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Free fall with and without air resistance

In free fall without air resistance, a body accelerates indefinitely (or until the object hits an obstacle).

In free fall with air resistance, the acceleration stops (and the velocity stays constant) when the force of air resistance $$F_{\text{air}}$$ rises so much that it offsets the gravitational force $$F_g$$ acting on the object: $$$F_g-F_{\text{air}}=0.$$$

The velocity at which gravitation and air resistance cancel each other out is called terminal velocity.

Before opening her parachute, a skydiver has very little air resistance. Her terminal velocity is very high.

After opening her parachute, the skydiver has much greater air resistance. Her terminal velocity is reduced and she slows down.

Military parachutists jumping from a plane.
Military parachutists jumping from a plane.