1

Physics, Astronomy & Cosmology / Re: How high and fast can a person skydive from? How fast?

« on: 13/10/2012 00:31:38 »

For those interested in the history, there is an excellent book, 'The Pre-Astronauts; Manned Ballooning on the Threshold of Space' by Craig Ryan, documenting ballooning efforts of the late 1940's through the mid 1960's.

2

Physics, Astronomy & Cosmology / Re: earths age 6000 years or 4.5 billion?

« on: 13/10/2012 00:10:36 »

The group of people who believe the Earth is only 6,000 years old use written historical texts of genealogies to measure back to the beginning.  The most common measure is using the books of the Old Testament.  Genealogies are included which state what father gave birth to what son, and at what age each died.  Using the ages provided, you can count backwards to approximately 6,000 years old.  A nice layout of the timeline appears at:  http://www.bibleistrue.com/qna/qna63dating.htm.

Experimental observations  point to an age of approximately 4,500 million years.

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Physics, Astronomy & Cosmology / How much force on a bicycle pedal?

« on: 12/10/2012 23:57:36 »

OK, science folks...

I enjoy riding my bicycle with an infant seat for my son on the back.  I weigh about 200 pounds, my son is about 30 pounds, and the bike with seat is around 40 pounds.  There are several hills in the area around my home, and while climbing one, a thought struck me.  My body connects to the bike at about four places, one foot on each pedal and one hand on each side of the handle-bar.  I was in a very high gear up a steep hill, but my legs were still tiring.  What I'd like to figure out how to calculate is how much force I was demanding from my thighs with each pedal?  It much be less than my body weight, as my downgoing foot stayed attached to the pedal, weight was still transferred from my hands to the top of the handlebars.  I have regular pedals, not click-in shoes, so the up-going foot is as light as possible.  Any ideas how to work the kinematics to figure it? 

If I maintain a constant speed going uphill, then the force out of my thighs should be the same as the portion of the weight vector parallel to the slope of the hill plus friction losses?

Thanks for any ideas!

-Cheese

4

Physics, Astronomy & Cosmology / Re: Does an aeroplane weigh less when flying at high altitude?

« on: 18/03/2012 21:46:44 »

The answer, it seems, is at best incomplete.  If you are traveling 7.9 km/s tangentially to the orbit of the earth, you will follow a near circular path around the earth.  The same 7.9 km/s directly away from the center of the Earth will fall back to the surface without orbiting.  This is a question in which the velocity vector is equally important as the magnitude.

There is a great flash demonstration of the importance here: 
http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::800::600::/sites/dl/free/0072482621/78778/Escape_Nav.swf::Escape%20Velocity%20Interactive

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Physics, Astronomy & Cosmology / Re: Does an aeroplane weigh less when flying at high altitude?

« on: 18/03/2012 16:37:59 »

Pardon my confusion, but someone asked how fast you'd have to travel to experience zero g and was answered with a velocity?  As g is a unit of acceleration, the answer can't be a velocity.

Gravity does vary with altitude, so the airplane will weigh very slightly less, though the mass of the airplane will be exactly the same at both altitudes. 

By setting the angular acceleration equal to g, I believe Wolfkeeper solved for the tangential velocity of hoop the size of the radius of the earth imparting a centripetal acceleration of 9.81 m/s/s.

6

General Science / Re: Proper pie to celebrate pi day?

« on: 17/03/2012 18:03:30 »

So a full pie on June 28?

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Physics, Astronomy & Cosmology / Re: Why do we orbit the sun if space is not flat?

« on: 17/03/2012 04:46:42 »

This is likely not a perfect analogy, but it is one that works for me.

Have you seen the coin whirlpool wishing wells?you drop a coin onto a giant funnel down a ramp that imparts an initial velocity around the funnel. A bit like an orbital velocity. The coin rolls along and maintains the velocity by falling down the funnel. As the coin rolls along, the angle around the surface of the funnel is dictated by the forward (orbital) velocity and the velocity due to acceleration toward the bottom of the funnel.  If you took away friction, the coins velocity would dictate an orbital path around the hole, and would be elliptical if the path was not perpendicular to the slope of the surface. 

That's the way the earth orbits the sun. In a near frictionless environment, the amount the earth falls toward the sun is exactly offset by the amount of forward motion as it falls, keeping the earth at nearly the same distance from the sun. Obviously, the earth's orbit is an ellipse, so it speeds up near the minor axis and slows near the major.  Just like the coins whose frictionless path around the wishing well is not exactly perpendicular to the local "downhill" slope of the wishing well.

A famous depiction by Newton illustrates the body moving forward offsetting falling toward the Earth is the same as the Earth orbiting the sun:  http://www.askamathematician.com/wp-content/uploads/2012/01/newtmtn.jpg

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General Science / Re: Why Does A Litre Of 100C Water Have MORE Mass Than When The Water Cools To 4c ?

