Isaac Newton

[Tim Streater]

Oh quite possibly. But in which bit?

[Martin Brown]

ITYM 92.5% of the precession is accounted for by Newtonian dynamics. GR frame dragging is a 7.5% second order correction.

Mercury is the only planet where it really matters 43" relativistic correction to the position of perihelion out of 574" drift per century.

Probes to Mercury use a GR corrected treatment for trajectory otherwise they would be out by a few tens of kilometres on arrival.

[Pinnerite]

I am pretty sure he meant gluing the scales to the wall and pressing against it while standing on an intergalactic friction-free skate board.

Well, that's what I would have done. Always the simplist solutions me. :)

[The Natural Philosopher]

"ALL computations for planetary probes"

ISTR that relatavistic effects had to be accounted for in SOME

[Martin]

You stand the chance of being decapitated if you hit a truck that has stopped dead after colliding with a stationary object.

[Tim Streater]

I was computing it via this:

"The perihelion precession of Mercury is 5,600 arcseconds (1.5556°) per century relative to Earth, or 574.10±0.65 arcseconds per century[105] relative to the inertial ICRF. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5,557 arcseconds (1.5436°) per century."

to be found here:

But perhaps that author is mixing his units.

And 5557/5600 I computed (apparently erroneously) as 0.97 (or so), hence 97%. Humph!

Perhaps I should have said: "The most pronounced effect in our Solar System requiring Einstein to explain is *the* *amount* *of* the precession of the orbit of Mercury ...".

OK. So that's an example where GR needs to be applied. What about probes to, e.g., Mars?

[The Natural Philosopher]

If you can control it evenly, the medians for death is around 350g.

I watched a man walk away from a racing mini that hit a tirewall head on at 100mph. stopped in about 4 feet.

Nigel Mansell at Indianoplois an estimated 250g. Broke some stuff, survived.

IF everybody had racing helmets and roll cages and a racing harness they would be in no danger of decapitaion - the frnt crumple zone on a car is good for about 100mph IF te passenger doesnt flow around inside.

After watching my first motor race I never ever failed to use a seatbelt.

A decision made more firm by watching a man who got thrown out of a spinning vintage car not so equipped get hit and killeed in front of me. And his family. :-(

[Martin Brown]

Only needed for Mercury. Frame dragging is undetectable further out.

Venus wouldn't need it although it is in such a circular orbit anyway that you would be hard pressed to measure it accurately if it did.

[whisky-dave]

Because rubber ones would absorb the enrgy rather than transfer it to the n ext ball.

[The Natural Philosopher]

What you want is low hysteresis steel is pretrtry elsatuic and al te energy that is lost is in the 'ping'

[mechanic]

How can it cool down to less than the temp of the Cosmic Microwave Background - about 3 deg k?

[Martin Brown]

By the time it gets there the microwave background will also have dropped in temperature. White dwarfs take a very long time to cool down. There hasn't been time since the big bang for any of them to have cooled down to the point where they are not radiating in the visible spectrum.

When the CMB was emitted not long after the big bang the microwave radiation we now see as a 3K background was visible light from a hot plasma at 3000K in a universe 1000x smaller than it is now.

As the universe gets bigger the surface of last scattering becomes ever more distant and moving away from us faster due to Hubble expansion.

[Robin]

Indeed. Start with the satellite's in-built clocks. (IIRC the Apollo missions to the moon involved clock differences of 0.5 sec or so.) Also I think time of flight of signals (which may matter when satellites are on t'other side of Sun). And a greater need for many near-Earth missions because of the Earth's rotation (Lense-Thirring acceleration). Maths way beyond me but a talk on JPL's program was fascinating for the clever stuff involved - eg the transformation from geocentric to barycentric frames.

[dennis]

You could read what I wrote.

[Jethro_uk]

Famously the Apollo 13 crew used Newton to get home ....

[Jethro_uk]

Isn't that elastic and inelastic collisions ? One preserves momentum, the other doesn't (although I'm not going to stick my neck out and try and remember which is which).

[Johnny B Good]

Rubber is less elastic than steel, or, put another way, rubber is more plastic than steel, at least in this particular setup. You can store a lot more elastic energy in rubber compared to using the same volume of steel to make a spring. The price for this being an increased hysteresis loss in the rubber band energy store.

In something like a catapult where the mass of a steel spring of equivalent power to a rubber band would severely reduce the acceleration force on the missile (marble, steel ball or or small stone), this makes the modest energy loss of a rubber band a worthwhile trade off for the vastly greater performance offered in such a simple missile launching system as a handheld catapult. However, when it comes to making a Newton's Cradle, the much lower hysteresis loss of steel makes for a much longer lived demonstration in such a low energy experiment.

Where steel springs are used as an energy store to launch missiles, it's usually the practice to employ them indirectly by having them operate a piston to rapidly compress a charge of air in a cylinder which is directed through a small hole matching the diameter of a lead pellet sat immediately in front of it and at the beginning of a rifle barrel, aka, an air rifle or gun.

The piston/cylinder acts essentially as an impedance matching device to translate the momentum of the spring and piston from low speed/high force into an extremely low mass high pressure impulse force to accelerate the pellet to around the 450/500 feet per second muzzle velocity limit permitted under UK regulations (the 18th century Austrian army's 20 shot repeating pump up air muskets fired a half ounce lead ball with a muzzle velocity of 500fps making them more deadly than the firearms of the day).