Laws of Motion/Rest?

[Kumar]

Billy T, thanks but. will molecular bonding in crystal be more relavant thought instead of atomic/chemical bonding? Does surface/closed atmosphere as in crystal. restrict motion/temp. of molecules in crystal and so the thermo-dynamic equib. with surrounding atmosphere?

 

[Kumar]

I think you are understanding now. As you have seen we use the terms inertial and non-inertial. In your example the "space frame" would be an inertial frame, while the "earth frame" would be non-inertial. A person resting would be at rest in the earth frame, but would not truly be at rest because the earth frame itself is accelerating. A person running would be moving in the earth frame which in turn is accelerating. An object at rest in the (inertial) space frame would be truly at rest, not accelerating in any way.

-Dale

Yes, thanks for better explaining.

"An object at rest in the (inertial) space frame would be truly at rest, not accelerating in any way."

Yes but, can an object remain in motion in the space(inertial) frame? If yes, how?

 

[Kumar]

Works for me. Of course, you would have to consider it potential energy.

Although to be honest I don't understand why Kumar is bringing it up. It seems irrelevant to the thread since I would imagine that a bunch of powder could be at rest just as easily as a solid crystal. I don't think that potential energy of any kind has any bearing on an object's state of rest.

-Dale

I just want to understand the differences in motion and energy/temp. of atoms/moecules in crystal and in its "pulverized" form? It will help to understand motion and rest in different substances and in different forms.

 

[Billy T]

Billy T, thanks but. will molecular bonding in crystal be more relavant thought instead of atomic/chemical bonding? Does surface/closed atmosphere as in crystal. restrict motion/temp. of molecules in crystal and so the thermo-dynamic equib. with surrounding atmosphere?

I should have been more clear. I was speaking as if no atmosphere, especially no oxygen containing one, was present. What would actually happen, perhaps even in an inert gas (argon etc.) atmosphere is almost instantly at least a monolayer of atoms from the atmospheric gas would form on the fresh surface.

As these molecule attach, fall down the force field of the Van de Walls atomic or even ionic forces, they will strike the surface and excite phonons in the crystal. That is they will lose most of the KE they acquired in the fall, but may "bounce" once or twice until they find a nice, probably "interstitial" bed to sleep in. The phonon will soon collide with others and scatter off lattice site if the crystal is not perfect etc and soon the "infall energy" is thermal. I.e. the work you did "pulverizing the crystal" is heat, perhaps even more than you did is now heat as the electronic binding of oxygen at least surely releases more energy that the symmetric binding of the Van de Walls forces had in the original unified crystal.

As for what you want to call this binding, energy release etc. I do not really care, but when one speaks of "chemical energy", I tend to think we are discussing at the level where some alternatives are "nuclear energy" "gravitational energy" not at the level where one speaks of Van de Walls and ionic binding energies etc.

My main point (and one I made in the thread suggesting metal dust as fuel for cars etc.) and one that few people fully appreciate, was that it takes significant energy to make (atomicly clean) surfaces.

 

[DaleSpam]

Yes, thanks for better explaining.

"An object at rest in the (inertial) space frame would be truly at rest, not accelerating in any way."

Yes but, can an object remain in motion in the space(inertial) frame? If yes, how?

The short answer is yes, you can remain at rest in an inertial frame. The answer about how is different according to general relativity and Newtonian mechanics.

For GR, as long as you are only experiencing gravitation then you remain at rest in an inertial frame. So if you received any net force due to collisions with e.g. photons or interstellar dust, then you would have to compensate for that in order to remain truly at rest. But if you are in a dust-free region not close to any star then you would be fine.

For classical mechanics you would still have to compensate for any collisions as before, but you would also have to compensate for any gravitational acceleration, since gravity is considered a real force and free-falling frames are considered non-inertial. You would be fine in a dust-free region not close to any galaxy.

-Dale

 

[Kumar]

Sorry I asked,

Yes but, can an object remain in motion in the space(inertial) frame? If yes, how?

 

[DaleSpam]

Sorry I asked,

Yes but, can an object remain in motion in the space(inertial) frame? If yes, how?

Sure, anything other than exactly compensating the forces as described above will lead to unbalanced forces and therefore acceleration. That is much easier to arrange.

