Science Fail

[Lunar Sea]

When doing the gravity scan in space near Duna the flavor text says gravity is lowest at the bottom of the canyon - but we all know gravity is stronger the closer you get to the center of mass! So the surface gravity would be highest in the canyon floor. 

Nice to be able to feel sanctimonious about something today...

[JoeNapalm]

8 minutes ago, Lunar Sea said:

When doing the gravity scan in space near Duna the flavor text says gravity is lowest at the bottom of the canyon - but we all know gravity is stronger the closer you get to the center of mass! So the surface gravity would be highest in the canyon floor. 

Nice to be able to feel sanctimonious about something today...

Uhm....

No?

As you get closer to the center of mass, the attraction would be distributed around you.

You'd be weightless at the center of mass.

-Jn-

[Lunar Sea]

Theoretically yes as distance would = 0, but I am talking about surface gravity here. The gravity would be stronger at lower elevations and weaker at higher elevations as per inverse square law. So when the science text says gravity is lower in the canyon floor it doesn't jibe.  

 

 

[Terwin]

 

8 minutes ago, Lunar Sea said:

Theoretically yes as distance would = 0, but I am talking about surface gravity here. The gravity would be stronger at lower elevations and weaker at higher elevations as per inverse square law. So when the science text says gravity is lower in the canyon floor it doesn't jibe.  

If you are over the canyon, then you are slightly farther away from the center of mass.  The center of mass is not at a point that is at the minimum possible elevation, but the point that has the lowest average distance from all the mass of the planet, so unless there is something heavy under the canyon that counters the lack of material in the canyon, gravity would indeed be less over a deep canyon than over a mountain.

 

If you are *in* a canyon, then you have the mass of the canyon walls actually pulling you *away* form the center of mass.

Acceleration from a given body only increases consistently with reduced distance so long as that mass is far enough away that the entire body is pulling you in a consistent direction.  finally leading up to zero gravity at the center of mass(as the mass of the entire body is balanced around you and it is all canceled out)

Edited June 17, 2016 by Terwin

[Corona688]

28 minutes ago, Lunar Sea said:

When doing the gravity scan in space near Duna the flavor text says gravity is lowest at the bottom of the canyon - but we all know gravity is stronger the closer you get to the center of mass!

That's an oversimplification, unless your planet is really an ideal point-source then there's more to it than distance to the center of mass.  Having a wide stretch of particularly low-density rocks beneath can reduce gravity a minor but measurable amount.  There's places on earth where 9.81 is really 9.77.  It's a useful tool to geologists.

Also, mun is a rotating object, which causes its own apparent forces.  This would matter more to the seismic detector than the "gravioli" one though (in real life you can't really separate those measurements).

Meaning, this is a chance to learn something fun!

https://en.wikipedia.org/wiki/Gravity_of_Earth

Wikijump to your heart's content.

Edited June 17, 2016 by Corona688

[cubinator]

57 minutes ago, Corona688 said:

unless your planet is really an ideal point-source

In KSP, they are. So, technically, the gravity scan is incorrect. But if KSP had our physics, then gravity would indeed be slightly less at the bottom of a deep canyon than at the top of a mountain.

[Snark]

Okay, a couple of things.

First, in the real world:  satellites have done high-resolution mapping of the ocean floor by taking super-precise measurements of the height of the ocean surface.  Big undersea mountains have lots of gravity that pulls water towards them, mounding the surface of the ocean slightly higher above them.  Big undersea canyons do the reverse.  So it's a real-world effect that real-world scientists and engineers have actually used to good purpose; it's given us amazing maps of the ocean floor that would have been very expensive and time-consuming to try to do with soundings, not to mention finding features that would have been invisible to traditional sounding techniques (e.g. a big canyon in high-density rock that's filled up with low-density silt).

Of course, that refers to the gravity field outside of the canyon.  What about inside?

The question came up in another thread, concerning "what happens to the strength of gravity if you go deeper and deeper underground".  Here's a discussion of the answer:

In short:  Whether the bottom of the Duna canyon would have higher or lower gravity to someone who's standing in it depends on a lot of factors, including the planet's mass distribution, the shape and size of the canyon, etc.  However, to a spaceship outside the canyon that's passing overhead... yes, the gravity will be slightly lower at that point.