Air Resistance.

Introduction:
Air resistance is sometimes referred to as a drag force. Experiments have been done with a variety of objects falling in air. These sometimes show that the drag force is proportional to the velocity and sometimes that the drag force is proportional to the square of the velocity. In either case, the direction of the drag force is opposite to the direction of motion.

Mathematical form of drag:

• The drag force depends on the square of the velocity. So as the body accelerates its velocity and the drag increase.

•  It quickly reaches a point where the drag is exactly equal to the weight.

•  When drag is equal to weight, there is no net external force on the object, and the acceleration becomes zero.

• The object then falls at a constant velocity as described by Newton's first law of motion.

• The constant velocity is called the terminal velocity.