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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. [Corel]

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 — 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 — and not into a crystal’s 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 doesn’t break then, it is still strained at room temperature.

That’s 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" — 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.

Slaughtering myths

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.

Today’s 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.

Batteries. [NOAA]

Most lead acid batteries need charging every two to three months when not in regular use. Perhaps, neglecting this precaution helps perpetuate the myth.

Further Reading:

AutoBatteries.com: How to jumpstart a car battery

How Stuff Works: How batteries work

Weighing Earth

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, Denton, Texas

Apollo 11 astronauts view the rising Earth, 1969 [NASA]

Your question shows insight. We can’t directly measure Earth’s mass so we calculate it.

I believe, though, you want to measure Earth’s enormous mass — 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, Earth’s 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. Earth’s mass, moreover, measures Earth’s inertia or sluggishness if we tried to stop or change its movement through space.

We calculate Earth’s mass from two relations (derived in 1665 by Sir Isaac Newton at age 23):

• how Earth’s 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.

That’s it.

We crank through the math and out pops the answer: the mass of the Earth is 6.6 billion trillion tons (6 trillion trillion kilograms) — a number so huge it’s difficult to imagine. But then again, imagine stopping Earth or changing its movement.

A word about the constants that we plugged into the equation. The Greek, Eratosthenes (276 - 194 BC), first determined the Earth’s radius (4182 miles, 6,731 km). Galileo (1564 - 1642) figured out that the acceleration due to gravity was a constant near Earth’s surface (and measured it). Henry Cavendish (1731 - 1810) first measured Newton’s gravitational constant. Since then, scientists have refined these measurements and we have used the latest values.

Further Reading:

Enchanted Learning: Calculating Earth’s mass

NASA, Goddard Space Flight Center: How Galileo measured g, the acceleration due to gravity

(Answered May 14, 2004)