From: Paul Doherty (pauld@exploratorium.edu)
Date: Tue Nov 28 2000 - 17:21:08 PST
Message-Id: <l03110719b64a060406d2@[192.174.2.173]> Date: Tue, 28 Nov 2000 17:21:08 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Student Questions
>Here are only some of the questions my students asked this month that I
>cannot answer. If you have thoughts on any of these, I would appreciate your
>response. Thank you! Jhumki Basu
>
>1. Why do we see only one face of the moon from the earth?
As the moon rotates the earth pulls up tides on the moon (yes rock tides.)
If the moon rotates faster than synchronous, these tides are pulled ahead
of the earth-moon line, there is a time delay between when they are pulled
up and when they reach maximum displacement. The earth then pulls on the
tidal bulges and slows down the rotation until it becomes synchronous.
>
>2. Do helium balloons feel gravity? Then why do they float in air? How does
>gravity affect objects that are less dense than air?
Yes they have mass, the earth pulls on them.
The earth pulls on air too.
the air is denser.
Air is pulled down under the balloon forcing it up.
So there is a buoyant force on the balloon.
think of the balloon like an air bubble in water.
>
>3. If I push hard on a book resting on a table, does the book break first or
>the table? Why do objects break?
Think of a book on an empty eggshell.
The eggshell breaks first under the book.
Think of the eggshell over the book the eggshell breaks first.
The order isn't important.
Every object has an ultimate breaking strength.
Breaking is complicated. Even more complicated than slippery ice.
>
>4. When my joints pop, is the sound from a liquid turning into a gas?
I've always heard this.
When you pull on the fluid in a joint you reduce the pressure so that the
joint fluid temperature remains the same but the boiling temperature drops.
The fluid then produces bubbles.
When you release the pull the temperature remains the same and the boiling
point becomes higher than the temperature. The bubbles suddenly vanish and
the walls of the bubbles crash in on each other creating a sound.
>
>5. Does water go down a drain in one direction in the Northern Hemisphere
>and in another in the Southern Hemisphere? If so, why?
In real drains the water goes down clockwise or counterclockwise at random
due to assymetries in the shape of the sink. In a perfect sink in which
water has benn sitting at rest for a day. Water will spiral down
counterclockwise in the northen hemisphere and clockwise in the southern
hemisphere due to the coriolis effect.
>
>6. Is there an evolutionary advantage to having a dominant eye?
?
>
>7. How do you measure mass without measuring weight?
In space they measure the mass of the astronauts by attaching them to the
end of a spring and finding the resonant frequency of oscillation. The
resonant frequency of a mass on a spring depends on the spring constant k
and the mass m. So you can measure mass independent of gravity.
>
>8. If I stand on a scale placed at an angle against the wall, why does it
>show a different reading than if I place it on the floor and stand on it?
>Why does the reading on a scale change if I stand on a corner instead of on
>the center?
Scales are complicated.
Turn one upside down sometime and see what it reads.
Different scales give different readings.
I can't predict what a given scale will read when rested against a wall.
>
>9. What is the name for units that are not SI or metric (pounds, feet,
>ounces, etc,)
The system used in the US is the English system of units.
>
>10. If I dissolve green food coloring in water uniformly and freeze the
>water, the ice cubes are not uniformly green. How come?
When water freezes, air in solution does not go into the ice it is forced
into the remaining liquid. Eventually it com,es out of solution and makes
bubbles.
These bubbles look white because they have air water interfaces. If there
is a lot of green ice between the outside and the bubbles the ice cube
looks green, if there is little ice it looks less green. The random
distribution of bubbles leads to a random distribution of light and dark
places.
>
>11. Why does water expand when you freeze it? I thought the particles in a
>solid were closer together than the particles in a liquid!
These are rather short answers.
Anyone want to add more?
Paul D
>
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Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
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