[Nuu™™]

Because Galileo said so.

[Zoo]

Well, at least between small and very large objects, the mass of the larger body (say the earth for this one) is so much bigger than the object that adding their masses together is essentially the same as the mass of the earth (really, what's 5g or 5kg more when you have the whole earth?), so mass is negligible.

[Neraphym]

Zoo is on the right track. This isn't something that is purely conceptual; it can be proven mathematically.

[Darkness]

Isn't it technically relevant though? Since speed is mass times time... XD

[Goto]

Yeah, Zoo is on the right track. My physics is a little rusty, since I haven't done this stuff in over a year and a half, but I'll take a stab at the mathematics and see if I can remember.

Let m1 be the mass of the falling body and m2 be the mass of the Earth. G is a constant, r is the radius between them.

F = m1 * a = (G * m1 * m2) / (r^2)

Take out m1 from both sides of the equation and you have...

a = (G * m2) / (r^2)
That is, acceleration due to gravity is the gravitational constant multiplied by the mass of the Earth, divided by the square of the radius between the earth and the falling body. The mass of the falling body is no longer part of the equation, and is thus irrelevant. As you can also see the acceleration differs based on how high you are, but the change in radius divided by the initial radius is so tiny that this isn't particularly observable. (What's a couple of meters, hundred meters or even kilometers compared with the radius of the planet?)

And yeah, Terminal Velocity is the point at which a falling body won't accelerate, because the force of gravity is balanced out exactly by air resistance. So it's not actually the same for all falling bodies, it's just the acceleration up to that point that is a constant.

[Neraphym]

Yup, perfect!

[Goto]

Whee, yay for guessing. Guess I have to come up with a question now, though.


Name a failing of the wave model of light.

[Neraphym]

The particle model also works.

[Goto]

How is that a failing?

In a lot of cases either model will work equally well, but the particle model is much simpler to work out. I want an example of something that the wave model fails to explain or account for.

[Ratty]

The wave model doesnt account for the momentum of light.

[Goto]

Hmm.. Really? My memory is a bit shaky I guess.

I mean under both models light doesn't have a mass, which is usually used to determine momentum. But since it certainly does contain energy, I wasn't aware that either model failed at explaining it. I'll take your word for it though, since I'm a little hazy on the details and you're probably right.

I was thinking more along the lines of any example where light needs to be thought of as discrete packets for the theory to match experimental results. The wave model applied singularly (as opposed to using wave/particle duality) fails to account for something like the photoelectric effect. Under the wave model any wavelength of light would eventually provide enough energy to eject electrons, while under the particle model an individual photon has to have enough energy to eject it on its own. Anyway, your question.

[Ratty]

I was just trying to remember what the teacher was saying, while sitting next to you drawing stick figures on your notes.

Anyway, that whole spinny vein thingie in a vacumm that when you shine the light on it, it starts turning.

My question, eh?

Hmm... What is an elements 'half-life'?

[Zoo]

the time it takes for half of a quantity of an element to decay

[Ratty]

Hmph, it deleted my reply saying you were correct. =P

Your question.

[skenasis]

Zoo is right...I remember doing half-lives in chemistry last year. Boring.

For those who are interested, half-lives are the reason that carbon dating works. There are different 'types' of carbon (isotopes), and one naturally decays into the other (carbon-14 to carbon-13). So, by working out the ratio of carbon-14 (because elements always decay to a more stable element or isotope, with the most stable being...I think iron) to carbon-13, scientists can figure out how old something is. The process only works for stuff up to 50000 years old, because after that there's not enough carbon-14 to for the calculation to be accurate.

Oh, and it also works because the rate of decay is fixed, and a known value of time.

So there's something to remember for those of you who do biology or chemistry at a secondary school level.

And for those of you who are interested in the subject, I suggest you check out this.