TBR pendulum problem

[greenseeking]

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Please see the attached for problem.

Answer is C.

I totally guessed on this problem because Ithought that the angles had something to do with it. But in the answer key it says that the angles had no bearing on the problem- just as long as the two balls are at its highest point and released at its max KE, they will be out of phase by pi.

How are we supposed to know whether they are released at its highest point??? Couldn't the angles easily have been like, 30 (for the upper) and 60 for the lower ball? That would have given the balls a bigger potential energy and higher kinetic energy...

 

[monkeyvokes]

Please see the attached for problem.

Answer is C.

I totally guessed on this problem because Ithought that the angles had something to do with it. But in the answer key it says that the angles had no bearing on the problem- just as long as the two balls are at its highest point and released at its max KE, they will be out of phase by pi.

How are we supposed to know whether they are released at its highest point??? Couldn't the angles easily have been like, 30 (for the upper) and 60 for the lower ball? That would have given the balls a bigger potential energy and higher kinetic energy...

pi radians out of phase.. sounds like waves that are 180 degrees out of phase. If both of the balls are stretched in opposite directions as far as possible, I would think they are at their highest point. Then when let go, they will oscillate back and forth 180 degrees out of phase.

two waves, pi radians out of phase

 

[pilotjoe]

How are we supposed to know whether they are released at its highest point??? Couldn't the angles easily have been like, 30 (for the upper) and 60 for the lower ball? That would have given the balls a bigger potential energy and higher kinetic energy.

You know the balls are at the highest point because the question implies the balls are stationary, i.e. "the balls are released..." From the conservation of energy we know that the balls won't get any higher unless acted upon. Since the force of gravity pulls downward on the balls and the balls are stationary at the opposite side of the pendulum, they will be pi out of phase.

 

[greenseeking]

Thanks got it!