The reaction shown is a free radical halogenation reaction. This method is unsuitable for the preparation of fluoroarenes because of the high reactivity of fluorine. Fluorine is so reactive that it is difficult to control the reaction, leading to multiple substitutions. In this reaction, the free radical is formed by using the metal Fe in dark conditions. For the other halogens, the reaction can be controlled to obtain the corresponding haloarenes.
A bob of heavy mass \(m\) is suspended by a light string of length \(l\). The bob is given a horizontal velocity \(v_0\) as shown in figure. If the string gets slack at some point P making an angle \( \theta \) from the horizontal, the ratio of the speed \(v\) of the bob at point P to its initial speed \(v_0\) is :