Saturday, February 13, 2010

Bullets and Blunders

Caught up on a bit of telly this evening, including the latest QI and Psych.

On QI, there was the old canard about if you fired a bullet and dropped it simultaneously, which would hit the ground first?  The standard answer is, they would hit at the same time.

Except, that's not strictly true.  They would essentially hit at the same time, but not precisely.  The fired bullet would hit ever so slightly after the dropped bullet.  Stephen even admitted that if the bullet were going FAST enough, it would leave the atmosphere.  So, at one speed does it go from hitting the ground at exactly the same time as a dropped bullet, and NEVER hitting the ground?  It only takes a moment's thought to work out that it must land slightly later.  The greater the speed, the greater the delay, until it reaches a velocity that takes it out of the atmosphere.  BUT, since a bullet's speed is never going to be THAT great, the delay is hardly measurable.  It's like how a spring weighs slightly more compressed than uncompressed; the difference is so small it is hard to quantify.  But it's there.

The bullet blunder on Psych was much more egregious, although technically it was a tree transgression.  In this episode, they found a bullet embedded high in a tree.  It got that high due to the tree's growth over the years.  However, trees do not grow in this manner.  Despite the widely held belief to the contrary, trees actually grow from the top.  All parts of a tree (branches, embedded evidence, etc) will stay pretty much at the same height, no matter how much time passes.

I watched these two shows back-to-back.  I'm beginning to wonder if too much education lessens the enjoyment I would otherwise get from television.


  1. I don't understand the bullet firing/dropping situation. Can you explain the setup?

  2. You have a gun pointed horizontally at the horizon. You're holding another bullet at the same height as the one in the chamber. You fire one and drop the other, simultaneously.

    The reason this example is given in school is because many people (intuitively, incorrectly) think that the fired bullet will land much later than the dropped one. That the momentum forward would somehow counteract the gravitational pull. After all, planes don't fall out of the sky, right? As long as they're moving forward fast enough.

    Bullets of course aren't designed like planes, and so the forward momentum of the bullet does not generate any lift. Which is why it lands at pretty much (but not precisely) the same time as the dropped bullet.

    Mythbusters did an episode on it. Their methods offended my scientific sensibilities. :-/