yes, g = -GM/r2 shows you the inverse square law. The Earth has a density roughly 8 times that of Saturn, yet surface gravity is almost the same. Earths density is 4 times that of Jupiter but has a much lower value of surface gravity. It is down to its density but there doesnt seem to be a correlation - Im not going to waste an hour working out the density to gravitational field strength ratio of all the solar systems planets tonight - but I could be wrong. Density is mass per unit volume so I guess if grav field strength is related to mass it is related to density. It is not, however, taught that way in England and I cant think of any equation that has density in it
Not that it matters, but after a moment of futzing with algebra, g = -4*pi*G*R*D / 3 where pi = 3.14 etc. G = Newton's gravitational constant R = planet's radius D = planet's density My original point was simply that gravity doesn't depend only on mass. You said it did. You have to have the inverse square of the radius. If you screw around with the numbers, you can have giant planets with many times Earth's mass, but less surface gravity than Earth. /nerdiness
Good point! But I suppose if it shows as a disk in a telescope, its radius can be measured even if you can't actually stand on it.
I said gravity depends on mass, I didnt say it only depends on mass. I see what you have done with the equation. You have rearranged it to take mass out and use the fact that mass = density x volume. Very good use of mathematics in an discussion, I like it. However, that is not the equation on the A-Level specification, nor the accepted equation for g, but very good all the same. Oh, and I suppose every forum has a tionA.
One more even nerdier point: My equation for surface gravity as a function of radius and density made the assumption that the planet is a perfect sphere. Real planets are not perfect spheres; they are closer to being oblate spheroids. That means the polar radius is smaller than the equatorial radius, and the surface gravity is actually a function of latitude (less gravity at the equator, more at the poles). So the OP might be interested to know that, if the flattening of the planet is significant, and he has a polar-dwelling character who wishes to lose weight, the character could accomplish that goal by moving to the tropics. /extreme nerdiness
Also, it's only the mass contained in the imaginary sphere with the radius from your position to the center of mass. any portion of the mass in the shell outward from your position makes no contribution, neither adding to nor subtracting from the gravity from the inner sphere. This becomes increasingly significant if you descend into a gas giant. /maximum nerditude
I think pupils lose out an awful lot these days in relation to the subject syllabus. Certainly here in England anyway. The syllabus is so rigid and without scope, or the timeframe, foe more indepth analysis. Hence "gravity depends on mass". This conversation has got me thinking about different things with regards to this topic, and thats a positive thing. Ive been very strict with how I teach, ie within the confines of the syllabus, and as a consequence students are missing out on so much real Physics. Which is a shame as I am sure they would be interested in this more real Phsyics. Ha,I was nearly going to call it everyday Physics but it isnt really
I was a little surprised at your post above commenting on my equation relating surface gravity to density. You said it was not "the equation on the A-Level specification, nor the accepted equation for g." If students are taught there's only one equation for g, and here it is on the A-level specification, they might develop some rigid ideas about what physics is, how it developed, how it's used, and how it relates to mathematics. They might not truly understand how quantities like mass, distance (or radius), gravitational force, density, and so on are related. They might resist trying to do the kind of thing I did (relating surface gravity to density) because it takes them out of their comfort zone, when in fact it shouldn't. They should learn to do things like that because it improves their understanding of the physical situation. When I derived that equation with density in it (yes, I have a deriver's license, in case you were wondering ), I thought momentarily that it would be cool if I could get rid of the radius entirely and have an equation for surface gravity as a function of density and nothing else. But of course, you can't do that, because you could have a grain of sand with the same density as a planet, but it sure won't have the same gravity. So I could write an equation with density and radius, or density and mass, but never with density alone. If your intuitive grasp of the physical situation doesn't make that clear to you, the math sure will. Students should be encouraged to play with physics equations like this. The algebra involved is very simple, and the exercise is very instructive. It's too bad if the syllabus discourages them from doing this.
Going back to the OP and the thread title, I wonder if there are any decent forums dealing in physics out on the internet? I know it's frowned upon to 'endorse' products here, but surely it would help if in this case suitable links could be given? I know I for one could use the help of an astrophysics adviser for some of my inane questions, and without access to such a resource a forum would be the next best thing, maybe.
it may surprise youfriend but its the truth. The sad thing about schools these days is that they are very results driven. To get the best results possible we need to keep to the syllabus as much as possible. What we have been discussing here is only the tip of the iceberg with where Physics could go. I would love to go through the basics of special relativity - time dilation and length contraction for example - with my pupils, as well as what we have touched on here and other topics. They would definitely understand it and some of them would get so much joy from it. Sadly, unless there is a complete overhaul of the syllabus, that wont be happening
I'm sure there are such forums. Certainly there are plenty of astrophysics discussions on science fiction sites. No need to post links. I'm sure a short google session will find plenty of good physics sites. There were always plenty of NNTP (newsgroups) in the pre-www days.
Both of those appear to be for students or workers in physics, something I'm not and never will be. At best I'm a casual observer of physics, and the kind of questions I have would send the more serious physicists up the wall! I think I've found a forum (via Google) that might suit my needs. I'll need to check it out further though to see if it can cope with my dunce level understanding.
Im not sure about that mate, there will be plenty of teachers on there who should be happy that "normal" people are asking Physics questions
Just thought I should point this out. From what I know, the surface of Venus is super-heated (even more so than mercury) due to its high atmospheric CO2 contents. It has nothing to do with a moon. The only major force our own moon exerts on Earth is the generation of tidal waves. The high concentrations of GHG's in Venus' atmosphere are thought to be caused by a large number of active volcanoes on the planet's surface.
Male giraffe adults are up to 5.5m in height, but typically live for around 25 years in the wild and can certainly surpass 30 years in captivity. (However, there are physiological adaptations which make this possible - very high heart rate, powerful (but compact) heart and high blood pressure - and a giant humanoid would likely have similar adaptations). (Maybe they'd have evolved two hearts like Dr. Who's gallifreyans) The Quetzalcoatlus - which lived in the late Cretaceous - arguably had a wing span of up to 15m (though no-one seems 100% sure). They would certainly have had behavioural adaptations to deal with this - flying almost entirely by gliding (riding thermals like vultures and eagles - or glider pilots - can be seen to do). I'd mark a bird with a 25m wingspan as "extremely unlikely" (I struggle to envisage the circumstances that would have made its huge size an evolutionary advantage - I think the environment would have to be rich in thermals and carrion) but I couldn't commit myself to saying it was impossible.
Isn't it meaningless (for our purposes) to talk about surface gravity when there's no solid surface? [EDIT]I see cogito beat me to that one[/EDIT] /should've read the whole thread
And there are tidal forces of a decent magnitude acting on Venus; the sun's. Here on earth, the sun and moon both exert tidal forces (which arise due to the pull on one side of the planet being greater than the pull on the far side due to the increased distance (the force falls of more sharply than the distance due to the inverse square law)). On earth, the moon's influence is greater than that of the sun. (Clearly, over time sometimes the moon and sun pull together, sometimes they are at odds). Venus while it has no moon will have a greater tidal effect from the sun, being closer and it will still be significant. If Venus supported liquid water, any ocean would still have tides.
