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Solar system

Straight answers to factual questions
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Clitheroekid
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Solar system

#284127

Postby Clitheroekid » February 14th, 2020, 12:42 am

Not sure this is the right board but ... I was reading today about a discovery that apparently discounts the traditional theory that the planets were formed by violent collision, and instead postulates that they were a result of a gentle coming together.

The basis for this theory appears to be a lump of rock that's formed of two separate rocks - https://edition.cnn.com/2020/02/13/worl ... index.html

But what I'd like to know is what physical force could achieve this?

I can easily understand how a new planet could be formed by violent collision, as the forces generated would create massively high temperatures that would melt the two rocks and enable them to merge as a molten mass. However, I'd have thought that if two lumps of rock simply bumped into each other "at about 7 miles per hour" they'd just bounce apart.

Because of their size, gravitational forces would be minimal, so how and why would they ever even stay together, let alone merge into each other?

Grateful thanks for any enlightenment.

servodude
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Re: Solar system

#284131

Postby servodude » February 14th, 2020, 3:54 am

Clitheroekid wrote:Not sure this is the right board but ... I was reading today about a discovery that apparently discounts the traditional theory that the planets were formed by violent collision, and instead postulates that they were a result of a gentle coming together.

The basis for this theory appears to be a lump of rock that's formed of two separate rocks - https://edition.cnn.com/2020/02/13/worl ... index.html

But what I'd like to know is what physical force could achieve this?

I can easily understand how a new planet could be formed by violent collision, as the forces generated would create massively high temperatures that would melt the two rocks and enable them to merge as a molten mass. However, I'd have thought that if two lumps of rock simply bumped into each other "at about 7 miles per hour" they'd just bounce apart.

Because of their size, gravitational forces would be minimal, so how and why would they ever even stay together, let alone merge into each other?

Grateful thanks for any enlightenment.


could it be as simple as the conservation of momentum?
like when you lift and drop two balls simultaneously on either side of a Newton's cradle?
- equal(ish) forces in opposite directions result in stasis(ish)

- sd

JohnB
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Re: Solar system

#284133

Postby JohnB » February 14th, 2020, 4:14 am

You get very high friction in vacuum because there us no air to lubricate, so perhaps things stick like snowflakes. When small these bodies are just jumbles of rock, ice and dust, so lots of surface area to interact, and ice to act as glue

GoSeigen
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Re: Solar system

#284137

Postby GoSeigen » February 14th, 2020, 5:29 am

servodude wrote:
Clitheroekid wrote:I can easily understand how a new planet could be formed by violent collision, as the forces generated would create massively high temperatures that would melt the two rocks and enable them to merge as a molten mass. However, I'd have thought that if two lumps of rock simply bumped into each other "at about 7 miles per hour" they'd just bounce apart.

Because of their size, gravitational forces would be minimal, so how and why would they ever even stay together, let alone merge into each other?

Grateful thanks for any enlightenment.


could it be as simple as the conservation of momentum?
like when you lift and drop two balls simultaneously on either side of a Newton's cradle?
- equal(ish) forces in opposite directions result in stasis(ish)

- sd


Also, they are not perfectly elastic spheres like the ones in physics problems! They are more like a car, which changes its shape in a collision, absorbing the kinetic energy of the two vehicles: you don't see cars bouncing away from each other like snooker balls! The loose material on the mini-planets' surface would similarly be displaced and make their collision relatively inelastic.

GS

Julian
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Re: Solar system

#284155

Postby Julian » February 14th, 2020, 8:22 am

GoSeigen wrote:
servodude wrote:
Clitheroekid wrote:I can easily understand how a new planet could be formed by violent collision, as the forces generated would create massively high temperatures that would melt the two rocks and enable them to merge as a molten mass. However, I'd have thought that if two lumps of rock simply bumped into each other "at about 7 miles per hour" they'd just bounce apart.

Because of their size, gravitational forces would be minimal, so how and why would they ever even stay together, let alone merge into each other?

