PC2021: Problem 2, week 5

Score “-1” means a major mistake, will bring a penalty factor 0.9
Score “-0.5” means a minor mistake or self-noticed major mistake, will bring a penalty factor 0.95.
Positive score: accumulated penalties for a correct solution (excluding the decreased base factor due to published hints).
Major mistake: the solution starts with wrong physical assumptions
Minor mistake: the basic assumptions are valid, but mathematics is not.
Uncorrected typo at the last step of the solution or a sign mistake for an otherwise correct solution: solution will be considered as correct, but additional factor of 0.9 is applied.