Physics help?

[halfracedrift]

True or false questions..

Orbiting astronauts are weightless because of the absence of a support force.

The outer planets in our solar system wobble in their orbits because the gravitational effects of the sun are so small.

As you move farther from the earth, its gravitational force acting on you decreases.

The gravitational field at the center of a planet is zero.

The earth is so far away the sun's gravity does not affect it.

The moon is more responsible than the sun for tides on earth because it exerts a greater gravitational force on the earth than the sun does.

 

[KieranMurphy]

Orbiting astronauts are weightless because of the absence of a support force.

False. Orbiting astronauts are weightless because the they are moving in circular motion just fast enough so that the centripetal force pushing them outward is balanced by the gravitational force pulling them down. The two forces balance each other out.

The outer planets in our solar system wobble in their orbits because the gravitational effects of the sun are so small.

I don't know.

As you move farther from the earth, its gravitational force acting on you decreases.

True, the gravitational force of an object inversely decreases with the square of the distance, so that at twice the distance, the gravitational force is one fourth and at five times the distance, the gravitational distance is 1/25th

The gravitational field at the center of a planet is zero.

False, the gravitational force is enormous at the centre of a planet.

The earth is so far away the sun's gravity does not affect it.

False, if the earth were not affected by the sun's gravity, then we wouldn't be orbitting it. The sun's gravity is what keeps us in orbit around it.

The moon is more responsible than the sun for tides on earth because it exerts a greater gravitational force on the earth than the sun does.

I'm pretty sure the moon's gravitational pull on the earth is far smaller than the suns but I don't know how either of them affect tides.


KM.

 

[halfracedrift]

Answers.. for those who are interested... This thread can be locked up if a mod wishes it to be!
I racked my brains out on this one. =.=

True: Orbiting astronauts are weightless because of the absence of a support force.
False: The outer planets in our solar system wobble in their orbits because the gravitational effects of the sun are so small.
True: As you move farther from the earth, its gravitational force acting on you decreases.
True: The gravitational field at the center of a planet is zero.
False: The earth is so far away the sun's gravity does not affect it.
False: The moon is more responsible than the sun for tides on earth because it exerts a greater gravitational force on the earth than the sun does.

 

[Famine]

True or false questions..

Orbiting astronauts are weightless because of the absence of a support force.

False. The key is "orbiting", as previously alluded to. They are in fact falling towards Earth, but their speed "forwards" is enough to prevent them actually falling directly down towards Earth.

Imagine shooting a gun. The bullet would follow a more-or-less straight trajectory, but be affected by gravity all the time and eventually hit the ground. Now imagine shooting the gun but 200,000 feet above sea level and at 24,000mph. The bullet is STILL affected by gravity but the curvature of the Earth is enough that as the bullet falls, the Earth falls away from it. The bullet is effectively weightless and in orbit.

However, that isn't the question. What the hell is a "support force"? It's not gravity, that's for sure.

The outer planets in our solar system wobble in their orbits because the gravitational effects of the sun are so small.

Truism - but broadly true. All of the planets in the Solar System wobble due to the gravitational effects of the Sun. However, the effects at Pluto are anything BUT small. The gravitational effects of the Sun reach out to the heliopause - three times the distance from the Sun of Pluto.

The Sun also wobbles due to the gravitational effects of the planets. This is how the first extrasolar planets were discovered - by observing stellar wobble.

As you move farther from the earth, its gravitational force acting on you decreases.

True - by the inverse square rule.

The gravitational field at the center of a planet is zero.

Truism - and broadly false. Gravity is a function of mass. At the centre of a planet, the mass beneath you pulling you inwards is zero.

However, the mass of the planet above and around you will be pulling you outwards. So the gravitational field acting inwards is nil, but the gravitational field isn't.

The earth is so far away the sun's gravity does not affect it.

False - otherwise why are we orbiting it? See the astronaut/bullet analogy earlier.

There are three points around the Earth where the gravitational forces acting on an object from the Earth and the Sun are perfectly balanced. These are the Earth-Sun LaGrangian points. They are the only places where you can say the Sun's gravity isn't having an effect - although that true would be false as it is in fact having the same effect as the Earth's gravity.

The moon is more responsible than the sun for tides on earth because it exerts a greater gravitational force on the earth than the sun does.

True. The Sun exerts roughly half the gravitational force upon the Earth compared to the Moon. Although the Sun is much more massive, the Moon is only 186,500 miles away - the inverse square rule plays merry hell with big forces over any sort of distance.

 

[Ev0]

What is the inverse square rule? I know I heard about it last year in physics class, but I'm pretty sure I ever had to do any calculations with it (knowingly at least).

 

[KieranMurphy]

What is the inverse square rule? I know I heard about it last year in physics class, but I'm pretty sure I ever had to do any calculations with it (knowingly at least).

It means that the strength of the gravitaional force between 2 objects is inversely proportional to the distance between their centres of gravity.

You are currently roughly 4000 miles from the centre of the earth. If you were 4000 miles above the surface of the earth, you would be twice the distance from the centre of the gravity so the gravitational force pulling you down would be only a quarter of what it is at the surface. If you were 8000 miles above the surface of the earth, you'd be 3 times as far away so the force of gravity would be 1/9 of what it is on the surface. At 12000 miles above the surface, you'd be 4 times as far away from the centre of gravity, so the force of gravity would be 1/16 of what it is now. Etc. etc.


KM.

 

[KieranMurphy]

True: Orbiting astronauts are weightless because of the absence of a support force.