Grateful thanks for any enlightenment.


could it be as simple as the conservation of momentum?
like when you lift and drop two balls simultaneously on either side of a Newton's cradle?
- equal(ish) forces in opposite directions result in stasis(ish)

- sd


Also, they are not perfectly elastic spheres like the ones in physics problems! They are more like a car, which changes its shape in a collision, absorbing the kinetic energy of the two vehicles: you don't see cars bouncing away from each other like snooker balls! The loose material on the mini-planets' surface would similarly be displaced and make their collision relatively inelastic.

GS

Also, don’t forget the massively long timescales. Even if all kinetic energy wasn’t absorbed and the bodies did bounce off each other at least some of the kinetic energy would be absorbed and so the rebound would be with less momentum than the collision. At that point the bodies are drifting apart but if there are no other significant masses in the vicinity then eventually, maybe over millennia, gravitational attraction will draw the previously collided bodies together again for a slightly lower energy collision than the one before, and then another rebound having shed just a bit more kinetic energy to heat, and so on until eventually they remain in contact.

- Julian

dspp
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Re: Solar system

#284169

Postby dspp » February 14th, 2020, 9:06 am

Clitheroekid wrote:Not sure this is the right board but ... I was reading today about a discovery that apparently discounts the traditional theory that the planets were formed by violent collision, and instead postulates that they were a result of a gentle coming together.

The basis for this theory appears to be a lump of rock that's formed of two separate rocks - https://edition.cnn.com/2020/02/13/worl ... index.html

But what I'd like to know is what physical force could achieve this?

I can easily understand how a new planet could be formed by violent collision, as the forces generated would create massively high temperatures that would melt the two rocks and enable them to merge as a molten mass. However, I'd have thought that if two lumps of rock simply bumped into each other "at about 7 miles per hour" they'd just bounce apart.

Because of their size, gravitational forces would be minimal, so how and why would they ever even stay together, let alone merge into each other?

Grateful thanks for any enlightenment.


At planetary scale the gravitational forces at the centre are enough to squish stuff together very well. I have no opinion on the theory you are alluding to, but in essence it is that gentle bumps and lightly squished together little bits ultimately become large enough accumulations that gravity takes over.

It is gravity that ultimately holds the nuclear reactions in the sun from escaping. Fortunately it is a 1/r2 force:

https://www.usna.edu/Users/physics/fink ... ter_13.pdf

regards, dspp

ErroneousBee
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Re: Solar system

#285236

Postby ErroneousBee » February 18th, 2020, 5:11 pm

Clitheroekid wrote:
The basis for this theory appears to be a lump of rock that's formed of two separate rocks - https://edition.cnn.com/2020/02/13/worl ... index.html

But what I'd like to know is what physical force could achieve this?


The physical force that brings them together is gravity, the reason they don't throw each other way like greased ice skaters is because they bump together gently. Under those gentle encounter conditions objects transfer rotational energy until after many such bumps they are not spinning fast enough to throw each other off.

The very small things (dust, individual atoms/molecules) clumps together via electrostatic forces. One grain gains a few electrons, another loses some (by being irradiated, or by friction), and dust will clump together. This is easily demonstrated at home by attracting small pieces of paper onto an acetate rod, or from the way flour and vacuum bag dust will clump. Physical (freeze/thaw/melt) and chemical processes over time can convert these fluffy collections of dust grains into rocks. Dust keeps accreting onto these bodies until they are km sized objects. At that point there is no more dust left to gather up, its been accreted, or blown out of the system by the solar wind.

At a much larger scale, moon sized bodies will attract each other gravitationally, they will perturb each other's orbits and fall into each other, building up to planet sized objects. Moon sized bodies can lose kinetic energy by interacting with smaller objects, throwing the smaller bodies out of the solar system. These processes are well understood and easy to numerically simulate: mass is conserved, gravity is proportional to mass and dominates at planetary distances.

Which leaves a big gap between 1km and and 1000km sized planetoids. How do the planetisimals clump together to form moons?

There were 2 leading theories:

Theory One: The rubble was flying around with the right amount of energy to deform or melt the planetesimals and weld them together, rather than have both objects disintegrate in a cloud of debris.

Theory Two: There were enough rocks in similar enough orbits that interactions would nudge them together. Boulders that did bounce away would steal a bit of rotational energy from each other and eventually piles of 1km sized objects would end up resting and rolling around on each other until gravity was enough to form them into moons.


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