-Dale

 

[Kumar]

Sure, anything other than exactly compensating the forces as described above will lead to unbalanced forces and therefore acceleration. That is much easier to arrange.

-Dale

What will be that "anything" as you mentioned? How a motion can be possible in absolute zero stiuation?

 

[DaleSpam]

What will be that "anything" as you mentioned? How a motion can be possible in absolute zero stiuation?

The type of motion associated with thermal energy is not bulk motion. They are independent of each other. Theoretically there is no reason why something of any given temperature couldn't be accelerated without increasing it's temperature.

-Dale

 

[Kumar]

The type of motion associated with thermal energy is not bulk motion. They are independent of each other. Theoretically there is no reason why something of any given temperature couldn't be accelerated without increasing it's temperature.

-Dale

What does then mean of "absolute zero" inertial frame, Ground state of energy, absolute rest, space....if motion is there in these absolute states? From where energy comes for motion in space?

Anyway, can you tell differences in motion, tempreture and energy in molecules/atoms on surface and on inner portion of a crystal?

 

[Kumar]

It looks some forces in outer space within solar system are still appliied as heat, cosmic forces etc. It is also not clear that "blank spaces" have some force by itself or not. With these, we can't say this as an "absolute inertial frame" so there may not be an "absolute rest" in outer space.

 

[DaleSpam]

I think we covered this before, remember the train station example:

You could say it that way if you wanted to, but I think that the modern view is a little more interesting. Lets say that the "train" travels through deep space and it passes through a deep-space "train station" without slowing. There are two passengers, one on the train and one in the station waiting for the next train. There are no forces acting on either one. If they close their eyes neither one has any indication of movement. No experiment they can perform can detect their own motion. They can therefore both claim that they are at rest. If they open their eyes they see other objects moving. They can therefore each claim that the other is not at rest. They are both right.

There is no one single prefered "absolute inertial frame" where you would define "absolute rest". All inertial frames can equally be considered a rest frame.

-Dale

 

[Janus58]

Think of clothes drying inside a commercial clothes dryer, the kind with the glass door.
if the observer is standing at the end of the cylinder, or in front of the clothes dryer, he will see the wet clothes thrown to the outside wall of the clothes dryer due to centrifugal force. A ball initially that is initially resting on top of a pile of clothes near the center axis of the dryer will migrate closer to the wall of the dryer due to centrifugal force. The observer standing in front of the dryer will see the ball move in a widening spiral as it rotates with the dryer.

The only reason your observer would see this is because the clothes are dragging the ball along with them and transfering their angular momentum to the ball.

Instead imagine a cylinder rotating in space. Suspended from the axis is a rod from which a ball hangs. The ball is released. To an outside observer the ball will travel in a straight line until it hits the cylinder wall. It isn't effected by a centrifugal force, since for it, no centrifugal force exists. It follows the same path as it would if the cylinder was not there. (after all how would it know of the cylinder's presence until it actually hits the cylinder.)

The ball then hits the cylinder and rebounds (we'll ignore any friction between the ball and cylinder and any "english" that might induce to the bounce air any air resistance).
It then continues in a straight line until it again strikes the cylinder and rebounds again.

Like this animation which starts at the moment the ball is released.

For someone inside the cylinder the ball will fall following a curved path, strike the cylinder rebound, rise back up with a path that mirrors the path of the fall until it reaches a point that is equal to the distance from the axis from which it started, then fall back along a curved path, etc.

like this:

This second animation was made exactly the same way as the first, with the exception that the "camera" rotates with the cylinder.

 

[2inquisitive]

Thanks, Janus58, but what you are illustrating is the Coriolis force. I have seen such animations many times before. It arises because of the different rotational velocities at different points along the radius of the circle. Here is an actual video of a ball being rolled on a rotating Merry Go Round (animation at bottom of page)
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/pw/crls.rxml

The centrifugal force is an inertial force that accelerates an object against the interior wall of a cylinder, both on Earth and in space. Think of a non-rotating cylinder in space with a loose pile of clothes floating near the axis of the cylinder, but not exactly the center. Now, start the cylinder itself rotating. Remember, the clothes are not 'connected' to any part of the cylinder, just floating near the center. Assume there are several thrusters located around the outside circumference of the cylinder to start its rotation without changing the cylinders lateral location, in other words, the cylinder just starts rotating 'in place'. Will the slightly off-center pile accelerate towards the wall of the cylinder, assuming a vacuum inside, or will the pile appear to orbit the cylinder's axis from the point-of-view an observer located against the wall?
I assume if the pile does not move towards either the center of the cylinder, or towards the wall of the cylinder, neither centrifugal force nor centripetal force are 'real' forces acting on the pile. Is that correct? The acceleration felt by the observer against the wall is due to a changing angular momentum from the rotation, correct?

