General > General Technical Chat
Force multiplier
electrodacus:
--- Quote from: karpouzi9 on February 08, 2023, 03:53:58 am ---This post is a great example of why physics questions that are poorly posed are almost unanswerable :-+
Torque isn't like any other force; it's an emergent property of flexible materials. You can't apply torque to objects with zero stiffness, and no solid objects exist with infinite stiffness that you can physically apply a torque to: https://scholarship.haverford.edu/cgi/viewcontent.cgi?article=1494&context=physics_facpubs
Towards the end of the video you posted, it's pretty clear that you've forced the smaller wheels of your roadster to spin (by very quickly applying force on the paper) before the rest of the system can react and stabilize -- before the front wheels can feel enough force from the rubber band to overcome their stiction and enough force to noticeably modify their rotational inertia.
The rubber band has elasticity; you can stretch it before it can react the same way that the surface of a musical drum has elasticity. If the drum wasn't elastic, your drumstick would shatter the second it touched it. In other words, there's a phase difference between forces applied at one part of an elastic object and forces/movement resultant on the other parts. All movement is a wave of some kind, after all.
Try repeating your experiment with a steel or chain belt and try to show us the same response. The front wheel will slip almost immediately because the steel is much less elastic than the rubber band.
--- End quote ---
This sort of vehicle can only move in the opposite direction of the forced applied due to a combination of energy storage and stick slip hysteresis as the trigger for the charge/discharge cycles.
What happens exactly can be seen fairly clearly in this slow motion video https://odysee.com/@dacustemp:8/wheel-cart-energy-storage-slow:8
You can see it starts with an increasing F2 = F1 as the belt is stretched (elastic energy storage).
When F2 = F1 is large enough that the input wheel can slip (it is easier to slip than output wheel as it is already in motion). And the moment wheel starts to slip the force needed to slip is much lower that the force needed to unstick the wheel in the first place so this allows the potential elastic energy to be converted in to vehicle kinetic energy.
Since there is friction the vehicle will slow down so to maintain an average speed this cycle of charge discharge needs to happen multiple times per second and much faster than our slow brain can see.
What I did in this other video you likely seen (first 15 seconds) https://odysee.com/@dacustemp:8/stick-slip-removed-from-front-wheels:0 is reduce grip at output wheels suficient so that they will be the first to slip instead of the input wheels and in this case at initial acceleration the belt is still streched as F2=F1 increase but they will not be able to increase that much before the output wheels slip and the stored enenrgy is not discharged the belt remains at that constant streached level while speed remains constnat with the vehicle being draged as the mechanism as it is is just a locked gearbox.
So eliminating the stick slip hysteresis from the input wheel removes the ability of the vehicle to take advantage of the stored energy thus it can no longer charge and discharge that to move in the opposite direction of the applied force.
If I were to replace the soft belt with a stiffer belt then displacement for same force will be lower meaning less energy will be stored before input wheel slips and that means the cycles of charge discharge will need to be faster (more cycles per second for same constant speed).
If the belt is to stiff then amount of energy may be to low and not be able to cover the frictional loss and to have extra to convert to kinetic energy so vehicle will no longer be able to move in the opposite direction so it will be dragged or maybe stay in place while the paper is dragged under the input wheels.
jonovid:
--- Quote from: electrodacus on February 08, 2023, 04:09:42 am ---
--- Quote from: karpouzi9 on February 08, 2023, 03:53:58 am ---This post is a great example of why physics questions that are poorly posed are almost unanswerable :-+
Torque isn't like any other force; it's an emergent property of flexible materials. You can't apply torque to objects with zero stiffness, and no solid objects exist with infinite stiffness that you can physically apply a torque to: https://scholarship.haverford.edu/cgi/viewcontent.cgi?article=1494&context=physics_facpubs
Towards the end of the video you posted, it's pretty clear that you've forced the smaller wheels of your roadster to spin (by very quickly applying force on the paper) before the rest of the system can react and stabilize -- before the front wheels can feel enough force from the rubber band to overcome their stiction and enough force to noticeably modify their rotational inertia.
