 In this video, let us talk about significant figures. Now to understand significant figures, we need to first understand what is the problem that these significant figures are solving? Why do we need to study about them? Why is this in our syllabus? Okay, let's see. Let's say you're performing an experiment in the lab and there's a ball in your lab and you're trying to find the density of that ball. Now to find the density, you need the mass of this ball. But you don't have the weighing scale right now. But in some other experiment, somewhere else has written that the mass of this ball is 27,000 grams. So it's a very heavy ball. Let's say it's a steel ball. Now you want to use this measurement in your calculations. You want to use this 27,000 grams. But to be able to use it, here's the problem. You don't know how much error does this measurement have. Who measured it, you don't know. Which device they used, you don't know. But you want to know how much error does this measurement have. And you want to know this error because depending on this error, you will be able to say how much error is there in your final result, in your result of the density of this ball. So you must know how much error this measurement has. Now maybe the mass of this ball was measured on a device that measures the mass in grams. So maybe the device said that the mass of this ball is 27,000 grams. And if it was one grams less, it would say that the mass would be 26,999 grams. And if it was one gram more, then the device would say that the mass is 27,001 grams. So we are very sure that the mass is 27,000 grams. We are sure of all of these digits 27,000. So all of these digits in that case would be significant digits. We trust all of them. So we say that all of them are significant. But there is another possibility. Maybe this was not the case. Maybe the device that this ball was measured on measures the mass in kilograms. And it said that the mass is 27 kilograms. And the person that wrote this measurement simply multiplied this by thousand and wrote it as 27,000 grams. In that case, if the mass was one gram less, then the device will not show 26,999 kilograms because it was made to measure only till the last kilogram, not below that. So it will only show 27 kilograms even if the weight was 26,999 grams. Or even if the weight was 27,001 grams. Even then it will show 27 kilograms. So in this case, if the person has written it as 27,000 grams, then we are only sure of the 2 and the 7. We are not sure of the other three zeros. Maybe the mass was 26,999 and the device showed 27 kilograms. Maybe it was 27,001. So we are not sure of these three. So in that case, these three will not be significant figures. Now in order to deal with such situations where there is ambiguity and we don't know which digits are significant and which digits are not significant, we devise the set of rules. These rules will tell us which digits we should consider significant when we are reading a measurement and which digits we should not consider significant. And these rules help us ensure that we are taking the right amount of error in our calculations. So for example, over here, we are only sure of 2 and 7. We are not sure whether the zeros are trustworthy or not because we don't know the device. So we will say that only 2 and 7 are significant and others are not significant. So based on this, we can make a rule that if there are zeros at the end of a number where there is no decimal point, then those zeros are not significant. Similarly, we can make other rules to make sure we are not underestimating our error. We will go through all of these rules one by one in the next video but now you know why do we need to talk about significant figures at all.