back into the seat as the airplane accelerates down the runway. Yet a physicist would say that you tend to remain stationary while the seat pushes forward on you, and there is no real force backward on you. An even more common experience occurs when you make a tight curve in your car—say, to the right. You feel as if you are thrown (that is, forced ) toward the left relative to the car. Again, a physicist would say that you are going in a straight line but the car moves to the right, and there is no real force on you to the left. Recall Newton’s first law.

In figure a, there is a girl driving a car turning toward right. A fictitious force vector is acting on her body toward left. In figure b, the actual force vector acting on the girl’s body is shown toward right.
(a) The car driver feels herself forced to the left relative to the car when she makes a right turn. This is a fictitious force arising from the use of the car as a frame of reference. (b) In the Earth’s frame of reference, the driver moves in a straight line, obeying Newton’s first law, and the car moves to the right. There is no real force to the left on the driver relative to Earth. There is a real force to the right on the car to make it turn.

We can reconcile these points of view by examining the frames of reference used. Let us concentrate on people in a car. Passengers instinctively use the car as a frame of reference, while a physicist uses Earth. The physicist chooses Earth because it is very nearly an inertial frame of reference—one in which all forces are real (that is, in which all forces have an identifiable physical origin). In such a frame of reference, Newton’s laws of motion take the form given in Dynamics: Newton's Laws of Motion The car is a non-inertial frame of reference    because it is accelerated to the side. The force to the left sensed by car passengers is a fictitious force    having no physical origin. There is nothing real pushing them left—the car, as well as the driver, is actually accelerating to the right.

Let us now take a mental ride on a merry-go-round—specifically, a rapidly rotating playground merry-go-round. You take the merry-go-round to be your frame of reference because you rotate together. In that non-inertial frame, you feel a fictitious force, named centrifugal force     ( not to be confused with centripetal force ) , trying to throw you off. You must hang on tightly to counteract the centrifugal force. In Earth’s frame of reference, there is no force trying to throw you off. Rather you must hang on to make yourself go in a circle because otherwise you would go in a straight line, right off the merry-go-round.

In figure a, looking down on the counterclockwise rotation of a merry-go-round, we see a child sitting on a horse rotating in counterclockwise direction with angular velocity omega. The fictious force is equal to the centrifugal force at the point of contact between the pole carrying horse and the merry-go-round surface, which is from the center of the round base toward outside. This is the merry-go-round’s rotating frame of reference. In figure b, the merry-go-round’s inertial frame of reference is given, where two horses carrying children are seen rotating with angular velocity omega in the counterclockwise direction. The net force of first horse is equal to the centripetal force, shown here with an arrow from the first horse toward the center of the circular base. A shadow of the second horse is shown going past the right side of the first horse in straight direction, whose net force is equal to zero. A dotted line from second as well as first horse are shown meeting at the center point making an angle.
(a) A rider on a merry-go-round feels as if he is being thrown off. This fictitious force is called the centrifugal force—it explains the rider’s motion in the rotating frame of reference. (b) In an inertial frame of reference and according to Newton’s laws, it is his inertia that carries him off and not a real force (the unshaded rider has F net = 0 size 12{F rSub { size 8{"net"} } =0} {} and heads in a straight line). A real force, F centripetal size 12{F rSub { size 8{"centripetal"} } } {} , is needed to cause a circular path.

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Source:  OpenStax, College physics arranged for cpslo phys141. OpenStax CNX. Dec 23, 2014 Download for free at http://legacy.cnx.org/content/col11718/1.4
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