The rubber band has elasticity; you can stretch it before it can react the same way that the surface of a musical drum has elasticity. If the drum wasn't elastic, your drumstick would shatter the second it touched it. In other words, there's a phase difference between forces applied at one part of an elastic object and forces/movement resultant on the other parts. All movement is a wave of some kind, after all.
Try repeating your experiment with a steel or chain belt and try to show us the same response. The front wheel will slip almost immediately because the steel is much less elastic than the rubber band.
--- End quote ---
This sort of vehicle can only move in the opposite direction of the forced applied due to a combination of energy storage and stick slip hysteresis as the trigger for the charge/discharge cycles.
What happens exactly can be seen fairly clearly in this slow motion video https://odysee.com/@dacustemp:8/wheel-cart-energy-storage-slow:8
You can see it starts with an increasing F2 = F1 as the belt is stretched (elastic energy storage).
When F2 = F1 is large enough that the input wheel can slip (it is easier to slip than output wheel as it is already in motion). And the moment wheel starts to slip the force needed to slip is much lower that the force needed to unstick the wheel in the first place so this allows the potential elastic energy to be converted in to vehicle kinetic energy.
Since there is friction the vehicle will slow down so to maintain an average speed this cycle of charge discharge needs to happen multiple times per second and much faster than our slow brain can see.
What I did in this other video you likely seen (first 15 seconds) https://odysee.com/@dacustemp:8/stick-slip-removed-from-front-wheels:0 is reduce grip at output wheels suficient so that they will be the first to slip instead of the input wheels and in this case at initial acceleration the belt is still streched as F2=F1 increase but they will not be able to increase that much before the output wheels slip and the stored enenrgy is not discharged the belt remains at that constant streached level while speed remains constnat with the vehicle being draged as the mechanism as it is is just a locked gearbox.
So eliminating the stick slip hysteresis from the input wheel removes the ability of the vehicle to take advantage of the stored energy thus it can no longer charge and discharge that to move in the opposite direction of the applied force.
If I were to replace the soft belt with a stiffer belt then displacement for same force will be lower meaning less energy will be stored before input wheel slips and that means the cycles of charge discharge will need to be faster (more cycles per second for same constant speed).
If the belt is to stiff then amount of energy may be to low and not be able to cover the frictional loss and to have extra to convert to kinetic energy so vehicle will no longer be able to move in the opposite direction so it will be dragged or maybe stay in place while the paper is dragged under the input wheels.
--- End quote ---
to impart oscillation in to the loop, a mechanical tank circuit oscillator. but still in need of continuous input
PlainName:
--- Quote from: electrodacus on February 07, 2023, 11:55:05 pm ---So now the treadmill is unpowered free to move and you push the vehicle body to the right. Of course since you apply a force to the right to the vehicle body the vehicle body will move in that direction.
--- End quote ---
OK, well that's a start!
--- Quote ---But it has basically nothing to do with the original problem where the only applied force is to the left (not to the right) and it is applied at the input wheel.
--- End quote ---
Do you want to learn that it does indeed have something to do with the original problem, or must you hide under your safety blanket?
Now, imagine the right wheel is not connected with a belt (and, sorry, I used 'horizontal belt' previously when I should has used 'treadmill' to save confusion). The wheel is locked rigid. You, the HoG or whatever push the vehicle to the right and that causes the treadmill to move to the right. I presume you are perfectly OK with that since it's pretty basic. So..
Let's say the vehicle moves to the right (via whatever strange force) at 2m/s. With a locked right wheel the treadmill will move to the right at 2m/s. Yes? Good. Clearly, any clockwise rotation of the right wheel will reduce the speed of the treadmill. We can thus have three distinct situations:
1. The connection between the wheels is 1:1 (that is, the left wheel drives the right wheel at the same speed). In this case, the vehicle moves right at 2m/s, the right wheel rotates at the same speed the vehicle is travelling and the treadmill is stationary.
