TBR Physics Ch.5 Passage 2 Q12

[brood910]

One of the answer choices is "the PE and KE are equal to one another at a time that is 1/8 of the period after it had all gravitational PE", and this statement was true. Can anyone elaborate on this?

Their answer key says "At a point IN TIME that is half way between these two states, the KE must equal PE."

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[IlyaR]

One of the answer choices is "the PE and KE are equal to one another at a time that is 1/8 of the period after it had all gravitational PE", and this statement was true. Can anyone elaborate on this?

Their answer key says "At a point IN TIME that is half way between these two states, the KE must equal PE."

Since this concerns gravitational PE I'll assume this questions deals with the simple harmonic motion of a pendulum.

A cycle of a pendulum is (arbitrarily starting from right): Dropped from right--> max height at left--->back to original height on the right
So half a period is the point where the pendulum reaches the max height at the left
A quarter of a period is the point where the pendulum reaches KEmax (at the bottom, pointing straight down, where all PE becomes KE)
An eighth of a period is the point where the pendulum is halfway between the time it was dropped to the time it reaches KEmax. At this point, half of the PE has been converted into KE.

 

[brood910]

Since this concerns gravitational PE I'll assume this questions deals with the simple harmonic motion of a pendulum.

A cycle of a pendulum is (arbitrarily starting from right): Dropped from right--> max height at left--->back to original height on the right
So half a period is the point where the pendulum reaches the max height at the left
A quarter of a period is the point where the pendulum reaches KEmax (at the bottom, pointing straight down, where all PE becomes KE)
An eighth of a period is the point where the pendulum is halfway between the time it was dropped to the time it reaches KEmax. At this point, half of the PE has been converted into KE.

Thanks a lot!