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| electrodacus:
--- Quote from: cbutlera on February 06, 2023, 07:29:21 pm ---So what is it that you think that people don’t understand about Newton’s third law? The paper describes the failure of many students to correctly identify Newton’s third law force pairs. Do you agree with this, or have you identified some other common misunderstanding? --- End quote --- I think my conclusion is the same as that of the paper but is valid for those that graduated from university. Even in the paper it seems there is a lot of emphasis on gravity but Newton's 3'rd law has nothing to do with gravity and that is just one special case where it also applies. In my 3 examples gravity is not involved as forces are horizontal. I was sure that example B is so simple that nobody can get that wrong and once again I was surprised. Even those very few that understand how B works still get confused by A and think that is different especially after they see real world examples where that device moves to the right. I will assume you claim to understand Newton's third law. Can you explain why this moves to the right (is the setup A but in real world test where more things than defined at A are in play). https://odysee.com/@dacustemp:8/wheel-cart-energy-storage-slow:8 |
| cbutlera:
--- Quote from: electrodacus on February 06, 2023, 07:44:50 pm ---... I think my conclusion is the same as that of the paper but is valid for those that graduated from university. ... I will assume you claim to understand Newton's third law. Can you explain why this moves to the right (is the setup A but in real world test where more things than defined at A are in play). ... --- End quote --- I’m glad to hear that you agree with the findings in the paper. The paper addresses the class of problem shown in your examples a) and b) with it’s discussion on objects in static equilibrium under the influence of just two forces. Here is the relevant paragraph from the bottom of page 5 of the paper. “Consider an object in static equilibrium, under the influence of just two forces. In the Newton’s Second Law – Net Force” (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 onnovice students; but the mental model which arises from the flawed sequence of reasoning is strong and resistant to instruction (Wilson & Low, 2015).” |
| electrodacus:
--- Quote from: cbutlera on February 06, 2023, 08:07:12 pm ---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 onnovice students; but the mental model which arises from the flawed sequence of reasoning is strong and resistant to instruction (Wilson & Low, 2015).[/i]” --- End quote --- Can you please elaborate on this ? In my diagrams (A and B) F1 can not exist without the equal and opposite F2. We are discussing non accelerating reference frames. The way both mechanism A and B are set up makes them no different from a single solid object that is compressed by the second object earth. So yes you can say F1 has a say F1' equal and opposite force provided by the treadmill with is reference do ground and then F2 has an equal and opposite F2' pusing on the red box that is also referenced to ground. But F1' and F2' can be eliminated as the interaction can be simplified as the interaction of just two objects the gearbox(A) or cylinder (B) and ground/earth. You can simplify everything as a hot cubic box with the top surface missing where you introduce a solid object like a cylinder almost touching the two opposite sides inside the cube and you wait until the cubes cools down a bit so cylinder is stuck with forces F1 and F2 pusing on the opposite ends of the cylinder. You remove your hand and all that is left are two solid objects cylinder inside the cube and cube as it cools applying an increasing force on the cylinder with F1 always equal and opposite to F2. Are this F1=F2 Newton's 3'rd law forces or something else. Keep in mind that the cube is a single object and you can not call one face of the cube one object and the other face another object. |
| cbutlera:
--- Quote from: electrodacus on February 06, 2023, 09:00:38 pm --- --- Quote from: cbutlera on February 06, 2023, 08:07:12 pm ---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 onnovice students; but the mental model which arises from the flawed sequence of reasoning is strong and resistant to instruction (Wilson & Low, 2015).[/i]” --- End quote --- Can you please elaborate on this ? ... --- End quote --- The point that they are making on page 5 of the paper is that the two forces acting on the object in the N2-NF misconception are equal and opposite when the object is in static equilibrium, but they are not Newton’s third law force pairs, and so do not have to be equal and opposite in every circumstance. Under another circumstance, such as when the object is accelerating, the two forces can and will be different (not equal and opposite). In that case, the vector sum of the two forces would be equal to the rate of change of momentum of the object. So if the the object is accelerating then the two forces must be different (not equal and opposite), or equivalently, if the two forces are different (not equal and opposite), then the object must be accelerating. Edit: added some equal and opposites for clarity. |
| electrodacus:
--- Quote from: cbutlera on February 06, 2023, 09:25:55 pm --- The point that they are making on page 5 of the paper is that the two forces acting on the object in the N2-NF misconception are equal when the object is in static equilibrium, but they are not Newton’s third law force pairs, and so do not have to be equal in every circumstance. Under another circumstance, such as when the object is accelerating, the two forces can and will be different. In that case, the vector sum of the two forces would be equal to the rate of change of momentum of the object. So if the the object is accelerating then the two forces must be different, or equivalently, if the two forces are different, then the object must be accelerating. --- End quote --- Fair point but in my examples as they are presented acceleration is not possible unless slip is allowed to occur and even then for case A if slip happens at right wheel (input wheel) then object of interest will still not move to the surface of the treadmill will be allowed to move. And in case output wheel (left one) is allowed to slip then vehicle will accelerate in the same direction as F1 (to the left) so for some small amount of time F1 will be larger if the object has mass as that will be converted to kinetic energy but once the speed of the readmill is equal with the speed of the object the forces will again be opposite and equal. None of the above explain what happens in the video I showed where vehicle moves in the opposite direction of the applied force so to the right. People just assume that F2 can be larger than F1 based on the experimental result not understanding that you can not have force amplification with just two points of contact. What actually happens should be easily spotted in the slow motion video https://odysee.com/@dacustemp:8/wheel-cart-energy-storage-slow:8 It is a combination of energy storage and stick slip hysteresis as the trigger for charge and discharge and not force amplification which is impossible in this case. I don't even care if somebody ever heard of Newton. As long as they can understand why this vehicle moves the way it moves in the video and it is not due to force amplification or force multiplication witch is impossible in all 3 cases I used as examples. Both case A and B can be modified so that F2 = 2 * F1 or whatever gear ratio is by connecting the now floating vehicle body to ground the blue part in case A and the cylinder body in case B. Then there are 3 connection points and F1 will no longer be relative to F2 (equal and opposite) but relative to the vehicle body and then F2 that can be multiple times larger will also be relative to the third point vehicle body and not relative to F1. Forces can not exist in isolation they need to be relative to something and in my 3 examples as they are F1 is relative to F2 and can not exist independent of each other in an non accelerating reference frame. |
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