Terminal Velocity

There's a bit of a lull in the fires here at work, so I was reflecting back on a conversation I had with someone (I don't actually remember who) about terminal velocity, and there was a lot of misinformation from their side.

Early in the conversation they mentioned the experiment that Galileo performed, and that the feather fell at the same rate as the ball. This is a common misconception; the experiment featured no feather (that was David Scott on the Apollo 15 moon landing), but a great illustration of the misunderstanding of terminal velocity.

Before I begin, it's worth defining the term. Terminal velocity is the maximum speed an object can reach under given conditions.

Firstly, it's important to understand that the presence of gravity (and it's effect) does not imply terminal velocity. Terminal velocity is achieved when the ACCELERATION due to gravity (32 meters/second²) is balanced by the RESISTANCE of the substance the object is falling through. Most of the time, that's air.

So, say a feather and a baseball are dropped from a hot air balloon. As they start to fall, they will both accelerate at a rate of ~32m/s² (apprx due to distance from center of Earth), but their rate of acceleration will immediately begin to decrease as they air they are falling through applies its resistance. Surface area and shape play a huge role here as well; the ball's acceleration will decrease slowly due to its more aerodynamic shape, whereas the feather will rapidly cease to accelerate and start to flutter.

This is where it starts to become obvious that there is no universal "Terminal Velocity." An even more compact example is a sky diver with a winged suit. Wings in, the sky diver has less surface area and a higher terminal velocity.

But, what happens when an object isn't falling through a substance? This is where we head back to the moon. Dr. Scott was in, more or less, an environment void of atmosphere; anything he dropped would be exposed to negligible external resistance, and so would be at the mercy of gravity alone. And he tested it. When he dropped a hammer and a feather they accelerated at the same rate throughout their decent and landed simultaneously.

To pull things back a bit farther; that baseball from the hot air balloon would hit a certain speed and stop going faster; that is, it would reach a terminal velocity. If Dr. Scott could have gotten further away from the surface of the moon before he dropped his items, that feather and hammer would have hit the moon going limitlessly fast. Well, limitless until you consider the gravitational pull of the Earth, the realistic distance at which the moon would apply its gravity and the strength thereof (bear in mind, the acceleration due to lunar gravity is only 1.6 m/s²)... so, you know, "nearly headless."