English

[Snowflake]

Now now, obs. Is this why you have nails all over your face?

[Kody]

Nah he went to get acupuncture and got attended to by the vac work kid.

[Enigma_Nova]

Originally posted by HongHu
And then forces are reciprocal also. When there is a force that pushes the spaceship away from the earth, the earth receives a force that pushes it away from the spaceship too. Then what determines who gets the work (acceleration) done would be the mass I suppose.

When a car drives by you, you don't fly off your feet in the opposite direction to the car's acceleration.

Let's make this clear:
Spaceship has Fuel.
Fuel goes out the back of the spaceship.
Spaceship moves forward.
Net effect:
Fuel is forced backward, Spaceship is forced forward.
Third law of mechanics holds!

If the fuel hits earth, then the Earth//Fuel would be forced back with the same momentum as the rocket.
Consider that a rocket weighs 5 tonnes and the earth weighs 5 * 10^21 tonnes, and the earth has 10^-21 times the acceleration.
Of course, most of that fuel will not directly hit earth.
It will most probably mix in the atmosphere or miss the planet entirely.

Tiger's question:
Why is the moon keeping up with us?
I asked a similar question when I was young.
My logical answer:
It's too far away to change its relative position.
I logically presumed that two cars travelling in opposite directions would observe the same moon, and that a moon can't keep up with two cars at the same time - therefore the presumption that the moon moved with the car was false.

Of course, I doubt Tiger is that intelligent.

[Illuminatus]

Nah, but I tried to play russian rulet with nailgun. I won, 278 times

[Snowflake]

Originally posted by Enigma_Nova

Let's make this clear:
Spaceship has Fuel.
Fuel goes out the back of the spaceship.
Spaceship moves forward.
Net effect:
Fuel is forced backward, Spaceship is forced forward.
Third law of mechanics holds!

Exactly. Although a spaceship itself may not carry fuel, not to say leak it. It may be that the force is actually produced by the engine of the rockets with the spaceship burning the fuel and throwing the products out of the end of the rocket.

If the fuel hits earth, then the Earth//Fuel would be forced back with the same momentum as the rocket.
Consider that a rocket weighs 5 tonnes and the earth weighs 5 * 10^21 tonnes, and the earth has 10^-21 times the acceleration.

Again, this is the point.

Of course, most of that fuel will not directly hit earth.
It will most probably mix in the atmosphere or miss the planet entirely.

The atmosphere is a part of the earth, I think.

Tiger's question:
Why is the moon keeping up with us?
I asked a similar question when I was young.
My logical answer:
It's too far away to change its relative position.
I logically presumed that two cars travelling in opposite directions would observe the same moon, and that a moon can't keep up with two cars at the same time - therefore the presumption that the moon moved with the car was false.

Of course, I doubt Tiger is that intelligent.

Actually, it has something to do with your relative frame of observing. (It is what is called an optical illusion). If you are looking at the moon in an open field, you will be less likely to feel that the moon is moving with you. The illusion is often caused when there are other objects that are closer to you, such as trees and houses.

If you look at two things (say a building and a tree) that are different distances from you and then walk sideways, you will see that the two objects shift, but that the closer one shifts to the side more than the one that is farther. The Moon is so far away that it does not appear to shift at all. The human mind interprets this in a funny way. It thinks that the Moon is closer than it really is, but that it is moving sideways at the same speed as you are. So you think that the Moon is following you.

[Kody]

The human mind interprets this in a funny way. It thinks that the Moon is closer than it really is

The optical illusion is to do with how humans always use a frame of reference when seeing objects of varying distances. We learn to judge how far away things using other objects as a frame of reference. Because the moon is so far away we don't have a reliable frame of reference anymore, hence our mind determines that it's closer than it actually is.

This optical illusion can also be observed when you see the moon near the horzion and when we see the moon directly above us. When the moon is directly above us it seems smaller than when it's hanging close to the horizon. This is not to do with refractive effects as the moon would seem elongated. Rather there's objects near the horzion that we can use as a frame of reference. The moon seems bigger as we can see the distant objects that we roughly know as being so and so large while the moon is behind them and hence obviously needs to be scaled larger (in our minds) than we scaled the objects we saw on the horizon.

PS: Didn't you bring this up at that newbie quiz at civgaming last year?

[Snowflake]

That's right. Just wanted to test if I can get a couple brilliant answers again. And I still haven't found a perfect answer for a five year old (who is very likely not as intelligent as Enigma).

[Maniac]

Thanks for making things more clear, Enigma_Nova.

[Kody]

I still think my idea of showing that distant objects appear to move slower than a near object is your best bet.

So pick a landmark and start walking down the road. Ask your son if he notices that the distant landmark doesn't appear to move compared to tree or something that you're walking past. Then explain that the moon is so much farther away that it doesn't appear to move.

The other part of the illusion of "the moon is following" comes from the fact that the moon appears in different places as the car changes in the direction it is facing. Also during a car trip all the surrounding scenery will change where the moon doesn't. So where the moon is, appears to change and it strengthens the illusion that the moon is moving about.

[Hercules]

Rubbish the man in the moon is keeping an eye on you, protecting you at night.

[Chaos Theory]

The twin paradox explained, and more about relativity:

http://www.physicsguy.com/ftl/html/FTL_part2.html

[Enigma_Nova]

An explaination for a young Enigma:
/me locates circular objects of varying sizes
A ping-pong, a dartboard, a satellite dish and The Moon.

/me arranges the items so they are pretty much lined up
"Okay, all these objects appear to be the same size, right?
Now move around a bit and see which one moves more."
/me watches the kiddie move around a bit
"The closest object moves most, right?
Well that moon thing is pretty far away. It doesn't look like it moves much."

If you need a wordy explaination:
"Why are we keeping up with that mountain?"
and let the kid work it out for himself.

[Snowflake]

Thanks you guys! I just need to find the time to do the right experiment with Tiger.

Originally posted by Hercules
Rubbish the man in the moon is keeping an eye on you, protecting you at night.

I think that Tiger knows that there isn't a man in the moon, but a tree, a beautiful girl (ChangE) and a jade rabbit.

When you are in a car or train and look out from the window, why do the trees moving away from you seem to be moving faster then the trees moving toward you?

[johndmuller]

Hey Hercules,
Your latest avatar would be real appropriate for the way my Aunt, a devoted fan, insists on pronouncing the good detective's name (Herc u lee), a kind of singular version of your handle; as for her version of his last name, I'm not going there

[Enigma_Nova]

Glad to know that I helped parent a child.
Educating the unknowing is what I take pride in doing.