Start reading pp. 51 - 88 from The Great Physicists from Galileo to Einstein. The material is on Isaac Newton.

We discussed the meaning of standard deviation which arose in the lab on the projectile motion. We looked at a related quantity called the root-mean-square deviation. After one takes the average x_ave of a set of values {x_i}, the deviation of a value is the difference of the value and the average (x_i - x_ave), which describe how far a value is from the average. One might consider averaging these to get an overall sense of how spread out the data is. But there will be both positive and negative deviations (values above average and value below average). So we square the deviattions to get all positive values, then we average that. But if our measurements were in cm for instance, the squared deviation would have units cm². To get back to the same units we started with, we can take a square root. Hence, you read "root mean square deviation" backwards: take the deviations, square them, take their mean, take the square root. The standard deviation is almost the same with one exception. If you had N values and thus N deviations, when taking the mean you would add up the squared deviations and divide by N. In the standard deviation, one divides by N-1. This difference is trying to take into account not only the values in your set, but the measurements you have not done.

We moved on to discuss Newton and his three laws of motion.