 

[DaleSpam]

Thanks Janus58 and 2inquisitive, those were both excellent graphics.

2inquisitive, the key point in the video clip you linked to is right at the beginning when the narrator says that to a stationary observer (above the merry go round) the path of the ball is straight. Everything about the motion in the rotating frame can be derived from that simple statement about the motion in the non-rotating frame.

Your video clip and Janus58's animations each show both the Coriolis force and the centrifugal force. The announcer was only talking about the Coriolis force on your clip, but both were clearly demonstrated. In the rotating frame the centrifugal force explains the acceleration in the radial direction while the Coriolis force explains the acceleration in the angular direction, but both accelerations are demonstrated in each example.

For your "clothes in a space station" example, this is a degenerate case where the straight line is actually a single point. The space station is simply spinning around and not interacting with it at all. In any case, you simply transform this "straight line" (actually a single point) into the rotating frame. You get a circular orbit around the axis.

-Dale

 

[2inquisitive]

by DaleSpam:

"Your video clip and Janus58's animations each show both the Coriolis force and the centrifugal force. The announcer was only talking about the Coriolis force on your clip, but both were clearly demonstrated. In the rotating frame the centrifugal force explains the acceleration in the radial direction while the Coriolis force explains the acceleration in the angular direction, but both accelerations are demonstrated in each example."
=============================================================

I understand centrifugal force is shown in Janus58's animation BEFORE the ball is released from the rod. After release, the ball simply follows straight-line motion, an inertial motion, no centrifugal force. The motion of the ball after release in the rotating frame is simply Coriolis motion due to differing velocities along the radial plane wrt an observer located at the CENTER of the circle, correct?

In the animation I linked to, the observer (camera) is located on the perimeter of the Merry Go Round in the rotating frame. The ball is accelerated by the guy sitting on the perimeter also, toward the center of the M-G-R. The guy will feel a centrifugal force, but the ball will not in its travels. Did you notice the last sequence where one guy pushed the ball toward the center with the ball curving back to him? Yes, I know that from the non-rotating frame, the ball appear to travel a straight line to meet with the guy as he rotates to the other side of the frame. Now, cover that same ball with paint so that it will trace its path on the surface of the Merry Go Round. The painted path will appear to make an ellipse curving back to near the starting point in the frame where the camera is rotating with the M-G-R. Stop the Merry Go Round and view the painted path from a frame in which both the above camera and the M-G-R are stationary. The ellipital path painted on the surface will then appear as curved to that camera also. So, which frame is the 'illusion' and which recorded the actual path of the ball? I realize physicists prefer to use the stationary observer/rotating M-G-R frame to explain the motions, but is it truely the best frame to use? I know it is much simpler.

My point in the 'pile of clothes, rotating cylinder' example was that NO forces were evident on the clothes, not centrifugal, coriolis OR centripetal. So, why is centripetal force accepted for use, but not centrifugal?

 

[DaleSpam]

Hi GMontag, I hope you will not mind that I quoted you from the Bremstrallung (sp) thread here. I think that your comments have a bearing on the non-inertial frames discussion we have been having here.

Just because a force *can* be a cause of acceleration does not mean that every acceleration must have a force causing it.

I think in this quote that you are refering specifically to accelerations caused by non-inertial frames and/or curved spacetime. I don't know of any other accelerations that can happen without forces.

In this thread we have been discussing non-inertial frames, specifically the rest frame of a rotating space station in deep space (to get rid of any GR spacetime curvature and other gravitational effects). 2inquisitive seems to think that the centrifugal force is a perfectly real force in the rotating frame so he doesn't like the term "ficticious force". I agree with him that it is necessary to explain some very real effects in the rotating frame, so I proposed the term "frame force". BillyT, on the other hand, seems to prefer the term "frame effect" because he doesn't like calling anything a force that doesn't come in an action-reaction pair.

