Binary Stars and Air Friction Videos

Hey, juniors - here are two links for learning how to do binary orbits and air friction (the hockey puck example, and a sky diver example). For the air friction, this could be review of our derivations in class. For binary orbits, check out the video and see what you can do with the AP problem over the weekend. Feel free to do this with a friend or study group, and help each other through it.

How to do Air Friction on a Sky Diver

Here is a case where air friction acts on a falling object, such as a sky diver. This is one of the trickier math problems we will do in physics, as it involves calculus (anything with air friction will, since it is a non-constant force: f = -kv). We specifically want to solve for the velocity as a function of time for the sky diver. Check this out to get a feel for how Newton's 2nd law sets up the equation, and then we do almost all algebra with a step of calculus to solve for velocity. Note that terminal velocity is the speed you reach when air friction matches the strength of gravity, and the person falls with a constant speed at that point. Also note that we do a very simplified model of air friction. Other factors we do not worry about here include the shape of the object, air density that varies with altitude, wind, air temperature that varies, the material of the object, the gaseous composition, and so on (for us, all this information is contained in the constant, k).

How To Deal With Air Friction Mathematically - Hockey Puck

Air friction is a different creature compared to static or kinetic friction. Static and kinetic frictions are forces between two solid surfaces, whereas air friction is between a solid surface and a fluid. Something like water friction behaves similar to air friction, where these friction forces depend on how fast you try to move through the fluid (think about how it is actually tougher to try and run in water than to walk in water). Check out how to handle this fluid friction mathematically...it is certainly more involved than dealing with static or kinetic friction, which we treat as constant forces. Air friction is non-constant, and calculus must be used to find an exponential behavior with time.