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Re: A Parents' Day gem -Reply

Re: the time for a bullet to drop, and Herb's comments:

Another thing we need to teach our students is how to calculate the
magnitudes of the effects they are talking about. For instance, Herb's
statement that a bullet might travel for "several minutes, hours, days, or
even weeks before it returned to the ground" could only possibly hold if the
"bullet" were put in orbit! Over a distance of even one mile, the vertical
drop due to the curvature of the earth is only about 8 inches--far less than
the local variations in the ground level. The time it takes an object to drop
that distance is about 0.2 seconds. Even the Mach-4 bullet Leigh was
remembering (I don't think they go anywhere near that fast, but take it as an
upper limit) would take 1.2 seconds to go a mile. In 1.2 seconds, an object
drops 23 feet. The curvature of the earth is clearly not a significant source
of error in what we are talking about. (Taking Mach 4 = 4400 ft/s. Orbit
speed near the surface is about 26,000 ft/s, by comparison.)
Rondo Jeffery
Weber State University
Ogden, UT

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Herb Gottlieb (<herbgottlieb@juno.com>) commented on an earlier post:

nguilber@peddie.k12.nj.us (nguilber) writes:

YOU were teaching that a bullet FIRED from a level gun and a bullet
DROPPED >from the gun when the shot was fired will both hit the ground at
the SAME TIME!"

"This is witchcraft!" the parent hollered. "WITCHCRAFT!!!" And
he turned and stormed out of the classroom.

No word yet on whether or not the kid has dropped physics.

On the other hand.... The parent might have been correct in her
insistance that the dropped bullet would strike the ground before the
fired bullet. It is very probable that she knew that the earth is
spherical rather than flat. Thus a bullet that is fired from a level gun
has a further vertical distance to fall before it reaches the earth's
surface than a bullet that is dropped straight down.

In fact, if the muzzle velocity of the gun was sufficiently great, the
fired bullet might travel for several minutes, hours, days, or even
weeks before it return to the ground.

Herb Gottlieb from New York City
(Where we no longer teach that the earth is flat .... and kids like the
one above go into honors physics)
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