So, what are your thoughts on it? From your comments I think you are inclined to agree with BillyT and not use the word force to describe any effects due to the non-inertial frame. What terminology would you use?

I thought of calling it a frame acceleration, but that falls apart when you are talking about static cases (in the rotating frame) where the centrifugal force balances the centripetal force causing strain but not acceleration in e.g. turbine blades or the earth's equatorial bulge.

-Dale

 

[DaleSpam]

I understand centrifugal force is shown in Janus58's animation BEFORE the ball is released from the rod. After release, the ball simply follows straight-line motion, an inertial motion, no centrifugal force. The motion of the ball after release in the rotating frame is simply Coriolis motion due to differing velocities along the radial plane wrt an observer located at the CENTER of the circle, correct?

Let's analyze the balls motion in each frame separately:

Stationary frame (inertial):
Before the release the ball is traveling in a circular path. The uniform circular motion is a continuous acceleration directed towards the center. This acceleration is caused by the unbalanced centripetal force. No other forces are acting on the ball. In particular, the centrifugal and Coriolis forces are not involved.

After the release the centripetal force is gone. No forces are acting on the ball so it travels in a straight line. In particular, the centrifugal and Coriolis forces are still not involved.

Rotating frame (non-inertial):
Before the release the ball is stationary. The centripetal force is exactly balanced by the centrifugal force. The Coriolis force is proportional to the speed, so it is zero at this point. That said, the centrifugal force and the Coriolis forces are both present and can be used to explain the motion of the ball.

After the release the centripetal force is gone. The centrifugal force continues to act on the ball so it begins to accelerate outward. Since it now has some speed the Coriolis force accelerates the ball counterclockwise also. The centrifugal and Coriolis forces must still be present in order to account for the non-linear motion of the ball.

Summary:
The centrifugal force is proportional to the distance from the axis and the Coriolis force is proportional to the speed so they are equal to zero on the axis and for stationary (in the rotating frame) objects respectively. That said, the centrifugal and Coriolis forces are always present, 100% of the time, for all objects in the rotating frame. That is regardless of wether or not something is touching the object etc.; the forces are a property of the rotating frame itself and must be consistently included at all times to explain the motion of any object in the rotating frame. On the other hand, the centrifugal and Coriolis forces are never present for any objects in the inertial frame. In each frame the motion of the ball is completely and consistently explained, both before and after the release of the ball.

-Dale

 

[MacM]

Bremstrallung (sp)[p/quote]

FYI: Bremsstrahlung

BillyT, on the other hand, .......doesn't like calling anything a force that doesn't come in an action-reaction pair.

Is not the centrifugal force one feels against the wall of the rotating carnival ride and it's counter force (centripetal force - frame stress) a force pair?

 

[DaleSpam]

Is not the centrifugal force one feels against the wall of the rotating carnival ride and it's counter force (centripetal force - frame stress) a force pair?

"Inertial force- (1) The force produced by the reaction of a body to an accelerating force, equal in magnitude and opposite in direction to the accelerating force. An inertial force lasts only as long as the accelerating force does. (2) A force that must be added to the equations of motion when Newton’s laws are used in a rotating or otherwise accelerating frame of reference. It is sometimes described as a fictional force because when the same motion is solved in the frame of the external world, the inertial force does not appear."

Definition 2 (aka frame force, fictional force, pseudo force) is the only one we have been using up to this point. Please don't try to introduce definition 1, since there is already enough confusion on this thread.

You are using the definition 1 that we are deliberately avoiding on this thread. As I told Kumar, please don't try to introduce definition 1 since there is already enough confusion on this thread using only definition 2. I suspect that the confusion resulting from these two definitions is one unfortunate reason that centrifugal forces and rotating frames are never taught in introductory physics classes today.

Btw, even using definition 1 you have it backwards. The force in this pair that "one feels" is the force of the frame pressing on your body. This force is pointed inwards and is the centripetal force. The force in the pair that causes "frame stress" is the force of your body pressing on the frame. This force is pointed outwards and is the centrifugal force.

-Dale