« on: 17/03/2012 04:24:51 »

If the mass decreases with temperature, we may have to reconsider the "Law of Conservation of Mass."

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General Science / Re: Proper pie to celebrate pi day?

« on: 14/03/2012 20:12:21 »

Spring time pie... I like a Rhubarb Strawberry pie!

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General Science / Re: Why Can We Feel Heat When Space Is So Cold Between Sun And Earth ?

« on: 11/03/2012 21:52:15 »

Joe,

You're absolutely right.  You're describing the Standard Atmosphere (the creation of which was pretty interesting history in itself).  The model uses a standard lapse rate up to 36,152 ft MSL.  Above which, the temperature is a constant -70 deg F until 82,345 ft MSL.  NASA has a high school level description of the standard atmosphere at http://www.grc.nasa.gov/WWW/K-12/airplane/atmos.html.

Every assumption made by an aerodynamicist in designing your airplane inherently relies upon the standard atmosphere.

It may or may not be interesting that's why so many jet liners want to fly around 36,000 ft MSL.  As you go higher, the density continues to decrease, but the temperature stays the same.  It is effectively the "sweet spot" for turbine powered aircraft. 

But, we're completely off topic for space...

11

Technology / Re: Is there a history of Solid-fuel Gas-turbines experiments?

« on: 10/03/2012 00:47:27 »

A Pulsed Detonation Engine could use a fine enough solid for combustion, but PDE's are still in the experimental stage...

http://www.afmc.af.mil/news/story_print.asp?id=123098900

12

Physics, Astronomy & Cosmology / Re: Why can I cycle faster than the fastest runner?

« on: 09/03/2012 23:48:57 »

I ran a few numbers to actually see how this works out.  I know I can comfortably riding a stationary bike at 80 RPM for 30 minutes.  I made some measurements on my actual bike and found (rounded to the nearest cm):

Radius of the front sprocket:  10 cm
Radius of the rear sprocket:  3 cm
Radius of the rear tire:  37 cm

Given the RPM and the radius of the front sprocket, the chain moves along the sprocket at 0.84 m/s.  The chain moves at the same speed along the rear sprocket at the same speed, but induces a rotational speed of almost 28 rad / sec (or about 270 RPM).  At the wheel tread, this gives me a very respectable 10 m/s (or 23 mph) of forward travel. 

In this case, the gearing gave me more than 3 turns out of the rear wheel for every turn on the pedals.  That speed is due to the gearing, not the friction.

The friction between the ground and the motive surface (tire or shoe) provides the forward moving force, right?  I'm assuming the coefficient of friction between a shoe and pavement is very close in value to the coefficient of friction between rubber and pavement.  If that's true, and I do not have an appropriate table of coefficients available to me, the body plus the bike should make more friction than the body alone.

13

General Science / Re: Why Can We Feel Heat When Space Is So Cold Between Sun And Earth ?

« on: 09/03/2012 22:45:06 »

The sun transfers its heat via electromagnetic radiation, and we named the modeled process "black body radiation."  The amount of radiation received and absorbed depends on the material composition of the body and the cross-sectional area of the body perpendicular to the path of the radiation.  As spacecraft travel to the inner solar system, the heat management becomes an immense problem.  To counter the higher energy, the spacecraft are coated in reflective materials to deflect the heat and/or coated in materials that can absorb the heat, but not transfer the heat into the body of the spacecraft. 

Where things get really interesting in spacewalks.  In the sunlight, temperatures can reach as high as 250 deg F, while plunging to -250 deg F in the shade.  (Thanks, NASA: http://www.nasa.gov/audience/foreducators/spacesuits/facts/facts-index.html)

Even though it is cold between the Sun and Earth, if you put a body there, it will heat up due to absorbing the radiation from the sun.  It is only cold, because it is empty space without a physical body there to absorb the energy.  I promise, if the Earth magically moved to Venus' orbit, the surface would be much, much hotter!

Attached is the spectral output of the sun based on black body radiation (thanks, wikipedia).  Notice how the power output of the sun at around 5,800 Kelvin, is nearly even across the visible spectrum.  Which is why sunlight breaks into a nice even rainbow out a prism.

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Physics, Astronomy & Cosmology / Re: Why can I cycle faster than the fastest runner?

« on: 08/03/2012 22:20:16 »

But the question was about a bike!  Not what different ways how he move faster than an elite athlete!

Totally agree with your observation though.  The reduced friction decreases his or her deceleration. 

15

Physics, Astronomy & Cosmology / Re: Why can I cycle faster than the fastest runner?

« on: 08/03/2012 15:00:07 »

It's all about the gearing ratios!  The governing equation here is:

V_t = r * w   (The tangential velocity is equal to the radius times the angular velocity)

As you push your feet on the pedals, it spins the front sprocket with the chain leading to the back sprocket.  The angular velocity you impart to the pedals, is the same angular velocity you impart to the sprocket.  That becomes an tangential (linear) velocity the chain moves back to the rear sprocket.  The chain imparts an angular velocity on the rear sprocket that is the  same angular velocity of the rear tire.  The edge of the rear tire moves quickly along the ground thanks to the advantage of the largest radius on the bicycle!