2. The connection is 2:1 (that is, the left wheel drives the right wheel at half the speed). In this case the vehicle moves right at 2m/s and the treadmill moves right at 1m/s (since the right wheel is rotating slower than is necessary to keep up with the vehicle speed).
3. The connection is 1:2 (that is, the left wheel drives the right wheel at twice the speed). In this case the vehicle moves right at 2m/s and the treadmill moves left a 1m/s (since the right wheel is rotating faster than is necessary to keep pace with the vehicle speed).
Do you agree with those? If not, which of 1..3 do you think is wrong? I can assure you that this will resolve your original problem if you stick with it.
cbutlera:
--- Quote from: electrodacus on February 07, 2023, 10:01:47 pm ---...
Did you miss the Newton's 3'rd law ? It is present in all non accelerating frames and of course when whe are in an accelerating frame the 2'nd law also applies.
...
--- End quote ---
From your responses to my own postings and those of a number of others, it seems that you do understand the arguments that we are making. However, in every case you throw a spanner in the works by bringing up your version of Newton’s third law.
Try a very simple experiment. Extend the forefingers of you left and right hands, then press the tips of those two fingers together. You will notice that the forces you can feel at your two fingertips are equal and opposite and there is no way of performing this experiment where that would not be the case.
You may regard this experiment as pointless, and that it just demonstrates a law of physics which is intuitive and completely obvious. So obvious in fact that there would be no point in giving it a name. But it does have a name, it is Newton’s third law of motion. A key characteristic of this law is that the two forces involved always act on different objects, in this case the tips of your left and right forefingers.
The thing that you have been calling “Newton’s third law” is in fact the ”Newton’s Second Law – Net Force” (N2-NF) misconception that I have mentioned before, and is discussed in detail in the Teaching Science paper. I have quoted the key paragraphs from page 5 below, because I think that it is so relevant to this discussion. A key characteristic of the N2-NF misconception is that the two forces involved act on the same object. In your example a), that object is the wheeled trolley. You always insist that it has a "locked gearbox" so that you can justify treating the trolley chassis along with its transmission and wheels as a single object, and thereby apply N2-NF. Hence your need to introduce this "stick slip" behaviour, because clearly (to you) the transmission and wheels need to move at some point if the trolley is to move at all.
Quote from page 5 of the Teaching Science paper:
“In fact, there is evidence that many students construct a flawed mental model of the Third Law by conflating it with the Second Law (Dedic, Rosenfield & Lasry, 2010; Wilson & Low, 2015). Unlike some misconceptions in physics, such as the Aristotelian idea that objects move until they run out of impetus, this is not a common sense alternative conception based on every day experience. Rather, it is something that is learned during the study of Newton’s laws. We call this the "Newton’s Second Law – Net Force” (N2-NF) misconception.
Consider an object in static equilibrium, under the influence of just two forces. In the N2-NF misconception, students note that the two forces must sum to zero (a correct application of the Second Law) and hence the two forces must be equal in magnitude and opposite in direction (also correct, as a direct mathematical result of the first statement), and thus are a Third Law force-pair (incorrect). This reasoning is similar to the (il)logical sequence, “All cats have four legs; my dog has four legs; therefore, my dog is a cat”. While a Third Law force-pair are equal in magnitude and opposite in direction, not all forces which are equal in magnitude and opposite in direction are a Third Law pair. This subtlety is often lost on novice students; but the mental model which arises from the flawed sequence of reasoning is strong and resistant to instruction (Wilson & Low, 2015)."
jonovid:
I remember this is somewhat similar to the infinity drive or torque amplifier conundrum. a quest for a better continuously variable transmission.
a mechanical system of interconnected differentials in a mechanical circuit were torque and velocity are controlled by a third input
an order of magnitude less then the power throughput.
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