High School physics questions

[Urban Ranger]

Originally posted by Starchild
In the space station, there is no gravity drawing the pendulum downwards. So there is no potential energy, just the kinetic energy. So when a pendulum is started off, it will swing around and around.

Not true, otherwise the ISS wouldn't be circling around earth now

[Kidlicious]

On number two, isn't the answer the Earth, because the moon orbits the Earth. Sure the moon orbits the Sun, but only as it orbits the Earth. I'm just asking because I'm not good at Physics.

[Urban Ranger]

Originally posted by Kidicious
On number two, isn't the answer the Earth, because the moon orbits the Earth. Sure the moon orbits the Sun, but only as it orbits the Earth. I'm just asking because I'm not good at Physics.

Spoiler:

Yes, you are right.

[FrustratedPoet]

Originally posted by Rogan Josh
Sykwalker was right for both. (I will explain why later but I have to dash of to a meeting now...)

But you (or most of you) are still not answering the question I asked....

And skywalker is (IIRC) one of the high-school kids we have around here. So I guess that proves they aren't too hard.

This thread also proves that some people tend to overthink problems.

[Chemical Ollie]

1) What drives a pendylum is gravity, so it will stop against the hull of the clock or circle around until friction stops it, depending on the construction of the clock.
2) The gravity of Earth has the largest impact on moon, as it is closer.

But can someone tell me what happens if you release a litre of liquid water into deep space? Will it instantly freeze into a chunk of ice or will it evaporate and then freeze into a fine mist of ice crystals? That's something I have been wondering for long.

[Rogan Josh]

OK, let me explain.

1. Maybe the wording of this is a little confusing. Of course, I wasn't really thinking about the pendulum being constrained by the box, but it doesn't really matter. Since there is no gravity to change the rotational speed it doesn't oscilate. It just goes round in a circle until air resistance stops it (or if constrained, to bounces against the walls until it dissipates its energy against the walls and stops, probably on the 1st bounce since wood on a grandfather clock isn't very springy).

2. The sun has more effect on the moon. This one is probably easiest to think of in terms of the energies involved. The energy of a rotating body goes like w^2*r where w is the angular velocity and r is the radius. w is 12 times larger for the moon round the Earth than for the moon round the sun (a year^-1 compared to a month^-1), but the Earth-moon distance is much less than a twelfth* of the moon-sun distance. So w^2*r is greater for the moon-sun and they must therefore have more gravitational energy.

Hmm... from the responses this thread has had I think these problems are probably too hard for school kids.

UR: Could you rephrase your question - I have not sure what our problem is. Whether he can beat his record depends on his starting velocity. Clearly if he starts from rest then he won't since 2*0=0. The minimum speed he would have to start with is (approx.) (1000m)/(2^14s)=0.062ms^-1
Is this what you mean?

Edit: *...err, a 144th obviousy. duh!

[Ecthy]

so I was right and everyone who said otherwise is a liar. burn.

[Urban Ranger]

Originally posted by Rogan Josh
1. Maybe the wording of this is a little confusing. Of course, I wasn't really thinking about the pendulum being constrained by the box, but it doesn't really matter. Since there is no gravity to change the rotational speed it doesn't oscilate. It just goes round in a circle until air resistance stops it (or if constrained, to bounces against the walls until it dissipates its energy against the walls and stops, probably on the 1st bounce since wood on a grandfather clock isn't very springy).

Not really, because if earth's gravity doesn't affect the ISS, it would not rotate around the earth, it would have gone into deep space. Hence, this question depends on the initial velocity of the pendulum.

Originally posted by Rogan Josh
2. The sun has more effect on the moon. This one is probably easiest to think of in terms of the energies involved. The energy of a rotating body goes like w^2*r where w is the angular velocity and r is the radius. w is 12 times larger for the moon round the Earth than for the moon round the sun (a year^-1 compared to a month^-1), but the Earth-moon distance is much less than a twelfth of the moon-sun distance. So w^2*r is greater for the moon-sun and they must therefore have more gravitational energy.

That doesn't make sense at all. If the Sun's gravational pull is greater, the moon would have stop orbiting the earth and start orbiting around the sun. Come to think of it, it is incorrect to assume the angular velocity of the moon around the earth is 12 times greater than that of the sun, because the orbit around the sun is much greater than that of around the earth.

[Rogan Josh]

Originally posted by Urban Ranger
Not really, because if earth's gravity doesn't affect the ISS, it would not rotate around the earth, it would have gone into deep space. Hence, this question depends on the initial velocity of the pendulum.

Of course the Earth's gravity affects the ISS - I didn't say it didn't. But anything inside the ISS is in the ISS's rest frame and doesn't feel gravity (just like being weightless in a freely falling elevator).

That doesn't make sense at all. If the Sun's gravational pull is greater, the moon would have stop orbiting the earth and start orbiting around the sun.

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.

[Kidlicious]

Now I'm confused. I think the Moon might still orbit the Earth if the gravitational pull by the Sun was greater, but wouldn't the ordit of the Moon around the Earth look quite a bit different. Wouldn't the Moon move farther away from the Earth during the day and closer to the Earth during the night?

[Kidlicious]

I'm looking at an Astonomy book and it appears to me that not only is the gavational effect of the Earth on the Moon is greater than that of the Sun, but the Moon actually has a greater gravitational effect on the Earth than that of the Sun. That's why tides are more affected by the Moon than the Sun

[Dauphin]

If anyone wants proof that the Sun pulls harder on the Moon than the Earth does then use real numbers. I figure the Sun has about twice the force the Earth does.

What many people are forgetting is that if the moon and Earth are rotating around the sun. In effect, if you look at the Earth-Moon system as being stationary you can ignore the sun's attractive force as it is cancelled out by the centripetal force of a rotating frame of reference.


Its exactly like ignoring the effect of the Earth's gravity in an orbiting shuttle or satellite. Its there but you don't notice it.

[Kidlicious]

Here's a quote from my Astronomy book.

"The gravitational force of the Moon, and to a lessor extent the Sun, raises the ocean tides of the Earth."

[Dauphin]

Effect =! Force.

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

[Neutrino]

From http://hypertextbook.com/facts/2002/AdaLi.shtml :
Mass of moon = 7.34 x 10^22 kg

Mass of Earth = 5.98 x 10^24 kg

Mass of Sol = 1.99 x 10^30 kg

Gravitation force = G[(m1*m2)/d^2)], where G is the gravitational constant, m1 and m2 are the masses of the 2 objects, and d is the distance between their centers of mass.

From http://scienceworld.wolfram.com/phys...lConstant.html ,
G = 6.672 x 10^-11 N m^2 / kg^2

From http://www.freemars.org/jeff/planets/Luna/Luna.htm ,
Distance from Earth to Moon:
Perigee 363,300 km
Mean 384,400 km
Apogee 405,500 km

From http://neo.jpl.nasa.gov/glossary/au.html ,
(mean) Distance from Earth to Sun:
149,597,870.691 km

Approx min and max distance from Sun to Moon (using mean dist of Earth-moon):
149,597,870.691 +- 384,400 km

So,
Earth-moon gravitational force :
1.98e26

Min Sun-moon gravitational force (assuming moon is on far side of earth to maximize distance):
4.33e26

Note that the distances are probably not distances between centers of mass, but I don't think that would change the results by that much. Feel free to check my results, I'm at work so I didn't have time to doublecheck.