You take the merry-go-round to be your frame of reference because you rotate together. Let us now take a mental ride on a merry-go-round-specifically, a rapidly rotating playground merry-go-round ( (Figure)). Different frames of reference must be considered in discussing the motion of an astronaut in a spacecraft traveling at speeds near the speed of light, as you will appreciate in the study of the special theory of relativity. Noninertial (accelerated) frames of reference are used when it is useful to do so. There is no problem to a physicist in including inertial forces and Newton’s second law, as usual, if that is more convenient, for example, on a merry-go-round or on a rotating planet. This inertial force is said to be an inertial force because it does not have a physical origin, such as gravity.Ī physicist will choose whatever reference frame is most convenient for the situation being analyzed. The car, as well as the driver, is actually accelerating to the right.
The force to the left sensed by car passengers is an inertial force having no physical origin (it is due purely to the inertia of the passenger, not to some physical cause such as tension, friction, or gravitation). The car is a noninertial frame of reference because it is accelerated to the side.
In such a frame of reference, Newton’s laws of motion take the form given in Newton’s Laws of Motion.
The physicist might make this choice because Earth is nearly an inertial frame of reference, in which all forces have an identifiable physical origin. Passengers instinctively use the car as a frame of reference, whereas a physicist might use Earth. We can reconcile these points of view by examining the frames of reference used. Only the normal force has a horizontal component, so this must equal the centripetal force, that is, Because this is the crucial force and it is horizontal, we use a coordinate system with vertical and horizontal axes.
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