Can anyone explain pendulum losing PE and then the period...

[MCAT guy]

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As the pendulum loses energy over time (to resistive forces, such as friction), its maximum displacement angle is reduced, and its potential energy at full displacement decreases. However, the period of the pendulum remains constant despite its loss of energy. Although the velocity of each swing decreases over time, the distance the bob travels per cycle also is reduced (by the same percentage), so the pendulum's frequency remains constant. No adjustment to the frequency of the strobe light must be made to keep it in phase with the pendulum, as neither frequency changes over time

So I was reading the above answer and got the question wrong because I reasoned that as acceleration had decreased (T = 2 pi x sqroot ( l / g) ) the period would be affected by this drop in acceleration, causing the period to increase. Maybe I misread the question, that when a pendulum loses PE it is not necessarily losing acceleration?

As the pendulum loses energy, what adjustment should be made to the strobe rate so that its flash continues to coincide with the arrival of the bob at its lowest point?

don't pay attention to the strobe, insignificant. The question basically wants you to figure if the period has changed.

 

[lmarina1]

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[lmarina1]

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[lorenzomicron]

why should period be affected by the change in potential energy? T= 2pi*sqrt(l/g). None of these things change. Frequency, and thereby period, are fundamental properties of the system, which depends on fundamental physical quantities: length of the string/wire and the acceleration due to gravity.

where PE comes in is this: the total energy should be conserved as of: E= PE + KE.
however, E decreases due to friction. so really it becomes this E(t)= PE + KE - friction(t)*deltax(t).

note that the PE, which at some points is max depends PE=mgh. thus, if PE decreases, then h decreases. thus, velocity also decreases because KE decreases in the same way as E decreases.

 

[lmarina1]

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