Free-Falling Bodies and Terminal Velocity    

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Fluid friction force will affect the falling motion of an object. Fluid friction force (air drag) is directed up which is opposite of object’s weight force direction. (Remember the object’s surface characteristics affect its air drag.) The object’s Fnet = -Fw + Ff . In the early stages of a free-fall event (high atmosphere altitudes) the air drag is small since the air density is low. So at the beginning of free-fall event, the object will be accelerated downward since its net force is directed down since the object’s weight force is larger than the air drag force.

As the object continues to fall, it experiences higher density air. Higher density air causes an increase in the object’s air drag force. As the object falls the net force directed downward also decreases thus causing a lower and lower downward acceleration. At some point the net force becomes zero since the force situation becomes balanced between the upper directed air drag force and the downward directed weight force Fw = Ff (equal in magnitude but opposite in direction). When the falling event becomes balanced, there is no more downward directed acceleration. The velocity that the object possesses when the situation becomes balanced is the maximum downward directed velocity that the object will achieve. The maximum downward directed velocity is called Terminal Velocity.