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# Free fall

## Free fall definition

Free fall happens when gravity is the only force acting on a body.

If you drop a glass in the kitchen, the glass is in free fall.

A ball thrown into the air is subject only to gravitational force. It is in free fall even when moving upwards. It is accelerating due to gravitation but its speed is decreasing.

If air resistance is ignored, a skydiver falling through the sky is in free fall.

With no air resistance, all objects fall to Earth with the same acceleration due to gravity.

The acceleration due to gravity has the symbol $g$ and points towards the Earth. Close to the surface of the Earth, $g = 9.81 \umps^{2} \approx 10 \umps^{2}$.

Displacement, velocity and acceleration during a free fall for a ball thrown vertically into the air. In this case, the upward direction is defined to be positive.

## 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.