Gravity Gone Ballistic

Some of my paternal grandfather's war stories involved his time in the army. The most action-packed involved a time on patrol when a spent round bounced off his upper chest, and simply fell to the ground because it had effectively lost all of its kinetic energy. More action-packed than stories of siphoning gasoline out of military vehicles, anyway. Strangely, the story has propagated disagreement among people who doubt its veracity. I can't vouch for the authenticity of the story, but I can vouch for its viability.

However, it wasn't enough that people simply believe me on my birthday, so I was forced to break out the W/t of SCIENCE (If I can have a dogion joke I can have a power = energy/time joke). Unfortunately, this led to a point of contention involving the physics of gravity.

Some people are surprised to know that the Aristotelian view of gravity is not true (despite how cool Aristotle is). Heavier objects do not inherently accelerate faster than lighter objects. This belief persists because lighter objects tend to be more buoyant and have more air resistance, and hence drop more slowly on Earth.

Current scientific thinking links gravity with the curvature of spacetime. Unfortunately, quantum mechanics disagrees (but quantum mechanics is like that frizzy-haired uncle no one likes to talk to because he seems to only speak gibberish). Despite all that, most situations still work fine with Newton's "simple" Law of Gravity. Here we also see that laws are meant to be broken, even when they're scientific, since relativity and quantum mechanics have proven that Newton's law does not apply to all possible scenarios. Basically, given a vacuum (to negate air resistance and buoyancy) and objects of negligible mass (relative to a planet), objects will fall to the ground at the same rate (both would fall at approximately 9.8m/s² on Earth).

Astronauts make everything better, so fortunately the concept was illustrated by our good friend Commander David Scott during the Apollo 15 mission.

Transcript for people without video: "Well, in my left hand I have a feather. In my right hand, a hammer. I guess one of the reasons we got here today was because of a gentleman named Galileo a long time ago, who made a rather significant discovery about falling objects in gravity fields. And we thought, 'Where would be a better place to confirm his findings than on the moon?' And so we thought we'd try it here for you. The feather happens to be appropriately a falcon feather...for our Falcon. And I'll drop the two of 'em here, and - hopefully - they'll hit the ground at the same time. [hammer and feather hit the ground simultaneously] How 'bout that? This proves that mister Galileo was correct in his findings."

Returning to our opening statements: people also tend to believe that horizontal motion negates gravity. If an object is shot horizontally and another object dropped simultaneously from the same height, both objects will hit the ground at the same time. Gravity's pull is uniform regardless of horizontal motion. This too, is illustrated by crazy science teachers around the world. Exhibit A:

(I apologize in advance for the lack of more astronauts)



...There is no Exhibit B.

Edit for 2013-2-06: There is an Exhibit B; I've since become aware that the Mythbusters have also performed this experiment using actual firearms with a result that is well within margin of error (basically the only improvements you could ask for is better timing on the drop/shot simultaneity and doing it in a vacuum).


Randomly teaching people about gravity since 2003; just another reason not to follow me when I walk home from school.