I'm no physicist but I'm sure that's wrong. What "support force" is present on the surface of the earth but not present in orbit that makes them weightless? The illusion of weightlessness is caused by the fact that they're moving at such a velocity that they're trying to escape the earth's gravitational pull with the same force as gravity is pulling them down. The forces are equal but in opposite directions so it seems like they're weightless.

If the astronaut was a couple of hundred miles up but not in orbital motion, then he'd plummet down to earth. Gravity a couple of hundred of miles up is only very slightly weaker than on the surface of the earth.


KM.

 

[halfracedrift]

I'm guessing the 'support force' is meant as Fn (normal)?

The answers (The ones I posted) are from Simon Fraser University. They were for a friend... *shrug* I have no clue.. I agreed with all the statements made by Famine and KM.. But for some reason, the University doesn't think so.

 

[Carlos_23]

I'm guessing the 'support force' is meant as Fn (normal)?

The answers (The ones I posted) are from Simon Fraser University. They were for a friend... *shrug* I have no clue.. I agreed with all the statements made by Famine and KM.. But for some reason, the University doesn't think so.

The equation for 'Normal' is Fn = m*g. Mass is constant so the only thing that can vary is 'g'. At orbiting heights and speeds, g is not zero (at the Earth's surface, g is approximately 9.8 m/s^2) so there's actually no "absence" of a support force. As KM and Famine already explained, Fn is 'cancelled' by the circular motion and the orbiting speeds, so, the object appears to be weightless.

 

[Purple Platypus]

Well, obviously, Simon Fraser University is wrong After all, who would doubt the brilliance of GTP?

As for that normal force equation, that's actually the equation for the force of gravity on an object. The normal force is the force of the ground pushing up on you (equal and opposite from forces pushing you down). It's usually the same as the Fg, but that's just when you're standing on the ground and not accelerating vertically. When you're in an elevator and it accelerates upward, Fn increases. It decreases when you decelerate at the top.
So, when you're in space, you experience no normal force, as there's no ground pushing up on you. Instead, you only experience the centripital and gravitational forces, which balance out and keep you in orbit.

There are three points around the Earth where the gravitational forces acting on an object from the Earth and the Sun are perfectly balanced. These are the Earth-Sun LaGrangian points. They are the only places where you can say the Sun's gravity isn't having an effect - although that true would be false as it is in fact having the same effect as the Earth's gravity.

That's pretty spiffy, never knew that. But, just what is the effect at those spots, weightlessness? And...do the spots move around the earth as it orbits and rotates?

 

[KieranMurphy]

I'm guessing the 'support force' is meant as Fn (normal)?

What's Fn (normal)? If you mean the force of gravity, it isn't absent at the heights that astronauts orbit at. The earth's force of gravity at the distance that astronauts orbit the earth at is only weaker than at the earth's surface. It's the fact that they're in motion at the right velocoty that the tendency to drift out into space is exactly balanced by the tendency to fall to earth. Thus the forces pushing them out to space and the forces making them fall downwards cancel each other out and it feels like there's no force on them. I'm not sure who Simon Fraser University are, but I don't think they've explained themselves very well.

edit: I read the previous post. I'd forgotten what the normal force was. It's the opposite and equal reaction that the ground exerts on you, as per one of Newton's 3 laws. I still don't know what they mean by "support force" though.


KM.

 

[Carlos_23]

Actually, normal force isn't just the force of the ground pushing up on you; it's present in every object (um, language doesn't help much here, but I'll try). If you sit on a chair, there has to be a force acting (Third Newton Law) so that you don't fall through the chair when you sit on it. That's the 'normal force'. The reason why Fn changes when you are going up -or down- in an elevator is because it's acceleration is applied to you also, so the forces add up, and you "feel" heavier -or lighter, depending on the movement of the elevator-. Gravity is the acceleration towards the Earth acting on other objects. (The equation for force is F=m*a, but since gravity is acceleration, it becomes F=m*g = Fn)

And the fact about the LaGrangian points, very interesting. I'll second the question...

 

[Famine]

That's pretty spiffy, never knew that. But, just what is the effect at those spots, weightlessness? And...do the spots move around the earth as it orbits and rotates?

Since you're in space anyway, yes you are effectively weightless. But there is no pull from either the Earth or the Sun, so you don't need to maintain an orbital motion to remain there. The spots do move around as the Earth orbits, but not as it rotates, so you have to keep up with the orbit - which isn't easy (I believe the Earth moves in its orbit at about 90 miles per second) - but the LaGrangian points are a very, very long way from the planet so it shouldn't require speeds as fast as the Earth's orbit

I believe NASA is placing/has placed a satellite at the second LaGrangian point, although I can't remember the precise function of it (edit: apparently it's an infra-red telescope).

LaGrangian points on the internet (and it turns out there are FIVE LaGrangian points in each two-body system):
http://www-istp.gsfc.nasa.gov/Education/wlagran.html
http://www-istp.gsfc.nasa.gov/stargaze/Sorbit.htm
http://zebu.uoregon.edu/~js/glossary/lagrangian_point.html
The ESA - http://www.esa.int/esaSC/SEMC4QS1VED_index_0.html, http://www.esa.int/esaSC/SEMO4QS1VED_index_0.html.

 

[wee_man]

Famines pretty much covered it all. Though with the centre of the earth question....false it's not. As Famine said you're actually pulled outwards and would feel an outward exerted gravitational force as the mass of the planet around you is pulling on you with it's gravitaitonal force. Infact everything with mass (apart from certain partciels, I forget which), will exhert a gravitaional filed/force. Most of the time you'll never feel it, unless it's a massive object such as a planet. So if you're in the centre you'll have mass all around you exherting a gravitational effect.

 

[free toes]

this thread makes isaac sad