2D spring-mass system

- So far we've limited our simulation to vertical motion only, but we want our hairs to sway in various directions so we'll need to update our code to include two dimensions of motion. It's fairly easy to do this, we just need to double-up many of our variables so that we'll have both an X and Y component. Right now we only have an initial vertical position called "position Y" So I am going to add a new one called "position X." I am also going to add a new variable to store the horizontal component of the velocity. I'll call this "velocity X" it should be initialized to zero, the same as velocity Y because our particle starts from rest. Now I'll move into our draw function and duplicate each equation, so we have a horizontal component as well. Currently, there is springForce Y, but not springForce X I'll add that now. SpringForce X equals negative K, times position X minus anchor X. I'm going to add a new variable called "dampingForce X" I'll set it equal to damping times velocity X. With these new variables I can add a new net force calculation in the X direction called "force X" it will be slightly different than force Y because we can ignore gravity, since the force of gravity only acts in the vertical direction. So, force X equals springForce X minus dampingForce X. Now lets move on to acceleration. I'll add a new variable called "acceleration X" and it will be equal to force X divided by mass. Let's do the same thing with the resulting velocity. I'll add a new variable called "velocity X" which is equal to velocity X, plus acceleration X, times timeStep. Finally we can update the position so it can have a horizontal component too. I'll do this with a variable called "position X" which equals position X plus velocity X times timeStep. Now we have X and Y components for force, acceleration, velocity, and position. We just need to update our draw code so that the circle representing our particle, and the line representing our spring, can move in the X direction. I'll simply add position X to the end of the line connected to the mass. And for the circle representing our mass, I'll add position X to its center position. Now watch what happens when I run the code. Ah-hah! 2D motion. To make our movies we do the same thing in 3D using a third Z component. That's a fun bonus project to start thinking about. But let's pause here so you can get comfortable with this code before moving on to the last step.