# Module 4.1: FallFriction

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The FallFriction model builds on the Fall model by adding air
friction. As you will notice, the stocks and flows for the
FallFriction and Fall models are the same, but the FallFriction model
adds a few more terms. Let's take a look at some of the new terms.

**Mass:** The object's mass can be adjusted using the Total Mass
slider. Mass impacts the object's weight and acceleration.

**Weight:** The object's weight is its mass times its acceleration due to
gravity. Acceleration due to gravity is as always, -9.81
m/s^{2}. Mass is controlled by the Total Mass slider. We need to have weight in the
model because it impacts the total force.

**Total Force:** Total force adds the object's weight and the air
friction, then is divided by mass to calculate the object's
acceleration.

Air friction is a slightly more complicated term, so we'll start by
looking at its term components.
**Radius:** Radius is controlled by the Object Radius slider.

**Projected area:** This term is calculated by squaring the radius
and multiplying it by π (it's a ball, remember?).

**Air friction:** Air friction is calculated using the value for Newtonian
friction, F = -0.65, multiplied by the projected area, velocity, and
the absolute value of velocity.

The graph and table show values for position and speed over time.