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its .315 instead of .135
Hi asifsoa, thanks a lot for your comment. Yes, I copied the length of stretching as 0.135m instead of 0.315m, but I plugged 0.315m into my calculator, so the results of this calculation are correct. Thanks for pointing out this confusion though, and I'll get around to redoing this video after I've posted the rest of the textbook solutions.
This video has been updated.