High School physics questions

[Rogan Josh]

Thanks Neutrino, I just worked it out with numbers and got the same thing.

The ratio of gravitational moon-sun force to moon-earth force is:

(Ms/Me)(Re/Rs)^2 = (2*10^30/6*10^24)*(4*10^5/15*10^7)^2 =(1/3)*10^6 * ((4/15)*10^-2)^2=16/(3*225) *100=2.37

So the moon-sun force is a little over twice the earth-moon force, confirming neutrino's answer.

Edit: this is actually quite a bit less than I was expecting...

[Kidlicious]

Originally posted by Big Crunch
Effect =! Force.

Analagously a small force on a lever crowbar is more effective than someone putting a lot of force by directly lifting.

I get it now.

[ixnay]

Originally posted by Berzerker
dannubis -

If you jump in the air, does the earth move in your direction as much as you move toward the earth when landing? If you were right, we'd be floating along the surface waiting to hit a bump and bounce off into space.

Actually, he's right. The forces between you and the earth are the same when you jump in the air. But since force = mass * acceleration, and the earth has so much more mass than you, the acceleration of the earth is negligible compared to your acceleration.

F(you) = F(earth)
m(you)*a(you) = m(earth)*a(earth)
small mass * large acceleration = large mass * tiny acceleration

[Traianvs]

what dannubis and ixnay just mentioned is high school physics, RJ (well I remember it somewhat from last year, heh)

[Adalbertus]

Edit: this is actually quite a bit less than I was expecting...

Rogan,

This is because you think in terms of rotational energy and neutrino calculated forces. In the case of energy, you've got m(sun)^2/m(earth)^2*d(earth-moon)/d(sun-moon), if I calculated correctly. This should turn out to bigger.

The problem with this question is that "effect" is not a precise term in physics. You should state what you mean in physically well-defined quantities (or accept a lot of different but well-explained answers). It's a mistake teachers make quite often (and then insist on their private interpretation).

That's a nice question, but not for "thinking" only, but doing the calculation - the problem here is that we look from earth and tend to neglect the influence of the sun on the moon entirely, because its the same as that on the earth.

As for those who thought of tidal forces: They don't depend on the force itself but on the rate of change of the force - or better graviational acceleration. On earth, the gravitational field of the earth-moon system changes faster than that of the earth-sun system, and therefore the tidal effect of the moon is stronger than that of the sun.

I wouldn't pose the ISS question for testing, unless the students made their first step into general relativity (or it is a really messy calculation in accelerated frames of reference! In classical physics, the ISS is not an inertial system, in GR, it is). I think it works more as a sort of guesswork to introduce general relativity.

[raghar]

Suppose you have an alien ship in L1. what is affecting that alien ship more earth, sun or moon? And what if we add jupiter? Could he answer it?
Could you post all his questions?
If he will try to get work as an teacher this questions could work, it could impress director of that institution. Well more or less. If he will try them on bored irritated students that have a lots of other problems, for example how we would pay for school, It probably wont be as big - how to say it nicely. Then again students in GB are different that from rest of europe, not too much however. Last time, I listened "...in prison is better than in school..."
Lets hope he would get experience rather quickly and don't forget to remind him about his ideals after few years. Or he could turn into some kind of maniac like most of teachers.

Lots of begining teachers are forgeting that teaching isn't just about stupid questions, but is also about common sense from teacher and what he could give to students. Like tolerance and lots of unpaid work.
As an ex astronomer amateur that had rather nasty experience with less capable teachers, I could tell the real difference between a teacher and a fool.

[raghar]

Originally posted by Berzerker
Well, the Moon goes around both, but since it goes around the Earth and doesn't float along with the Earth, I'd say the Earth's attraction is greater. But I understand the Moon's orbit is lengthening, so wouldn't that mean something is pulling it away a bit?

Moon is slowing down earth rotation and rising its own orbit. It's best explainable on some models or in an animation.
It might be better if people would be tought an astronomy in schools than indoctrinated by pseudoscience by theirs teachers.

[Cruddy]

Originally posted by Rogan Josh

The moon is orbiting the sun too. If fact both the Earth and the moon are orbiting the sun. The orbit of the moon around the Earth is not affected (much) by the sun because both the Earth and moon are in freefall, so in their rest-frame they don't feel the gravitational effects of the sun.

The reason for the tides being caused by the moon is because the orbit of the moon around the Earth is not circular. The same effect in the Earth around the Sun is what causes the seasons.

Yeah, but surely the highest gravity effect win?

If the sun had a greater gravitational effect on the moon than the Earth, why doesn't the moon get further away from the Earth and closer to the sun?

Of course the sun has a bigger gravity well than the Earth - but as the Earth's gravity well is stronger on the moon, in the moon's current position, that makes the Earth have a bigger gravitational effect?

Maybe the wording doesn't come over in the translation...

Anyway, here's an alternative question -

Assuming the ISS is in geo-sync orbit - it stays over the same spot of Earth all the time - and an astronaut throws an object at the earth - assuming it survived re-entry - would it hit near that local spot, or much further away?

[Dauphin]

Originally posted by Cruddy
Assuming the ISS is in geo-sync orbit - it stays over the same spot of Earth all the time - and an astronaut throws an object at the earth - assuming it survived re-entry - would it hit near that local spot, or much further away?

You'd have to throw the object pretty fast, it would more likely end up in an elliptical orbit than fall to Earth.

[Pekka]

The speed of sound in the air is directly proportional to the absolute square root of temperature. At temperature of 20 Celsius the speed of sound is 343 m/s (meters per second).
What is the speed of sound when temperature is -10 Celsius (0 Celsius = 273K)?

Is this too easy? I'm sorry, my english gets in the way when talking about physics

[Pekka]

An engine consumes 1.4kg of gasoline in one hour and gives the power of 3.5 kW. What is the engines efficiency, when gasolines heating value is 43 MJ/kg?

Ship of aliens do analprobe on you. There are 8 aliens, and they are about human size. It will take about 9 hours. How big of embarrasment you are to the world?

Seriously though, some pointers what kind of questions and what level, I'm not sure what kind of level you have in high school.

[Cruddy]

Originally posted by Big Crunch


You'd have to throw the object pretty fast, it would more likely end up in an elliptical orbit than fall to Earth.

Correct. Very unlikely to hit the Earth at all - because it's already in orbit, all you would get is a changed orbit.

Bit of a trick question really...

[Pekka]

A block of ice has a mass of 400 g and a temperature of -15 C. It is dropped into a thermally isolated container, which containes 2800 kg of water at temperature of 60 C. Determine the final temperature of water after the system has reached thermal equilibrium.

[Cruddy]

You forgot the "at standard air pressure" disclaimer Pekka.

You try that trick in a vacuum and you are in for a BIG surprise.

[Pekka]

It would only confuse students.. they are not Einsteins yet!!!!!