Have you ever seen an apple fall from an apple tree? If you have, you may have noticed that the apple, while falling, will move faster and faster as it approaches the ground. This is because of Gravity.

Let’s increase the drop height of this imaginary apple, though. In fact, let’s imagine that you have the opportunity to drop one from a plane. Because you are curious to learn if gravity still holds up at such an altitude, you designate a drop zone for this theoretical experiment. With a speedometer to measure the object’s speed, you let go of the apple. As expected, gravity causes the apple to accelerate downward at a rate of 9.8m/s^2 at first. The further the apple’s displacement from its initial position, the faster it moves. However, it's not quite that simple with real-world conditions where constant values are rare. Patient observation will show you that the apple’s acceleration will begin to decrease from 9.8 m/s^2 to 0. The reason is air resistance.

Air resistance is a drag force that acts in the opposite direction of an object’s motion; however this force is only relevant when an object is in the air. Once the object’s acceleration is 0, it will move with a constant maximum velocity known as terminal velocity. Newton’s Second Law states that acceleration is equal to net force divided by mass. Terminal velocity is when air resistance and gravity balance each other out. When the net force on the apple is 0 newtons, the acceleration is also 0. Your test subject will move at this same velocity until it finally hits the ground.