Robittybob1

I often find myself trying to find the balance point on an unstable object and even when you momentarily find it, it then tips to one side or the other. Be it the natural tremors in the ground from movements of machinery or the wind or even a heartbeat. There are these forces that we are not really aware of that will knock the object from the zero point on top of the dome.

If the train's acceleration is along force B the magnitude of B would increase (change) by factor (a=f/m) proportional to the acceleration. With force A pushing at 90 degrees to force B and inline with the motion of the train (accelerating in the direction of B) at some stage the mass will fall off the train at an angle to the direction of motion.

Link won't open very well. There is nothing but some headings in my view. What am I doing wrong? I see you have to click the "First Reading Expand All" button.

OK it matters not how fast the truck goes in that circumstance. I should have said "accelerating surface" rather than just "moving surface", for the surface of turntable is accelerating even when the angular velocity stays the same. I am just trying to get you to declare whether you thought an accelerating surface gives you a different result than what your diagram shows. There is nothing wrong with the physics of your diagrams but it doesn't cover the situation of circular motion.

Does your diagram show a moving surface? With a spinning turntable with a mass held onto it by friction alone. At a certain rotational speed it does slip outward but since the surface further out is rotating at a higher tangential speed the path it takes is the resultant of being slowed by friction in the radial direction but accelerated in the tangential direction which in turn accelerates it in the radial direction further overcoming the slowing due to friction. I am certain of this. Are you saying that is impossible? There is only one resultant direction so it moves in a curve across the turntable surface. With respect to the ground I am uncertain as to its path, but it isn't slipping on the ground, and even Swansont and I thought it might be possible the mass could do a full "circle" during the slip phase. That will be my next level of investigation (the motion WRT the ground).

"Centrifugal force is a fictitious force that is only appropriate to create pseudo equilibrium. It cannot and does not move anything outwards." When the radial component of the dynamic friction is less than the required centripetal force to enable the object to travel in a circle, the object no longer travels in a circle but a spiral. I am trying to see if the results of friction can be explained without using the word "centrifugal force".

Were you going to share the link to the discussion?

With rubber friction is supposed to be different. Commercial - Firestone Tires - Where the rubber meets the road! Drive down memory lane for a minute! There is the motor sport of drifting where what you describe is the purpose and the art.

That could be true if the mass starts off very close to the center of rotation. Thanks.

One aspect of the idea is bothering me. What would happen in the situation of an extremely large disk, would the object be carried around for more than one turn of the spiral once it has dynamic friction?

What I am coming to understand is that the resultant direction the object takes determines the alignment of the dynamic friction force. This dynamic friction force vector could be broken down into the radial and tangential components, but the components are less than the dynamic friction force needed to be overcome to make the object slide in any one component direction. So does that mean any centripetal force being provided by the dynamic friction (the radial component of it) declines as the object takes on a more transverse trajectory? * Does this failing centripetal force contribute to the apparent increase in the radial speed that the object obtains as it reaches the edge? * * * This could be a very important factor that I haven't heard being discussed before.

But that goes in the face of one of the questions at the beginning of the thread in that we had concluded the orthogonal forces combine vectorially and add together, so how would you get the force to always be orthogonal? Push down on a car stuck in the mud isn't doing any useful work, if the others are trying to push it out, OK it works there, but when the object can move freely in the xy plane forces in the same plane at right angles to each other will add to produce a stronger force. I am wondering if you can have a force in the xy plane that is orthogonal to displacement in the xy plane. [i know that is not quite answering what you said, but just a thought.] In the rotating turntable the distance in the radial direction is r(final) - r(initial). The object slides that distance so that amount of work is done in that action of dragging the mass across the surface. In the other direction (tangential) it is the arc length back to where is slips off the rim. Now if someone is paying for work done, the force of friction on the diagonal is the same as the pushing the mass in any individual direction on the surface, yet the combined distances (x and y components added) are less than the resultant! I'm a bit lost with that one.

Well that is my point; if there two friction forces that combine into one force why not just keep on analysing it as if the two forces keep acting separately? What I am needing to do is to understand the maths of positive work being done by friction. Formulas for work done by friction - here please.

Do you have just one frictional force or none at all? I can't seem to snap my thinking away from the two frictional forces at the moment. How many do you have? The table had a list of materials with their coefficients of friction and I can't recall the exact figure (either 0.1 or 0.2) but there was no difference between the static and the dynamic friction coefficient of glass on teflon, I was surprised but I suppose that can happen.

You would need to take pressure into account as well.

Plus we've got two frictional forces one slowing it down in the radial direction, and the other speeding the object up in the tangential direction. I took the table's word it, that on this type of surface the static friction and dynamic friction are the same. It can only accelerate at the larger radius if the force of friction acts on it with a component in the tangential direction. When F = mr0w02 exceeds the force of friction then it moves, then what? I've got that formulated for a 1 kg mass with a mu of 0.2 on a radius of 0.5 m. Now increase the omega till it slides (I was working with tangential velocity actually) and that happens at 0.991 m/sec. If there were no more friction forces after that it would take a curved path described by the Coriolis Effect, but there is the thought of how much did the tangential velocity exceed the velocity necessary to get it to slip. That thought was countered by the thought it is not possible to exceed this by varying amounts for the object will just slip when the forces tip the balance. Does that make sense? Which is the right thought? In the Centrifugal force experiment it felt like you could affect the rate it slips off the turntable, but this was probably due to adding energy by increasing the angular velocity after it begun slipping.

