A dumb question

[steve_bank]

Identify all external and internal forces and sketch a diagram to show the resultant force vector. Does it oppose rotation?

I ca;'t do it off the top of my head on the scale of the Earth a, geology, and astrophysics, but that is how I would approach it.

The vector sum of all applied forces must equal the resulant force.

 

[bilby]

I take the descent into abject punnery as strong evidence that the question has been adequately answered.

While that is likely the case in this instance, I would warn you that I rarely need any such excuse.

 

[Elixir]

Yes the effect may be immeasurable by our instruments, but unless you abandon LOT and conservation there is an effect.

It does say “ directly effect”. I think the imprecision is in considering the pendulum/earth a closed system, which of course it’s not. There is still an effect but no reason that it has to accrue to or against the earth’s angular momentum: it can all be expressed as heat, right?

(IANAP)

 

[steve_bank]

Looking at it from conservation.

Motion of the Earth supplies energy to the pendulum. Earth's kinetic energy is converted to the pendulum kinetic energy and some is lost in friction showing up as heat. Entropy says all of the energy transferred to the system cannot be used to do useful work in the system. Some is lost. The heat however small is transferred to the mass of the Earth, and heat is radiated into space.

In the end kinetic energy is lost from the Earth. Within a finite volume or boundary perpetual motion or a lossless system can not not exist.

It is not theoretical for me, when I was befuddled by a problem that is one way I approached it. LOT.

A continuity equation defines where all the mass and energy goes. The assumption is causality and conservation of mass and energy are true. It is a very powerful analysis tool because it applies to all systems regardless of the processes.

Continuity equation - Wikipedia

en.wikipedia.org

A continuity equation or transport equation is an equation that describes the transport of some quantity. It is particularly simple and powerful when applied to a conserved quantity, but it can be generalized to apply to any extensive quantity. Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described using continuity equations.

 

[steve_bank]

A PDF thermodynamics text if anyone is interested.

https://soaneemrana.com/onewebmedia/Thermodynamics%20by%20PK%20Nag.pdf

 

[Loren Pechtel]

It will very slowly lose it's spin.

1) Tides. Sure, it's tiny. It is not zero. A tiny force over a sufficient period eventually adds up.

2) Jeans escape. The atmosphere will be slowly lost to space. The molecule that escapes will carry away a disproportionate amount of the angular momentum. Note that the vapor pressure of rock is very, very low, not zero.

 

[steve_bank]

Again a question of immeasurable small changes.

What happens when the mass of a space probe leaves the Earth?

 

[Bomb#20]

A lot of people don't get that the issues with motion and work on the surface of Earth have many more barriers than in space. Things work against action on the surface, wind resistance, friction, illegal immigration, transgender issues, and electrostatic force among other things. In a vacuum, these factors aren't applicable.

These guys might disagree about one of those...

 

[Bomb#20]

The thing to ask isn't whether it'll stop spinning, but rather, what force is acting on it to stop spinning?

PERPLEXITY DAY...

Conservation of Angular Momentum​

Despite the energy loss through thermal radiation, the ball's angular momentum must be conserved in the absence of external torques. ...

The thermal radiation applies an external torque. Photons leaving a point on the spinning object's surface in the direction of the spin are blue-shifted; photons leaving in the direction opposite the spin are red-shifted; the momentum of a photon is proportional to its frequency.

 

[steve_bank]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

 

[Shadowy Man]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

 

[steve_bank]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

The Earth rotates so radiation from the sun equally roasts the Earth line a chicken on rotating spit.

The CMBR looks like a black body at round 2.7 kelvin radiating in all directions.

When a photon leaves a laser is there an equal and opposite reaction?

Researchers break Newton's third law -- with lasers

A team of researchers has managed to (very technically) break Newton's third law of motion -- that every action has an equal and opposite reaction -- by accelerating laser pulses around a loop, seemingly without any corresponding push-back.

www.extremetech.com

Researchers break Newton's third law -- with lasers
A team of researchers has managed to (very technically) break Newton's third law of motion -- that every action has an equal and opposite reaction -- by accelerating laser pulses around a loop, seemingly without any corresponding push-back.

 

[Shadowy Man]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

The Earth rotates so radiation from the sun equally roasts the Earth line a chicken on rotating spit.

sure, but I didn’t say “on one side”, I said “from one direction”. And since you were talking about momentum not energy I thought that would be relevant.

 

[bilby]

Photons get murky. No mass but possessing momentum.

No rest mass. Energy is mass; Just not much of it. If:

E=mc²

then:

m=E/c²

The mass of an energetic object is the sum of its rest mass plus its energetic mass. For a photon, the first term is zero, but the second is not:

E = hf

Where h is Planck's Constant (6.62607015×10^-34Js) and f is the frequency in s^-1.

So the mass of a photon is:

m = hf/c²

As h is very small, and c² is very large, m is minuscule, even for the most energetic of gamma rays. But minuscule > 0, even if it's close enough to zero for most mechanical purposes.

As photons move at c, and momentum is mass multiplied by velocity, the momentum of a photon is:

p = hf/c

(your link derives this same result via what to me seems an unnecessarily tortured method, whose only justification appears to be their dislike of considering energetic mass and rest mass as though they were both mass, but as the SI unit for both is the kg, it is hard for me to understand that reluctance).

A spherical black-body at constant temperature radiates photons at a characteristic mean frequency in all directions, but as @Bomb#20 points out, the photons from a rotating sphere are blue or red shifted by the rotation, depending on which side of the axis they are coming from. So the average momentum of the photons differs from one side to the other, causing a reduction in angular momentum.

It ain't much, but it ain't zero.

 

[bilby]

When a photon leaves a laser is there an equal and opposite reaction?

Yes. Yes there is.

 

[Bomb#20]

An object is radiating heat as EM energy while at the same tine is absorbing heat from the surrounding environment.

An object and its environment want to go to equilibrium.

Photons get murky. No mass but possessing momentum.

If Photons Have No Mass, How Can They Have Momentum? – Profound Physics

profoundphysics.com

But the Earth is only absorbing heat from one direction. There may be a power equilibrium but not a directional one.

The red-shift/blue-shift effect applies to incoming photons too, not just outgoing ones -- the solar heating that hits the morning side of the daytime hemisphere is a little bit bluer than the solar heating that hits the afternoon side. Actually the same principle applies to everything that interacts with the earth from outside. Micrometeoroids hit randomly so half of them speed up our rotation and half of them slow us down, but the half that slow us down hit harder on average.