King of Prussia, Fall 2016.
I took this photo from the top of a very tall building.
There was no railing.
The mall is in the center-left, with Mount Misery & Mount Joy in Valley Forge
Park behind. Valley Forge Towers is on the right.
I love deadlines.
I especially like the
whooshing sound they make
as they go by.
- Douglas Adams
As we settle into this distance learning thing, you may find that
you're having a harder and harder time getting your schoolwork done.
Maybe it's that as teachers get the hang of this, the demands are
increasing. Maybe it's the scheduled Google Meets during what
used to be your very flexible schedule. Maybe it's the occasional
nice spring days we are having between the rain and the cold.
If you find yourself getting behind, at least in physics it's not
really a problem. You can turn the work in any time. The only thing
you would not get is DU credit.
It's never too late, so don't give up on the physics!
Explosions
After the Rocks from Space bit, Billy Moser was asking for more
details about the science of explosions. I thought it
might make a good midweek topic.
When large amounts of energy are converted quickly into heat
(whether it's chemical PE or nuclear energy or KE),
that heat will cause surrounding air to expand very rapidly,
and then contract again. That push-pull is a shock wave/pressure wave
(a very loud sound) that travels outward at the speed of sound.
So for a regular explosion, there's two hazards:
the heat of the explosion which can burn things,
and the shock wave which can destroy things.
The outward spreading dissipates the energy of both as
you get farther away, though.
For a nuclear explosion, the heat is intense enough to
set things on fire even a mile or more away.
But there are extra hazards in that the flash
contains radiation traveling outward at the speed
of light, and the dust that gets thrown up and
eventually comes down contains radioactive material.
The first video is from a cable show called Destroyed in Seconds.
It shows the explosion of a rocket fuel plant. You can see the shockwave
spreading across the desert. You shouldn't hear the
explosion until the shockwave arrives, because the shockwave IS the sound.
I assume that an explosion sound effect was added by the TV people.
The second video is newsreel footage of one of our atomic bomb tests
in Nevada.
To test the effects, the military had houses and structures built in the desert and had
placed soldiers nearby. The soldiers were in trenches
farther way, but closer than I'd like to be. Unbelieveable, isn't it?
In the video, you can first see smoke coming off of objects, which is the
burning caused by the flash. Then you see the push-pull of the shock
wave.
Again, you can tell the explosion sound effect must have been added
to the video afterward because the sound would not have come before the shockwave.
The shockwave IS the sound.
Worth a Closer Look
The rain this week and the idea of explosions reminded me of
when my family and I were in Maine in 2014. We walked from our hotel
about a mile to our favorite chowder place,
Billy's Chowder House
in Wells, Maine. After we had eaten, we walked outside and saw a massive downburst
heading our way. Downbursts
are very violent thunderstorms. I pulled out my phone for a quick video
before we ran back to the hotel. We just made it before the storm hit.
My video captured a lightning strike. Lightning
is a massive amount of electrical current that superheats the air as it
flows, creating that same expansion/contraction shockwave - which is
of course the loud thunder you hear afterward. The flash of lightning
travels at the speed of light, but the shockwave travels at the speed
of sound which is slower. That's the reason you see lightning first and
then hear the thunder later.
Sorry, I deleted the sound from the video; I'm not sure why.
