Let's take some of that "frictionless" material and make a track around the Earth's equator for a ball (also frictionless) to roll. Or slide. (Why a ball if it be frictionless? Aesthetic considerations, of course!)

Let's put a frictionless ramp on the track. We put the ball at the top of the ramp and hold it there while we put on our space suits. Next, we suck all of the air out of the air in order to get a world-wide frictionless environment. (We could have simply built a container for the track, but we're on a tight budget.)

We let go of the ball. It picks up speed as it rolls/slides down the ramp. It goes off westward toward sunset. We remove the ramp before our ball comes back again from the east.

Does our ball come back again from the east? Why? Or why not?

And would the ball actually roll or just slide?

 

The ball will slide unless you make it roll.

The ball can come back from the east unless it is metallic because induced eddy currents from the earths magnetic field will slow it down. It may do one or more rounds of the earth depending on the magnitude of the eddy currents and the momentum of the ball.

 

The ball can come back from the east unless it is metallic because induced eddy currents from the earths magnetic field will slow it down. It may do one or more rounds of the earth depending on the magnitude of the eddy currents and the momentum of the ball.

'Ideal' world please!!

 

'Ideal' world please!!

Ideal world does not mean no magnetic field. He said ideal regarding friction.

 

I apologize for not specifying which material we coated with the frictionless paint. Mik3 did address both magnetically influenced and non-magnetically influenced materials, however.

 

I have this image of a really big Hot Wheels setup.

 

Ideal huh?

Are we to ignore Coriolis?

Remember also that the point of contact of the ball with the track is not moving.

 

Ideal huh?

Are we to ignore Coriolis?

QUOTE]

My guess is that's why he specified "at the equator".

 

Remember also that the point of contact of the ball with the track is not moving.

Why do you mention this? How is it relevant?

Also, define what you mean by contact point, because the statement doesn't seem correct. If the ball is spinning, then more than one point on the ball must contact the track. Further, there must be more than one contact point, otherwise there is no track. Further, the type of track has not been defined.

We could have a double rail track. If we have a doube rail track and the ball is not spinning, then there are two contact points and they are not moving. Unless of course, the track is not perfectly shaped, in which case the contact point must move a little.

Or we could have a wide track where the ball bounces off the sides. If we have a wide track and the ball is not spinning then the bottom contact point is not moving, but the side contact point is moving from north side to south side. We could consider this side-contact as one time dependent contact point that moves, or two non-moving contact points that never exist simultaneously.

It's also possible to conceive of a track that tries to conform to the shape of the ball, but is not perfect. Hence the contact points move as the ball slides over imperfections.

We could also conceive of a track that does perfectly conform to the shape of the ball, in which case the contact points are infinite in number and should be referred to as a line. Whether or not this contact-line on the ball is moving would again depend on whether to ball is spinning.

 

It goes off westward toward sunset.

Something's fishy. Does the answer depend on the speed of the sliding ball?

 

The whole point of my mechanical analogy in the other thead was to try to lead people to to realize that friction is necessary to cause my ball to stop once se in motion. My rolling ball would climb the next hill and then roll back down, oscillating forever between two hills.

I was looking for someone to state that you need a mechanism, such as friction, to dissapate energy.

 

I have this image of a really big Hot Wheels setup.

We could put in some loop de loops...

Something's fishy. Does the answer depend on the speed of the sliding ball?

Isn't speed of the ball relative to the planet? Would direction matter?

The whole point of my mechanical analogy in the other thead was to try to lead people to to realize that friction is necessary to cause my ball to stop once se in motion.

Indeed. That's why I put my question in a new thread.

I have trouble with the whole idea of ideal.

 

I have trouble with the whole idea of ideal.

We all have trouble with it, but isn't consideration of ideal situations useful in Physics?

Why did it take so long for someone like Galileo Galilei to note that all objects accelerate at the same rate in a gravatational field? Because it's hard to imagine a feather falling the same way as a lead sphere. We are not used to the ideal situation of no air-friction, and our minds reject any visualization of a feather falling like a stone.

This little fact lead Einstein to completely reformulate the theory of gravity. A simple fact about an ideal situation led to one of the most original discoveries in history. It's simply amazing.

 

The whole point of my mechanical analogy in the other thead was to try to lead people to to realize that friction is necessary to cause my ball to stop once se in motion. My rolling ball would climb the next hill and then roll back down, oscillating forever between two hills.

I was looking for someone to state that you need a mechanism, such as friction, to dissapate energy.

I should think that just about everyone reading was aware of this fact. However, you worded your analogy in a strange way. You said there was no friction and that the ball stopped at the bottom. This is a little confusing/misleading. When you prompted me to comment, I said that either the ball should keep moving or there is some unknown force to stop the ball.

 

Well my intent was to stimulate thought.

Perhaps I succeeded after all.

 

Well my intent was to stimulate thought.

Perhaps I succeeded after all.

You always do!

 

Thingmaker,

Where did you find that problem?


Also, if you remove the earths atmosphere everything will die very soon of harmful radiation from the sun.

 

More thoughts...

Ideally, wouldn't a track around the world be ideally gravitationally flat, i.e. go right through hills, or be high enough to go over all of them?

Wouldn't an ideal ramp be either zero high or infinity high? (either of which degenerates the problem completely).

Zero friction, with the ball not initially rotating... as it moved around the Earth, the contact point would also move around the ball. A non-rotating ball would not keep the same face pointing toward the center of the earth, so the contact point would rotate around the ball as the ball rotated around the Earth.

Ideally (and because I couldn't find where it was a steel ball), the ball is either completely non-ferrous/magnetic, or made out of the ideal substance I wish I could find for my torroids.

And ideally, when the Earth was put in a vacuum, our ideal space suits would also protect us from the radiation .

Ideal sure does make things easy! It gets to be whatever you want it to be.

 

I suspect that after a period of time, the ball would become tidally locked to the sun and move along the track in synchrony with it.

 

Thingmaker,

Where did you find that problem?

It was just something rolling around in my head.