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SECTION 6.3 • Motion in Accelerated Frames 161 θ T mg Inertial observer Noninertial observer θ T mg (a) (b) F
fictitious a she writes Newton’s second law as F ! T ( mg ! ma, which in component form becomes According to the noninertial observer riding in the car (Fig. 6.13b), the cord also makes an angle " with the verti- T and claims that the net force on the sphere is zero! In this noninertial frame of reference, Newton’s second law Noninertial observer
$
! F x + ! T
sin " ' F fictitious ! 0
! F y + ! T
cos " ' mg ! 0 Inertial observer
$ (1) ! F x ! T
sin " ! ma (2) ! F y ! T
cos " ' mg ! 0 ! We see that these expressions are equivalent to (1) and fictitious ! ma, where a is the acceleration according to the inertial observer. If we were to make this substitution x above, the noninertial observer ob- tains the same mathematical results as the inertial ob- What If? Suppose the inertial observer wants to measure the acceleration of the train by means of the pendulum (the Answer Our intuition tells us that the angle " that the cord Figure 6.13 (Example 6.8) A small sphere suspended from the ceiling of a boxcar accelerating to the right is deflected as shown. (a) An inertial observer at rest outside the car claims that the acceleration of the sphere is provided by the horizontal component of T. (b) A noninertial observer riding in the car says that the net force on the sphere is zero and that the deflection of the cord from the vertical is due to a fictitious force F fictitious that balances the horizontal component of T. Example 6.9 Fictitious Force in a Rotating System Suppose a block of mass m lying on a horizontal, frictionless Solution According to an inertial observer (Fig. 6.14a), if 2 /r, where v is its linear speed. The inertial observer concludes that this centripetal acceleration is |