Quickly, do you know why this example has been given? Do we have to use the formulas from the last example?
No sir. We have to this equation.
Yes fine. Sir the object is falling freely. We have to calculate the time from A to B.
Which equation we have to use? We shall use the second equation. S = ut + 0.5at2. Fine? Those who want to study, ways are made for them. So write down all these 5 quantities. Then see what quantities you know and find out what is your target. You know “u”, you know displacement, you know acceleration, and you have been asked “t”. Isn’t it? So we shall use second equation. So prepare yourself systematically in this manner. So sit idle. Put in your efforts. Now displacement; sir had told us to use displacement with proper sign, negative or positive. So what sign will you use for displacement? Displacement is directed downwards, so how will you show it? Positive. So I will write it “+H”. Initial velocity is zero. Now how will show acceleration? Because it is also directed downwards, positive and then t2. So we have 2h/g=t2. So done. I cannot express my feeling in words. I cannot express the importance of that small formula here. I cannot express. I am telling you it is God-like important formula in physics. If you don’t remember this equation, you don’t deserve to prepare for IIT. It is infinitely important formula, there are such two formulas that you should remember always. Second one is speed at this point. Shall we use the third equation? We need “v”. We know “u", “s”, “a”, we need to find “v”. If you know the value of time, you can directly use first formula. But if you want to calculate speed directly, then? If you don’t know the value of time then what will you do? You have to find “v”, we know “u”, “s”, “a”, we must use the third equation, yes or no? So let’s apply the third equation and once you will apply the third equation, you will reach to this point, v=√2gh. And again God-like important formula.