EK Physics Exam #120

[IlyaR]

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I'm having a hard time grasping this. On one hand, I can understand that at that exact point of time, it is not accelerating "downhill or uphill" and thus the acceleration would be 0, thus force would be 0. I also just substituted 0 degrees for theta, thus g would be 0 at that point.

However, the fact that the passage relates the motion similar to a pendulum confused me, since a pendulum at that point would be experiencing a centripetal force greater than MG, thus there would be a net force on it manifested as tension. Was I just too caught up in the pendulum analogy and missed the big picture (which is explained in the answer)

Thanks!

 

[milski]

In short - yes. 😉 The equivalent of the pendulum tension will be whatever force is holding the train in the center of the tunnel - normal from rails, air pressure, whatever. They do want you to ignore that and it is perfectly ok to do it here since the train is moving in a straight line and the net force is actually zero, unlike the pendulum.