High School physics questions

[Ecthy]

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.

[Berzerker]

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.

[Ecthy]

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

[CerberusIV]

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.

[Kuciwalker]

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.

[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?

[Ecthy]

Berzerker - we must be twins separated at birth.

[Kuciwalker]

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).

[Berzerker]

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.

[Berzerker]

Berzerker - we must be twins separated at birth.

You've detected something I haven't.

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?

[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....

[Eli]

RJ :

The questions are ok.

[Berzerker]

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:

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.

[Starchild]

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.

[Urban Ranger]

Re: High School physics questions

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?