[lunar7 (OP)]

I was doing a Physics quiz and got this question wrong.
There is gravity on satellites. Those on board feel no gravity because they are in free fall.
So, surely I did get this question right, though it was marked incorrect.
Have I missed something?

[alancalverd]

You are absolutely correct. The reason the satellite stays in orbit and does't hurtle into space is because it, and everything in it,  is in the earth's gravitational field.

The reason the pendulum doesn't work is because the satellite and everything in it is in free fall.

The formula for the period of a pendulum is T = 2π√(L/g) where g is the net acceleration due to gravity and/or any other force acting through the pendulum bob when it is in the neutral position.

Inside a satellite in stable orbit, there is no net force and no single "neutral" position. A mass stays exactly where it is in relation to the pendulum pivot, no matter where it is released. Therefore g = 0 and T is undefined. 

[charles1948]

You are absolutely correct. The reason the satellite stays in orbit and does't hurtle into space is because it, and everything in it,  is in the earth's gravitational field.

When you refer to the Earth's gravitational field, how far does that extend.  And how would anyone on a satellite orbiting the Earth, actually know that they were within the field.

I mean, suppose there were people living deep inside our Earth's satellite, the Moon.  These people wouldn't know the Earth existed.

If these lunar people developed Physics, would it be a different Physics from the one we have on Earth?

[Halc]

When you refer to the Earth's gravitational field, how far does that extend.

The strength of the field is given by GM/r² which means that at any radius (r), there is still a gravitational field.  So it extends indefinitely.

And how would anyone on a satellite orbiting the Earth, actually know that they were within the field.

Per the equivalence principle, there's no local way to tell.  A non-local way is to observe tidal effects (the difference in gravitation from one side of the room to the other), but that's real subtle for a small room thousands of km from the center of Earth.

I mean, suppose there were people living deep inside our Earth's satellite, the Moon.  These people wouldn't know the Earth existed.

Probably not, no.

If these lunar people developed Physics, would it be a different Physics from the one we have on Earth?

Cosmological physics would probably be different for anybody not capable of looking outside the window.  They'd probably be stuck in something more primitive than Newton/Kepler since gravity would be unknown. Everything falls at some uniform rate, because that's simply what it does. They'd have no reason to suspect it had anything to do with mass or distance or anything.  Maybe if their domain varied in depth enough (a big enough room so to speak) to notice the difference in acceleration rates (growing less with depth, not more), they'd need a new model.

[lunar7 (OP)]

So, I did get the question correct, but was marked wrong.

[alancalverd]

So, I did get the question correct, but was marked wrong.

Worth a complaint. People are entitled to teach conflicting views about history or sociology, but not about elementary physics!

[alancalverd]

And how would anyone on a satellite orbiting the Earth, actually know that they were within the field.

Because they are always at the same altitude. If there were no gravitational field they would fly off at a tangent.

[chris]

So, I did get the question correct, but was marked wrong.

No, sorry.

It's a craftily-worded question. The satellite, it says, will be in a state of "zero gravity".

That cannot be true, because a satellite is an orbital body, orbiting under the influence of gravity. Therefore it cannot be in a state of zero gravity.

Your physics was right but you've been caught out by nuanced wording!

[lunar7 (OP)]

I'd say badly worded rather than crafty, because, a satellite is a body that orbits a larger mass. So, there has to be gravity

[alancalverd]

It's a craftily-worded question. The satellite, it says, will be in a state of "zero gravity".

Sorry, Chris. And, sadly, Lunar too!

The question asserts that a pendulum will not work in a satellite (which is true)  then offers the reason that the sattelite is in zero gravity (which is false). The correct choice is therefore option 3, as Lunar stated.

If a body is in zero gravity, i.e. "deep space", far from any other massive body, it is not a satellite because in the absence of an external force it obeys Newton's first law and travels in a straight line at constant speed.

The examiner has made the elementary mistake of equating g with gravity.

As any pilot or motorcyclist knows, g is the vector sum of acceleration and gravitational field strength. Inside a satellite g = 0 because the centripetal acceleration is equal and opposite to the gravitational field strength.

A teacher is someone who takes a subject he understands and phrases it so a pupil can understand it. An educationalist is someone who takes a subject he doesn't understand and phrases it so nobody else can.

I think the question is a very good one, but it was marked by an educationalist.