Berkeley Review (pg. 255) Physics part I

[wareagle87]

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I am having trouble understanding why the answer is correct. It seems to me that BR is contradicting themselves. I am sure they are not, but I need someone to help me understand why without the nail, the period would be less. Anyone with BR physics part I, it would be appreciated with any help. Thanks in advance.

 

[lorenzomicron]

post the question?