4.4 Drag and terminal velocity

What is drag?

A frictional force that opposes the motion of the object

What does the magnitude of drag depend on?

Speed, shape, roughness or texture of the object, and the density of fhe fluid through which it travels.

What are the two most important factors that affect the magnitude of the drag?

The speed and cross sectional area of the object. Large cross sectional areas result in greater drag force. For most objects the drag force id directly proportional to speed².

When the speed of an object is doubled. The drag force increases by a factor of 4.

Describe the motion of a falling object:

During a vertical fall through air or another fluid the weight of the object remains constant but the drag force increases as the speed increases.

At the instant the object starts to fall there is no drag force on the object. The total force is equal to the weight. The acceleration is g, the acceleration of the free fall.

As the object falls, its speed increases and this in turn increases the magnitude of the opposing drag force. The resultant (net) force on the object decreases and the instantaneous acceleration of the object becomes less than g. Eventually the object reaches terminal velocity.

What is terminal velocity?

When the drag force is equal and opposite to its weight. At terminal velocity the object has zero acceleration and its speed is constant.