$4-)$ For the pendulum as shown below, mass $m$ is released at $t=0.$ At the lowest point there is a very thin blocker that cause the string roll. What is the maximum value of R $($in terms of I.$)$ so that the string is to remain taut as the mass $m$ swings around the blocker?

$5-)$ Some electric power companies use water to store energy. Water is pumped from a low reservoir to a high reservoir. To store the energy produced in $1\mathrm{.}0$ hour by a $180-$MW electric power plant, how many cubic meters of water will have to be pumped from the lower to the upper reservoir? Assume the upper reservoir is an average of $380$ m above the lower one. Water has a mass of $1\mathrm{.}00**{10}^{3}kg$ for every $1\mathrm{.}0m^3.$