A physics question

Gene Howe · Post by **Gene Howe** » Sun Oct 14, 2012 10:22 am

That guy that wants to fall to earth from space in order to break the sound barrier is about to try it today.
My question is: How does he achieve 700+ MPH? I thought terminal velocity was somewhere around 185 MPH.
At any rate, I wish him Godspeed....so to speak.

dusty · Post by **dusty** » Sun Oct 14, 2012 10:57 am

I too wish him well though I do not understand why he's doing this ! I am just thankful that I don't have to go along.

letterk · Post by **letterk** » Sun Oct 14, 2012 11:16 am

Becuase of the lack of air and moisture at the highest altitude he will achieve it with the first minute to 90 seconds of the jump. The comments I read about the original jump were that at first it is completely still until they get low enough and then they start to hear the wind rushing around them and wind resistance will start to slow them down.

What I think is more amazing then this feat is that the first person to do this did it more than 50 years ago. At that point I think he reached IIRC like 632 mph. This jump is going to be higher to reach the speed of sound and to set the record. I would guess the sonic boom will be non-existence since there is no air to carry the sound, but the speed will still be there.

Gene Howe · Post by **Gene Howe** » Sun Oct 14, 2012 12:15 pm

So, gravity and the lack of appreciable atmosphere will allow him to reach the speed of sound? Isn't the speed of sound lower at higher atmosphere? So he might not actually reach over 700 MPH and still break the sound barrier?

letterk wrote:Becuase of the lack of air and moisture at the highest altitude he will achieve it with the first minute to 90 seconds of the jump. The comments I read about the original jump were that at first it is completely still until they get low enough and then they start to hear the wind rushing around them and wind resistance will start to slow them down.

What I think is more amazing then this feat is that the first person to do this did it more than 50 years ago. At that point I think he reached IIRC like 632 mph. This jump is going to be higher to reach the speed of sound and to set the record. I would guess the sonic boom will be non-existence since there is no air to carry the sound, but the speed will still be there.

letterk · Post by **letterk** » Sun Oct 14, 2012 12:32 pm

Gene Howe wrote:So, gravity and the lack of appreciable atmosphere will allow him to reach the speed of sound? Isn't the speed of sound lower at higher atmosphere? So he might not actually reach over 700 MPH and still break the sound barrier?

It does vary based on altitude. Don't know if this link will come through, but it says the speed of sound at 120,000 feet is 700 mph.

http://m.wolframalpha.com/input/?i=soun ... ft&x=0&y=0

The sonic boom is sound waves compressing together which likely won't happen in an environment with little to no air.

stephen_a._draper · Post by **stephen_a._draper** » Sun Oct 14, 2012 1:20 pm

I teach physics.

In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance. Sound travels faster in liquids and non-porous solids than it does in air. It travels about 4.3 times faster in water (1,484 m/s), and nearly 15 times as fast in iron (5,120 m/s), than in air at 20 degrees Celsius. The speed of sound also changes with temperature and air pressure.

In a vacuum there is no such thing as sound or the speed of sound since there is no medium to propogate the sound waves. The speed of sound they are referring to is its speed in air at sea level at 68 degrees Fahrenheit on planet earth.

Gene Howe · Post by **Gene Howe** » Sun Oct 14, 2012 1:58 pm

That explains why I hear the iron dinner bell quicker than I hear the wife's to do list,:D

stephen_a._draper wrote:I teach physics.

In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance. Sound travels faster in liquids and non-porous solids than it does in air. It travels about 4.3 times faster in water (1,484 m/s), and nearly 15 times as fast in iron (5,120 m/s), than in air at 20 degrees Celsius. The speed of sound also changes with temperature and air pressure.

In a vacuum there is no such thing as sound or the speed of sound since there is no medium to propogate the sound waves. The speed of sound they are referring to is its speed in air at sea level at 68 degrees Fahrenheit on planet earth.

trainguytom · Post by **trainguytom** » Sun Oct 14, 2012 2:05 pm

Gene Howe wrote:That explains why I hear the iron dinner bell quicker than I hear the wife's to do list,:D

That's the best part of this whole thread.

paulrussell · Post by **paulrussell** » Sun Oct 14, 2012 3:42 pm

Terminal velocity assumes near-sea-level air density. He can go faster due to lack of atmospheric interference. On the other hand, he is counting on being slowed down as he nears the planet (prior to chute opening) as the atmosphere becomes more dense.

Yesterday my wife told me a great story. Back in the 60's her dad was explaining to the kids how the speed of sound was much slower than the speed of light. Her mother exclaimed "Oh, now I understand! That explains why when I turn on the radio the light comes right on, but the sound takes a while!"

Gene Howe · Post by **Gene Howe** » Sun Oct 14, 2012 4:36 pm

No Tom, Paul just claimed that honor.

paulrussell wrote:SNIPPED
Yesterday my wife told me a great story. Back in the 60's her dad was explaining to the kids how the speed of sound was much slower than the speed of light. Her mother exclaimed "Oh, now I understand! That explains why when I turn on the radio the light comes right on, but the sound takes a while!"