# Module 4.1: FallSkydive

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The FallSkydive model is based on the FallFriction model. This model
has two phases, one where the diver is in free-fall, and a second
phase where the skydiver's parachute has opened. The larger surface
area of the parachute results in more air resistance.
Let's break this model down into its components. The real complexities
in this model come from its terms. We'll start there.

**Mass:** The mass of the diver is controlled by a slider.

**Acceleration:** Total force/mass

**Acceleration due to gravity:** -9.81 m/s^{2}

**Weight:** mass * acceleration due to gravity

**Total force:** Total force is equal to weight + air friction

**Air friction:** Air friction in this model is equal to -.65 (air
friction estimate) * projected area (of parachute) * velocity *
absolute value of velocity (in Nova, this is shown as
Math.abs(velocity)).

**Position open:** This is controlled by a slider that indicates where in the dive the parachute is opened.

**Projected area:** The contents of this term, projected area of
the parachute, contains logic to show when the diver opens her
parachute. If the diver's position is higher than the value indicated
by the position open slider, then the projected area is .4. If the
diver's position is less than or equal to the position open, the the
projected area is 28. In Nova, this looks like `(position >
position_open) ? . 4 : 28`

**Speed:** Absolute value of velocity

**Adjusted speed:** If the position is less than 0, the speed is 0,
otherwise, the adjusted speed is the same as the speed: `(position < 0) ? 0 : speed`

**Adjusted Position:** If the position is less than 0, then
position = 0, otherwise, position is equal to whatever position is
indicated on the position slider.

Stocks and terms in this model are much simpler. This model contains a
stock for velocity and a stock for position. Initial velocity is 0 and
initial position is controlled by the initial position slider. The
change in position slider is fed by acceleration (it's all about
derivatives!).
The graph shows position and velocity over time.