General > General Technical Chat
Newton's third law problem.
electrodacus:
--- Quote from: IanB on November 20, 2022, 11:42:10 pm ---
Is the orange line a rope? And are the yellow circles hinges? And if so, how can it be locked with so many movable elements present?
--- End quote ---
Yes and yes.
There are movable elements but treadmill can apply F1 to the left so the mechanism as it is, is no different from a solid rock.
And I could add the other rope below the green beam but it makes no sense for the forces in this direction as that plays no role here.
IanB:
--- Quote from: electrodacus on November 20, 2022, 11:46:51 pm ---Fortunately I can imagine what happens if you have the left belt stationary.
--- End quote ---
No need to imagine, we know that the cart moves to the left.
--- Quote ---I guess you need to do the math to know that.
--- End quote ---
Yes, it is the math that gives us that result.
--- Quote from: electrodacus on November 20, 2022, 11:58:34 pm ---There are movable elements but treadmill can apply F1 to the left so the mechanism as it is, is no different from a solid rock.
And I could add the other rope below the green beam but it makes no sense for the forces in this direction as that plays no role here.
--- End quote ---
Why are you bringing forces into the analysis? This is just a misdirection to send the discussion in the wrong direction.
This is a simple problem of mechanics. The only important variables are the (x, y) coordinates of the pivot points in the mechanism and the connections between them. If you write down the linking equations you will see that the mechanism is not at all fixed and is quite free to move.
electrodacus:
--- Quote from: IanB on November 21, 2022, 01:51:10 am ---No need to imagine, we know that the cart moves to the left.
Yes, it is the math that gives us that result.
--- End quote ---
So you have the fixed ground on the left and the treadmill that moves to the right 3 squares.
How many squares does the vehicle move to the left ?
I think you need to make that animation to see the problem.
--- Quote from: IanB on November 21, 2022, 01:51:10 am ---Why are you bringing forces into the analysis? This is just a misdirection to send the discussion in the wrong direction.
This is a simple problem of mechanics. The only important variables are the (x, y) coordinates of the pivot points in the mechanism and the connections between them. If you write down the linking equations you will see that the mechanism is not at all fixed and is quite free to move.
--- End quote ---
So asking to prove by showing how F2 can be larger than F1 is a misdirection ?
"free to move" how ? the only two places you can interact with that are at F1 and F2
Since you can not have F1 without F2 and F2 is opposite and equal it means it can not move.
Just build that mechanism have the leg with F1 in one in right hand and the leg with F2 in the left hand.
Keep the left hand stationary and push with the right hand towards the left hand and see if anything moves.
Whatever force F1 you apply with the right hand will be equal and opposite to the force you will feel against the left hand.
IanB:
--- Quote from: electrodacus on November 21, 2022, 02:07:36 am ---So you have the fixed ground on the left and the treadmill that moves to the right 3 squares.
How many squares does the vehicle move to the left ?
I think you need to make that animation to see the problem.
--- End quote ---
The animation is already made, it is posted above.
With the fixed ground on the left the treadmill moves to the right one square. When this happens the vehicle moves two squares to the left.
--- Quote ---Just build that mechanism have the leg with F1 in one in right hand and the leg with F2 in the left hand.
Keep the left hand stationary and push with the right hand towards the left hand and see if anything moves.
Whatever force F1 you apply with the right hand will be equal and opposite to the force you will feel against the left hand.
--- End quote ---
You have the question, you build it. This one is simple to make, you can construct it out of cardboard, paper fasteners, sticky tape and string.
electrodacus:
--- Quote from: IanB on November 21, 2022, 02:19:31 am ---
The animation is already made, it is posted above.
With the fixed ground on the left the treadmill moves to the right one square. When this happens the vehicle moves two squares to the left.
--- End quote ---
I guess you can not see the problem as you concentrate only on the math with zero connection to physics.
It is OK for you that an invisible hand pushes the vehicle and is not actually powered by the treadmill.
Also do you observe that in reality ? The vehicle with this gear ratio moves at 2x the speed relative to ground vs the treadmill 1x relative to ground ?
--- Quote from: IanB on November 21, 2022, 02:19:31 am ---You have the question, you build it. This one is simple to make, you can construct it out of cardboard, paper fasteners, sticky tape and string.
--- End quote ---
Why will I need to build that ? I already had the vehicle which is the same thing.
You just seems to not understand what the problem is.
F1 is the only force acting on the vehicle there is no other magical force or energy source.
The direction of F1 is towards the left in my diagram so the vehicle no matter what it will move to the left if at all without involving energy storage.
I drawn all forces in that mechanism 6 of them in total each group of two equal according to Newton's 3'rd law.
F1=F2 F3=F4 and F5=F6
You need to show how F1 different from F2 if you want to claim the vehicle can move.
And I already showed how F2 becomes larger for short bursts due to energy storage released by slip at the front wheel.
There is no alternative explanation and I showed in the video that is exactly what happens. What more can I do ?
Is the same reason a vehicle driving at 30m/s with no wind experiences the same drag as if it drives at 10m/s in a 20m/s head wind again something that to many people get wrong.
As far as I can see is from not understanding that every action has an equal and opposite reaction. I can only imagine what Newton had to deal with.
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