Drag Forces and Terminal Velocity

Drag is a resistive force that acts on any object moving through a fluid (liquid or gas). Air resistance is drag in air.

Key facts about drag

• Drag acts in the opposite direction to motion.
• Drag increases as the object’s speed increases.
• Drag depends on the object’s shape, size and the fluid’s density.

Reaching terminal velocity

When an object falls through a fluid:

1. At the start, weight $>$ drag, so there is a net downward force and the object accelerates.
2. As speed increases, drag increases.
3. When drag = weight, the resultant force is zero.
4. The object moves at constant velocity: this is terminal velocity.

Factors affecting terminal velocity

Factor	Effect on terminal velocity
Greater mass/weight	Higher terminal velocity
Larger surface area	Lower terminal velocity
More streamlined shape	Higher terminal velocity
Denser fluid	Lower terminal velocity

Skydiver example

A skydiver initially accelerates at $g$. As speed builds, air resistance increases. Terminal velocity ($\approx 55$ m/s) is reached when air resistance equals weight. Opening the parachute dramatically increases surface area, so drag temporarily exceeds weight, causing deceleration to a new, lower terminal velocity ($\approx 5$ m/s).

Common errors and how to correct them

• Saying the object “stops accelerating because there is no force.” The forces still exist; they are balanced, giving zero resultant force.
• Confusing “constant velocity” with “zero velocity.”

How to apply this in an exam

Draw a free-body diagram showing weight and drag at each stage. Explain what happens to the resultant force and therefore the acceleration at each stage.