Introduction to coefficient of friction: mu = force of friction over the normal force

Not completely. What will go wrong? Increasing from zero is a very good answer, I can't argue against that. Which coordinate system do I have to use? 1. make the coordinates point to marks on the perimeter of the turntable. (noninertial rotating frame). 2.make the coordinates point to due North. (the ground frame or lab frame.) 3 make the coordinates point to the sliding mass. All of these would have one axis in common.

So that mass added from this energy adds to the gravitational and inertial mass too does it? If it does that is interesting.

So far we seem to be understanding one another. I am still having trouble with the sentence "the coordinate system is not rotating". For in my view the coordinates are rotating the same rate as the system (the turntable) or I might even have to rotate the coordinates the same speed as the object in an attempt to get the maths right. For aren't we allowing the coordinate system to go backward wrt the turntable so we can look at the radial component? When you say "the coordinate system is not rotating" in reality if we pointed the "y" coordinate toward magnetic North is "y" always pointing to the North even when we spin the turntable? The opposite of this would be to make the coordinates point to marks on the perimeter of the turntable. If the object is placed on the turntable while it is stationary the straight line drawn from the centre through the object to the perimeter becomes our "y" axis (this is the starting radial line) but not all radial lines are going to be parallel so why not call "y" the line that always exists from the centre through the COM of the object to the perimeter? OK we are close enough to start working on some figures and formulas. Did you want to hazard a prediction regarding your statement "the radial component of the speed changes"? Will that be an increase in speed or a decrease? Personally I can't imagine it decreasing unless we were to slow the turntable., but that is against the experimental design for we hope to keep the rotation of the turntable constant once sliding movement occurs (well we will in the analysis for sure. In real life all one can do is stop applying any further force to the perimeter, so in reality there is in fact a little slowing for the whole turntable has a little bit of friction from wind resistance and friction within the central bearing. One more assumption will be that the speed of rotation of the turntable is not significantly slowed by the work done by friction on the object.

Is that a generally known fact or is it a subject of recent research? Who did this experiment?

Your second sentence "The radial slipping should increase", now is that the same as saying "the rate of radial slipping should increase"? Does the rate change? If you agree it is spiraling toward the outer edge, I agree with you for I have no problem saying it is slipping in and travelling in a spiral manner. The object is sitting on a surface which is rotating in a normal circular motion fashion and this is the surface which is providing the friction forces to the sliding object. It is the direction wrt the turntable that I describe as tangential or radial so if it moves in a slight diagonal (small increment of spiral) that diagonal surely could be thought of as partly radial and mostly tangential. If it moves in a strong spiral where it goes from the center region to the outside in less than a half rotation are you saying we can't look at its instantaneous motion and break that down wrt the surface? Even you seem to say this "The motion is not radial. It's spiraling, so it has both tangential and radial motion", so if there is both tangential and radial motion surely we can measure those component speeds. I could tie a tablecloth to a motorbike and get it to travel at a constant (but variable) speed and whip it out from under a mass. We could measure the distance the friction moved the object forward easy enough. I could graph the amount of forward movement against speed. I'm predicting the slower speeds should drag the object the longer distances.

I was asking about increased sliding in the radial direction. After tangential acceleration it speeds up so what direction do the combined frictional forces make the object leave at? It leaves at a higher rate but not at tangential speed for it is slipping. Because it isn't going at tangential speed it can't leave via the tangent (of the whole apparatus) but in the no friction hypothetical situation it could be likened to leaving at a tangent to the smaller circle, but then when you think about it how did you get the object travelling in a circle in the first place if there was no friction? You could start it off on the very edge of the turntable, then when it moved there would be no friction and no surface to accelerate on either. We can't have absolutely no friction or start it off on the very edge. They are not examples that show what friction does in this case. I went out to dinner and experimented with pulling the tablecloth. I did it in a small way with my glass of beer on a doily. There is no easy way to estimate the force used to pull the cloth or the rate it moves, so it too is a complex problem to put the maths around it. The circular motion is so much easier for we will keep the rotational rate constant once the object moves, so it doesn't matter what this speed is. The glass keeps on accelerating till the doily was pulled right from under it, then it slides a little further on the tabletop slowing down. That is wrong.

Did you draw the diagrams? (There is no indication of source.) It would be not much more difficult just to have the barycenter as the point orbiting the Sun and the both the Moon and the Earth doing a wobbling curve around that, criss-crossing each other during a lunar month.

Obviously you don't want coarse fabric (as there would be too much friction) and probably not a plastic table cloth either (for it might crinkle forming a barrier). Is it true that friction is not proportional to velocity, so if it moves slowly the force of friction will be acting for a longer time? Good point I think we will have to take time into account in the rotating turntable concept as well?