Pendulum Experiment TBR Physics ch. 5, Psg II, Q2

[Futbol99]

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I don't know why I'm having a derp moment. Can some one just clear up this concept that's not clicking?
I got this question right, due to elimination, but I'm having trouble with this concept in general - the "conservation" of energy and momentum in Pendulum's. I read a thread earlier that helped with the momentum concept.
As for energy - I thought Kinetic energy is not "conserved" in the way that it doesn't change because of increasing V from the point it starts at rest. But I thought that it converts between Potential and Kinetic (according to the ball's position).
Is all this question saying is that- we need a rigid cord so that the cord is not involved in the conversion of KE into PE and vice versa of the bob, as that would cause a change in the velocity and motion of pendulum? (like if the bob starts at high PE, it would elongate the cord, thus ruin the pendulum motion, so we need a rigid cord to prevent that from happening?)- But this still would mean that in a pendulum PE + KE = const. correct?

Sorry about the rambling - I think it's one of those questions that the wording gets me. Thanks

Q. The cord in the experiment should be:
A. elastic, to allow the tension to establish a gradient.
B. rigid, to prevent tension in the cord.
C. elastic, to prevent conversion of kinetic energy of the bob into potential energy by elongating the cord.
D. rigid, to prevent conversion of kinetic energy of the bob into potential energy by elongating the cord.

Answer: D

 

[dudewheresmymd]

I don't know why I'm having a derp moment. Can some one just clear up this concept that's not clicking?

Is all this question saying is that- we need a rigid cord so that the cord is not involved in the conversion of KE into PE and vice versa of the bob, as that would cause a change in the velocity and motion of pendulum? (like if the bob starts at high PE, it would elongate the cord, thus ruin the pendulum motion, so we need a rigid cord to prevent that from happening?)- But this still would mean that in a pendulum PE + KE = const. correct?

Answer: D

Yes! If you were to have an elastic cord, some of the bob's kinetic energy could go into making the bob go higher than the initial amplitude (think if the cord is super loose, as the bob passes through equilibrium on it's way up it might stretch the cord out a bit and at the top it would be 0.1 m higher let's say than the original amplitude b/c the bond lengths are stretching a bit farther than they could with a taught cord), we want a FIRM RIGID cord so that does not happen.