 Let's say you're taking a physics class and are given a falling body problem, where you drop an object 250 meters, and you have to figure out how long it takes the object to fall. There's a formula for that. The time is the square root of twice the distance divided by g, the acceleration of gravity, where g is 9.8 meters per second squared on Earth. Here's the start of a program to help us with this problem. How do we do square roots in Python? Python comes with some built-in math operators and functions, but square root is not one of them. Instead, it's in a module called math. You can think of a module as a small library of related functions. In order to access a module's functions, you have to import the module. Once you do that, you can use the square root function by giving the module name, a dot, and the function name. In this case, sqrt, the abbreviation for square root. In parentheses, we'll put the division, two times the distance divided by gravity, and then print the result with three digits after the decimal point. We'll say time to fall, three decimal places for distance, and the time. Now we can run the program and find that it takes a little over seven seconds to fall 250 meters on Earth. Now we're going to cover a slightly more advanced topic in the math module, trigonometric functions. If you've never heard of these functions, sine, cosine, and tangent, you may want to look at a small tutorial about them, or do a web search for tutorial sites, or skip ahead to the summary of this video. Let's go to the Python shell to examine these functions. If you've studied trigonometry, you might remember that the sine of 30 degrees is one-half. We'll import math, and then evaluate math dot sine of 30, and that result is definitely not one-half. The reason is that the sine, cosine, and tangent functions do not take their arguments in degrees but in radians. So trigonometric function arguments must be in radians. And it turns out that 180 degrees is the equivalent of pi radians. Let's convert that 30 degrees to radians by hand. We'll take the sine of 30 times math dot pi using the pi constant that's part of the math module, and dividing by 180. And that's much better. It's not exactly equal to one-half because math dot pi has a finite number of decimal places, but it's a lot closer than negative 0.988. If you don't want to convert to radians by hand, you can use the radians function that's also a part of the math module. We can say math dot sine and call the math dot radians function, give it 30 degrees, which will convert it to radians, and it will come up with the same answer. In summary, you need to import modules to use functions that aren't built into Python, and you access the function by giving the module name, a dot, and the function name.