# Thermodynamics and Mechanics questions

by nasar176
Tags: mechanics, thermodynamics
 P: 22 For Q2, think about the "worst" case (since it asks for the minimal speed - which will barely keep the string taut). When the block is at the top of the circle, both gravity and tension contribute to the centripetal force, therefore $F_g + T = \frac{mV^2}{R}$ In the worst case scenario (the minimal speed), the string is barely taut - which means it is barely pulling the block - so you can say T = 0 for the minimal speed. Therefore $mg = \frac{mV^2}{R}$ $V = \sqrt{Rg}$ which results in 9.8^(1/2) :) For Q3, the "easy way" is to remember that, for constant acceleration, $\Delta S = v_ot + at^2/2$, so as Vo = 0, $S \alpha t^2$, where S stands for the displacement. Therefore, if you double the time, the distante will be multiplied by four (since it depends on the square of the time), which gives you 4m. Did i make any sense? :P