27-06-2003 15:01									#1
	High School physics questions Enter the AD-FREE zone

A friend of mine is going off to start a career as a High School physics teacher (God knows why). For his application to a private school (which are good in the UK - and much safer than state schools) he has to come up with some final year high school level questions which will made the kids think a little.

He told me two of his questions so far, but I wasn't sure of the level. Since some of you here go to high school, maybe you can let me know if the level is 'right'. Here are the questions:

1. How is the period of a grandfather clock's pendulum affected by putting it on the international space station?

2. Which is stronger: the gravitational attraction of the Earth on the moon or that of the sun on the moon?

Are these too easy, too hard or what?

	27-06-2003 15:04									#2
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my guess would be a little too hard. i did college in the netherlands in a scientific study, but i cant answer these 2 with certainty.

	27-06-2003 15:28									#3
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2 is easy. Just use common sense. You don't even need numbers.

	27-06-2003 15:32									#4
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1) Period being the time it takes for the weight to make a swing and back? I have to ask, it's been awhile. Assuming it's not a mechanically driven pendulum, I'd say the period would be shortened.

2) The second, I'd have to calculate the distances and gravitational pulls of the objects, but I'm not into it right now (or ever ).

To answer your question, those questions are easy if the subject matter was taught during the year. If it wasn't, they still aren't that good for your/his stated purpose. They reflect more memory than intelligence. If your friend requires the students to work out the math for those questions, that would show they can handle the material instead of just memorising a yes or no answer (this would be ridiculously hard if the material wasn't covered though).

	27-06-2003 15:33									#5
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quote: Originally posted by Big Crunch 2 is easy. Just use common sense. You don't even need numbers.

Which was why I didn't give any.

	27-06-2003 15:35									#6
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quote: 2 is easy. Just use common sense. You don't even need numbers.

What is it, and why?

	27-06-2003 15:35									#7
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1) Wont it go in circles?

	27-06-2003 15:37									#8
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It's a grandfather clock.

	27-06-2003 15:39									#9
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But the pendulum is what's important. I assumed that the clock itself is immaterial.

	27-06-2003 15:46									#10
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To make the question logical, the clock part would have to be removed. That's why I concluded a period was just one swing since it can't make a circle as part of a clock.

	27-06-2003 15:50									#11
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answer to question 2 : they both pull at each other with the same force. it is only because the mass of the moon is much smaller of that of the earth that the moon gets the much bigger acceleration...

even you attract the earth with the same force as the earth attracts you ...

	27-06-2003 15:51									#12
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Then what are we arguing?

	27-06-2003 15:57									#13
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quote: Originally posted by dannubis answer to question 2 : they both pull at each other with the same force. it is only because the mass of the moon is much smaller of that of the earth that the moon gets the much bigger acceleration... even you attract the earth with the same force as the earth attracts you ...

I think you misunderstood the question.

	27-06-2003 16:09									#14
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quote: Then what are we arguing?

I guess the validity of the question. If a period is a circle, then attaching the pendulum to something that doesn't turn and asking for changes in the period wouldn't make sense.

dannubis -

quote: even you attract the earth with the same force as the earth attracts you ...

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.

	27-06-2003 16:12									#15
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He is right though, just not answering what the question asked.

	27-06-2003 16:12									#16
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of course the Earth's gravitational effect on the moon is bigger because the circulates around Earth not around the sun. but then it converges.

	27-06-2003 16:16									#17
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quote: of course the Earth's gravitational effect on the moon is bigger because the circulates around Earth not around the sun.

That seems like the obvious answer, but I sense a trap.

	27-06-2003 16:25									#18
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BC said use common sense... maybe the convergence is the key, or it's even a lot easier

lateral thinking might be involved O_o

	27-06-2003 16:30									#19
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Think a bit more laterally. The Sun and Moon both cause tides in the Earth's oceans but the Moon's effect is greater (that is why the spring and neap tides occur on a 28 day cycle). If the Moon has a greater gravitational effect on the Earth than the Sun does then it is pretty safe to infer that the Earth (which has a larger mass than the Moon) has a greater gravitational effect on the Moon than that of the Sun on the Moon.

Going back to the original question, these questions are probably at about the right level or maybe a little difficult, not because of their technical demands but because they require the student to think about the question. That is the hardest skill to teach, getting the student to analyse the question and answer what is actually being asked.

	27-06-2003 16:32									#20
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1. There is no period, at least in the normal sense. The pendulum will just spin around and around (grandfather clocks have free-swinging pendulums, right?), slowing down with air resistance (ignoring things like hitting the side of the clock).

2. The sun's effect on the moon. I think.

	27-06-2003 16:36									#21
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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?

	27-06-2003 16:38									#22
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Berzerker - we must be twins separated at birth.

	27-06-2003 16:42									#23
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quote: Originally posted by Berzerker But I understand the Moon's orbit is lengthening, so wouldn't that mean something is pulling it away a bit?

Actually no. It just has a slightly higher velocity than necessary to stay in orbit, thus it spirals away (slowly).

	27-06-2003 16:45									#24
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quote: Think a bit more laterally. The Sun and Moon both cause tides in the Earth's oceans but the Moon's effect is greater (that is why the spring and neap tides occur on a 28 day cycle).

There you go, that makes sense. And even if the Moon is being pulled away a little, all the planets would have to be excluded from the outward pull on the Moon, not just the sun.
But I think acceleration, or deceleration has something to do with the Moon's changing orbit...It's been so long since I had to think about stuff like this.

	27-06-2003 16:48									#25
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quote: Berzerker - we must be twins separated at birth.

You've detected something I haven't.

quote: Actually no. It just has a slightly higher velocity than necessary to stay in orbit, thus it spirals away (slowly).

Ah, is that why astronomers believe the Moon was once alot closer to the Earth? Oh, and would this higher velocity indicate the Moon was once captured in the past?

	27-06-2003 17:00									#26
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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....

	27-06-2003 17:04									#27
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RJ :

The questions are ok.

	27-06-2003 17:19									#28
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RJ - I sure was stumped if skywalker is right on both. But how does a pendulum draw a circle when it's attached to a clock? And did you say the Sun's attraction is stronger?

Skywalker Your name suggests knowledge of celestial matters, but:

quote: grandfather clocks have free-swinging pendulums, right?)

I figured they don't, the question is a bit confusing because it implies a fixed pendulum. But I imagine a free swinging pendulum would move in small circles with the Earth as the focus.

	27-06-2003 17:39									#29
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Bezerker: The first question may be throwing you because you're visualising the pendulum inside the body of the clock and constrained by the four sides. Simplify the image so you have a fixed pendulum, say a mass of X grammes tied to a string that's tied to a cylinder.

Ignoring air resistance and anything else that would cause a loss of energy, the energy in the pendulum system in constant. On Earth, because we are in a gravitational field, the energy in the system is switched between two types. The kinetic energy of the pendulum's motion and the potential energy of it's height. As the pendulum falls, potential energy becomes kinetic and as it rises, kinetic energy becomes potential.

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.

	27-06-2003 17:43									#30
	Re: High School physics questions Remove this text

quote: Originally posted by Rogan Josh Are these too easy, too hard or what?

Too easy, at least for me

I think one of the brain teaser question would be interesting. The one about this guy running 1000m in 14s or less. He can double his speed every second, and a friend of his will fire a gun behind him every second. The question is, can he do it?