So, without going through and carrying the subscripts through a bunch of equations, the power stroke on a bicycle is normally the down-going leg.  It's a relatively short motion, but through the gearing, rotates a portion of the circumference of the rear tire determined by the ratio of the radii of the chain sprockets and radius of the rear tire.

Back to running, no gearing to be had, so the only velocity the athlete can achieve is out of his own legs.  A bicyclist can get a higher velocity than what his or her legs can generate thanks to the gearing.

16

Question of the Week / Re: QotW - 12.03.11 - How would I know if a meterorite was falling towards me?

« on: 05/03/2012 02:05:04 »

You can always tell if you're on a collision course with anything by a simple rule known as "constant bearing, decreasing range."

17

General Science / Re: What happens when light travels through a transparent object, like glass?

« on: 05/03/2012 00:36:49 »

Not sure how in-depth an answer you are looking for... but here goes.  The speed of light is a constant around 3.0 x10^8 m/s in a vacuum.  It is close to that value in air, but it is truly slightly less.  As light passes through a transparent medium, like air or glass, it slows.  The rate of slowing is proportional to the frequency.  Which is why white light entering a prism, exits as a rainbow.  The glass slows each frequency of the incident white light at a slightly different rate.  So when it exits the prism and the entire wave speeds up to the speed of light in air, the light has spread out in spectrum.  The effect is well defined by Snell's Law, and there is a very good discussion at:  http://en.wikipedia.org/wiki/Prism_%28optics%29 .
 

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General Science / Re: What stops the blades of a propeller-driven aircraft from icing?

« on: 05/03/2012 00:06:59 »

There was a discussion earlier about air friction heating. 

Windchill is only a factor if one has wind flowing over something warmer than ambient temperature.

Assuming a non-heated propeller, the propeller would be at ambient temperature.  Movement, and air friction would then tend to increase the temperature of the propeller. 

How much?  A couple of degrees might help.

There is a well defined function based on absolute temperature and mach number which dictates the heat rise.  Wikipedia displays it nicely here:  http://en.wikipedia.org/wiki/Total_air_temperature

Based on a given airspeed, a heat rise calculator is here:  http://www.luizmonteiro.com/Altimetry.aspx#TemperatureRise

It is interesting to know see that you get 10 deg C heat rise at 270 knots, but 20 deg C heat rise at 380 knots.

There are stories of the SR-71 Blackbird leaking fuel on the ground, but not in flight, because the skin heating due to friction caused the joints on the airframe to expand and seal airborne.  I don't know if the stories are apocryphal, but they're fun.

19

General Science / Re: What stops the blades of a propeller-driven aircraft from icing?

« on: 04/03/2012 23:30:13 »

First, each aircraft will have different systems for anti-ice (prevention of ice formation) or de-ice (removal of formed ice) in-flight.  By each aircraft, I don't mean different power-plants.  I mean each different aircraft type.  A common issue for all types of power plants is to ensure no ice is ingested due to foreign object damage (FOD) concerns.

I happen to have quite a bit experience flying propeller driven aircraft.  My particular airplane happens to be all-weather capable.  When icing conditions are encountered in flight, engine anti-ice valves are opened.  These valves open a tap of hot air from the last stage of the compressor section to flow towards the engine intake.  This heats the inlets around the engine air intake, and intake vanes.  The heat ensures no ice forms within the air intake of the motor.  Immediately after the intake is the compressor section.  As the air compresses, it heats, and serves to keep the water in vapor form.

The procedures for my airplane call for using wing and propeller de-ice once 1/4 to 1/2 inch of ice has formed on the wing.  The same hot air that heats the leading edge of the wing.  As the skin heats, the ice flakes off and is blown back over the wing.  The wing de-ice is secured until the more ice forms. 

Lastly, the propeller is de-iced electrically.  My props are about 8 feet in radius and spin at just over 1,000 rpm.  As you can imagine the outer section of the prop doesn't collect ice, as it is spinning too fast, and simply sheds any accumulated ice outward.  The inner-section of the prop is coated in a heavy plastic with heating elements called the "prop boot."  Of the four blades on each prop, the each prop boot is heated serially for approximately 20 seconds.  Each boot is heated on a 25% duty cycle, and sheds the ice radially.  The central hub of the prop is heated continuously. 

It is a very old airplane, but works exceptionally well and in all weather conditions. 

20

The Environment / Re: Changing the temperature of water (this is alot more interesting that you think)

« on: 04/03/2012 23:03:31 »

http://www.usatoday.com/weather/resources/askjack/wfaqhurm.htm

Interesting Q and A from the USAToday newspaper about what has been attempted and tested to weaken hurricanes.  There are discussions about icebergs, water absorbing materials, nuclear warheads, and coating the ocean with chemicals to limit water evaporation.

-Cheese