Shattering glass, slaughtering myths, weighing Earth
My glass ashtray suddenly broke. Prior to this, there was a single pop sound as
if a pebble hit the empty ashtray. Can you explain why this happened?
Louise, College Park, Georgia
Glass breaks if it has a flaw and tension in the flaw.
The glass was under too much tension so it broke much like
a snow-laden branch will snap, startling the quiet forest.
"Most probably the ashtray was not properly annealed after it was made," says Siegfried Herliczek, safety
glass consultant of Glassig, Inc., Petersburg, Michigan.
Glass forms only under extremely high temperatures ó high
enough to mix and fuse the basic ingredients (sand, soda, and limestone or
chalk) together. Then, the trick is to cool the melt slowly enough that the
glass atoms lock into a disordered, almost-liquid state ó and not into a
crystalís perfect arrangement. If done right, we get glass: a substance that is
not a liquid and not a crystal but something in between.
The process is downright tricky. If we cool glass too quickly,
it will be highly strained and may even break while cooling. Even if it doesnít
break then, it is still strained at room temperature.
Thatís what happened to your ashtray. "There was too much
tension on or near the glass surface. The addition of a small scratch eventually
caused the ashtray to break," says Herliczek.
To reduce that surface tension, the glass manufacturer should
have cooled the thick ashtray slowly enough that its surface temperature did not
differ much from the interior as it cooled.
By the way, in Mesopotamia (present-day Iraq), we found
"secret instructions" written on clay tablets 3300 years ago for making glass.
First, the instructions said, "Placate the gods" ó sacrifice a sheep, burn
juniper incense, and pour a honey and melted butter libation. Only then build a
fire in the glass furnace and start the capricious business.
Is it true that concrete will drain batteries left sitting on it?
Morgan, Savannah, Georgia
No. This myth had validity back when batteries used glass
separators between the lead plates. Then, if you dropped the battery on a
concrete floor, the glass separators cracked or broke, thus violating the solid
barrier between the plates. That allowed the positively (+) and negatively (-)
charged lead plates to touch and the charge leaked off through the connection.
Therefore, people thought that "concrete caused the battery to
discharge when, in fact, it had nothing to do with it," says EverStart Batteries
of Wal-Mart Stores.
Todayís batteries use paper (instead of glass) envelope
separators to keep the (+) and (-) plates from touching. The paper separators
are highly unlikely to tear even if you drop the battery on concrete.
Most lead acid batteries need charging every two to three
months when not in regular use. Perhaps, neglecting this precaution helps
perpetuate the myth.
AutoBatteries.com: How to jumpstart a car battery
Works: How batteries work
How much does Earth weigh and how are you able to come up with that answer
without having our planet on a scale??? Dillon, age 8,
Apollo 11 astronauts view the rising Earth, 1969 [NASA]
Your question shows insight. We canít directly measure
Earthís mass so we calculate it.
I believe, though, you want to measure Earthís enormous mass ó
not its weight. The "weight" of an object normally means the force that Earth
attracts it with. In this case, the object is the Earth. So, Earthís
weight is not a concept that has meaning in this context.
Its mass, on the other hand, is the matter that makes up
Planet Earth. Earthís mass, moreover, measures Earthís inertia or sluggishness
if we tried to stop or change its movement through space.
We calculate Earthís mass from two relations (derived in 1665
by Sir Isaac Newton at age 23):
- how Earthís force of gravity tugs on an object
- how an object resists acceleration due to gravity.
Also we need three known constants that an ancient Greek, a 16th
century Italian, and an 18th century Englishman first measured.
through the math and out pops the answer: the mass of the Earth
is 6.6 billion trillion tons (6 trillion trillion kilograms) ó a number so huge
itís difficult to imagine. But then again, imagine stopping Earth or changing
A word about the constants that we plugged into the equation.
The Greek, Eratosthenes (276 - 194 BC), first determined the Earthís radius
(4182 miles, 6,731 km). Galileo (1564 - 1642) figured out that the acceleration
due to gravity was a constant near Earthís surface (and measured it). Henry
Cavendish (1731 - 1810) first measured Newtonís gravitational constant. Since
then, scientists have refined these measurements and we have used the latest
Enchanted Learning: Calculating Earthís mass
NASA, Goddard Space Flight Center: How Galileo measured g, the
acceleration due to gravity
(Answered May 14, 2004)