One of the teaching units I enjoyed most in my physics classroom was on wave motions. Almost everything is capable of some sort of waving motion, or oscillation. The motions come in a wide range of frequencies and amplitudes dependent upon the object. Smaller objects tend to have high frequencies and small amplitudes. Large objects tend to have low frequencies and large amplitudes of motion. It is a fascinating field of study.
Some objects respond to an input of energy of some specific frequency and begin oscillating with the same frequency as the source. Their motion can grow in amplitude as the source of energy continues. A simple example is a pendulum with a child on a swing. Pushing the child at the right time inputs energy to drive the amplitude larger. The energy of drawing a violin bow across the strings of a violin sets some of the strings into vibrations that are large and produce sound. This response to the driving action goes by the name of resonance. A previous post of mine on resonance can be found here.
The resonance response can be so strong that large vibrating objects reach amplitudes that can be easily seen or felt. Between July and November 1940, the new suspension bridge across Tacoma Narrows responded to various strength winds by doing some gentle waving motions as motorists crossed it. Some called the bridge Galloping Gertie.
On 7 November 1940, a half mile long section of the suspension bridge was set into a waving motion none had seen before by a steady 42 mph wind from the side. It collapsed and fell into the narrows. The video of that event is a classic one used by physics teachers every year to dramatically demonstrate the effect of resonance.
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by Jim and Melanie
It was a little past 7 pm. Dinner was finished. The three of us decided to go to the waterfront park for the evening to enjoy the bay and watch people.
“I rode 65+ miles yesterday morning with the bicycle group. My legs are tired from that hard ride. I could use a slow ride to stretch and relax. You two go ahead to the park and find a bench in the usual place. I’ll follow up and meet you a few minutes later on my bicycle.”
We drove down the steep hill to the waterfront. I could see him farther back in the distance in the rear-view mirror going slowly down the rough streets. We reached the parking area and crossed the street to get onto the walkway. It was a beautiful evening. Many people and some other bikers were out. Cars were going along the street, some entering and leaving driveways to restaurants. One car leaving in front of us backed up a few feet to allow us to walk past. We waved our thanks.
We were perhaps 20 feet past that driveway when we heard a strange crashing noise behind us. A bicyclist was sprawled face down on the pavement where we had just walked. He was not moving. The car that allowed us to pass was still sitting there, waiting to see what he would do. Then the realization hit us. That was our son on the pavement!
What happened next?
Sky conditions could be excellent this year for the Perseid meteors. The moon will not be bright. If your sky is clear the night of August 12 into the morning of August 13, you have the opportunity to see several meteors per hour. The peak will occur at 3 am on the 13th CDT.
That is quite early. You can see them the night before in smaller numbers. Watch for their radiant near Cassiopeia in the eastern sky well after dark.
I invite you to listen to this audio tour of the August sky events brought to you by Sky & Telescope.
My five previous updates to the Dawn Mission to Dwarf Planet Ceres are located at this link.
As of 4 August 2015
On 17 July 2015, the NASA Dawn spacecraft started the spiral descent from survey mapping orbit #2 to the much closer high altitude mapping orbit (HAMO) #3. The descent will move Dawn from 2,700 miles (4,400 km) above dwarf planet Ceres to 915 miles (less than 1,470 km) by mid-August 2015. Dawn’s view of Ceres will be equivalent to looking at a soccer ball from a distance of 13 inches (33 cm).
This graphic describes the descent of Dawn to HAMO. In this point of view, Dawn orbits counter-clockwise. Blue shows ion-thrusting to leave survey orbit #2. Red shows ion-thrusting upon arrival at HAMO. It will take over 20 orbits to complete the transition. The red dashed sections show where Dawn is coasting for telecommunications. Once a week, Dawn pauses ion-thrusting and points toward Earth. Updated flight plans and status checks occur during these communications sessions. Click images to embiggen.
NASA | JPL
The latest images of surface details can be browsed at this link, updated almost daily. Below is an example of the fine detail in an image posted 3 August 2015. The HAMO will increase detail by 3x.
NASA | JPL-Caltech/UCLA/MPS/DLR/IDA
Mission scientists have converted elevation detail into this color animation of the rotating dwarf planet Ceres. The color scale extends 3.7 miles (6 km) below the surface in purple, to 3.7 miles (6 km) above the surface in red. The bright white spots are as yet un-explained highly reflective material. Some scientists think they are freshly exposed ice due to recent impacts.
Unlike the New Horizons mission which gathered data during a short time of high-speed flyby, Dawn will remain at Ceres permanently in close low-altitude orbit. From the Dawn Press Kit:
The resource that will ultimately limit Dawn’s lifetime is its hydrazine fuel. Once the fuel is exhausted, the spacecraft will no longer be able to point its instruments at the surface. It also will be unable to point any of its ion engines for maneuvering purposes, nor point its antenna at Earth or its solar arrays at the sun. The battery will be depleted in a matter of hours. The spacecraft will remain in orbit around Ceres, but it will cease operating.
My previous post about the New Horizons flyby of Pluto was dated a few hours before the event of 14 July 2015. Communication with Earth by the spacecraft was turned off so it could execute a large number of commands during the few hours of flyby. The data is to be transmitted to Earth in the months to come. Late in the day, New Horizons phoned home to say it was healthy and did succeed in carrying out the commands. It continues to gather data and return data sets each day. Where is New Horizons now? It is coasting beyond Pluto into the Kuiper Belt. It has more work to do.
There are 50 gigabytes of data stored onboard. It will take 500 days to send it to Earth. People wonder why it will take so long? They want to see the results NOW! Here is the answer from a member of the team.
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