EPOW - Ecology Picture of the Week

Bug, Bat, Bird, or Bullet?

Upper left: Leafcutter honeybee (Megachile sp.) on lavender.
Upper right: Unidentified bat, northern India.
Lower left: Australian pelican (Pelecanus conspicillatus).
Lower right: What not to do with forest signs.
(First three photos © Bruce G. Marcot; sign photo public domain.)

Explanation:  This week is a lesson in biophysics.

Ready for the quiz question?  Here goes.  Which would you most rather not be hit by:  a flying bug, a flying bat, a flying bird, or a (flying) bullet? ... and why?

Well, the quick answer is bullet, because we know they can kill you, and that's because they are designed for penetration.  OK, let's dispense with the bullet ... so now, which is worse, being hit by a bug, bat, or bird ... say each flying at their top speed?

One way to gauge the degree of impact is to calculate the momentum of each object in flight.  Don't lose me now, but momentum, remember, is mass times velocity.  So what is the momentum of each flying projectile?  (And sorry, I'm doing all this mostly in metric units; you can convert here, as needed.)

The mass of an object is gauged by its Earthly weight.  It turns out that a typical bug, say a honeybee, weighs about 90 milligrams for the worker bees (the queen and drones weight about 160 milligrams).  Let's use the 90 milligrams figure.  Bats vary in size tremendously, but a typical range at least in North America is 7 grams for a little brown bat and 20 grams for a big brown bat.  How about birds?  Let's again go for a range; a sparrow weighs about 28 grams; a peregrine falcon weights 720 grams; and a pelican 7.5 kilograms (that's 7,500 grams).  And to be complete, a typical .30-06 ("thirty ought six") bullet weighs in at an average of 165 grains or 10.7 grams.

How fast do they go?  The maximum velocity of a honeybee is about 8.9 meters/second or m/s (ok, I'll give you one: that's about 20 mph).  Bats can fly up to about 2.68 m/s.  A sparrow can fly 8.9 m/s; a falcon 44.7 m/s (remember, the peregrine falcon is the fastest bird on the planet); and a pelican a more leisurely 8.9 m/s).  And our bullet can dash along at 823 m/s.  

So ... momentum is mass times velocity, so the momentum of each object turns out to be:

 

OBJECT	MOMENTUM (kilogram-meters per second)
Honeybee	0.001
Little Brown Bat	0.019
Big Brown Bat	0.054
Sparrow	0.25
Peregrine Falcon	32.2
Pelican	67.1
Bullet	8.8

(The full calculations for this and numbers below are available here in a spreadsheet.)

Interestingly, the bullet seems to have an order of magnitude less momentum than the two bigger birds.  Among the animals, the pelican carries the most momentum because of its great mass, although I think I'd rather get hit by a pelican than by a peregrine falcon screaming along at a hundred-mile-per-hour diving stoop.  

We're not done.  Take a breather, then come on back.

Back?  OK.

Well, momentum might not be the best measure of the possible effects of an impact from an identified flying object.  A better metric might be the amount of energy contained in the moving object, or its kinetic energy.  

Kinetic energy is calculated as one-half the mass times the square of the velocity.  So let's do the calculations (mumble, mumble) and now see what shakes out:

 

OBJECT	KINETIC ENERGY (joules)
Honeybee	0.004
Little Brown Bat	0.03
Big Brown Bat	0.07
Sparrow	1.12
Peregrine Falcon	719
Pelican	300
Bullet	3,623

Hmm (again), now the pelican has lost its first place, replaced by the rocketeering peregrine falcon.  (But the bullet now has screamed well into first place.)

Kinetic energy is measured here in a unit called the joule.  One joule is the same thing as a watt-second, that is, one joule is equal to the power of one watt operating for one second.  I bring up watts because that's usually what we often think of in terms of light bulbs and electrical appliances around the home.  

In fact, let's put these results in those terms.  A 60-watt incandescent light bulb consumes 0.06 kilowatts (60 watts) of electricity in one hour, or 1 watt per minute, or (mumble, mumble) 0.017 watts per second.  So here's the kinetic energy of each of our flying projectiles, now converted into the length of time each could light up a 60-watt light bulb:

 

OBJECT	ELECTRICAL ENERGY (length of time a 60-watt incandescent light bulb could be lit)
Honeybee	0.0001 seconds
Little Brown Bat	0.0004 seconds
Big Brown Bat	0.0012 seconds
Sparrow	0.0187 seconds
Peregrine Falcon	12 seconds
Pelican	5 seconds
Bullet	60 seconds

OK, so NOW which would you least want to be hit by?  

Me, I'm still going with the bullet.  

If you downloaded the spreadsheet with these calculations, you'll also see the data on some additional flying objects -- a baseball, an SUV, a meteorite striking the ground, and a meteor entering the Earth's atmosphere.  (Please double-check all my numbers and calculations!  And if correct, have fun inserting your own flying objects...)

The baseball (thrown as a 90-mph fastball) can light a 60-watt bulb for 2 seconds; a 2-kg meteorite with a terminal velocity of 300 mph could light the bulb for 300 seconds (5 minutes); a 3-ton SUV at 70 mph could light the bulb for 18,289 seconds (304 minutes, or about 5 hours, yikes!); and the 2-km meteor first entering the Earth's atmosphere at 19.5 km/s (43,620 mph) could light the bulb for 6,342,570 seconds, or 73.4 days!  

If only we could harness the kinetic power of those SUVs ... or especially of those outer space meteors, eh?...