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Thread: Tension

  1. #1 Tension 
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    Hello everybody. Sorry if my question seems too stupid, but I am new to Physics so please tolerate with me.

    Well recently a problem involving tension has been troubling me. Well there's a table with two pulleys at each side of the table. There are two strings, both connected to a 25 kg weight, with the other end connected to a spring balance (spring scale). When I hung the two weights with strings on the pulley, one on each side, with the spring balance on top of the table, what will be the reading of the spring balance? Please explain, as all I think of is 500 N, although the correct answer is 250 N.

    I understand that this question was posted up in one of the topics but I am kind of confused as the problem is slightly different.

    So am I right in thinking that the problem is actually referring to 'what is the tension of a string when both side is exerted a force of 250 N? So please help me in answering:

    1) Answer of the question above, and explanation;
    2) What is the definition of tension, and;
    3) How a spring balance works(Is the reading on the spring balance same as the tension of string?);
    4) If the two questions are not the same, what's the difference?

    Thanks a lot.


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  3. #2 Tension 
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    The system you have described is in equilibrium because you have it set up in such a way that they oppose each other. Think about it this way, if you hang 25kg off the roof, the system is in equilibrium (not moving) and the roof must supply a reactional force of 250N to sustain the equilibrium. So you have two forces of equal magnitude opposing each other, resulting in a tension of 250N. In your system, the forces act against each other to keep it in equilibrium, just like hanging the weight from the roof and 250N will be the tension in the rope. I hope that helped.

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  4. #3  
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    You are correct in thinking that the spring scale is measuring the tension in the string. The scale reads 250 N (taking the acceration due to gravity as approximately 10 meters per second per second).

    The explanation is simply that the tension in the string has to be 250 N to counterbalance the force of the 25 kilogram weight.

    Suppose there were only one pulley and one weight, with the string anchored to a peg in the middle of the table. I assume you would have no problem agreeing that the tension is 250N. The peg is providing the counter force of 250N. Add another 25 kg weight and pulley on the opposite side of the table and anchor it to the same peg. Nothing has changed. The scale still reads 250. The force on the peg is counterbalanced by the weights on each side. You can pull the peg out of its hole in the table and nothing moves.
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  5. #4  
    Forum Bachelors Degree Demen Tolden's Avatar
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    Here are some helpful visuals in case anyone would like to a nice picture to show what is going on:

    The most important thing I have learned about the internet is that it needs lot more kindness and patience.
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  6. #5  
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    Hi all again.

    Thanks a lot for your help, I suddenly realized that after a night of thought.

    However, is there any definition for tension? Because I sometimes found it confusing to explain things with tension without its definition.

    Thanks a lot again. :-D
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    Quote Originally Posted by dbs.lin
    However, is there any definition for tension? Because I sometimes found it confusing to explain things with tension without its definition.
    You could just do a google search and find the dictionary definition, but tension is the force on a structural element that tends to stretch it. The opposite of tension is compression. In a structure, some rigid elements may be under either tension or compression. A string or rope can only have tension, because it simply collapses when a compressive force is applied.
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  8. #7 Re: Tension 
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    Quote Originally Posted by dbs.lin
    Hello everybody. Sorry if my question seems too stupid, but I am new to Physics so please tolerate with me.

    Well recently a problem involving tension has been troubling me. Well there's a table with two pulleys at each side of the table. There are two strings, both connected to a 25 kg weight, with the other end connected to a spring balance (spring scale). When I hung the two weights with strings on the pulley, one on each side, with the spring balance on top of the table, what will be the reading of the spring balance? Please explain, as all I think of is 500 N, although the correct answer is 250 N.

    I understand that this question was posted up in one of the topics but I am kind of confused as the problem is slightly different.

    So am I right in thinking that the problem is actually referring to 'what is the tension of a string when both side is exerted a force of 250 N? So please help me in answering:

    1) Answer of the question above, and explanation;
    2) What is the definition of tension, and;
    3) How a spring balance works(Is the reading on the spring balance same as the tension of string?);
    4) If the two questions are not the same, what's the difference?

    Thanks a lot.
    The spring scale is only calibrated to calculate one of the loads. One of the forces. It is not calibrated to take into consideration what is on the other side of the spring scale. Either supporting it from below or above.


    I will make an experiment to show this.

    Sincerely,


    William McCormick
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  9. #8  
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    A scale is designed to measure the pull or push from an object hung or placed upon it. It is not designed to measure the tension created internally upon or away from the scale.

    The scale measures half the tension the object placed upon the scale or hung from the scale actually creates.



    Here you can see that certainly the two 25 pound weights are lifting or creating 50 pounds of lift. Meaning that there must be 50 pounds of tension from two 25 pound weights.

    This image is a complication to the simplicity however it might help you to see what I am saying.


    We ran through this in the sixties, that is why I never wanted any of my friends to go to college.

    Here is the same thing without the 50 pound weight. Now the rope must support that 50 pounds along every inch of the rope.





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    William McCormick
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    Quote Originally Posted by dbs.lin
    Hi all again.

    Thanks a lot for your help, I suddenly realized that after a night of thought.

    However, is there any definition for tension? Because I sometimes found it confusing to explain things with tension without its definition.

    Thanks a lot again. :-D
    Your tension is always twice your weight.

    That means when you go to lift with a crane, 25 pounds. You actually divert, 25 pounds of weight from the crane itself out to the end of the boom. So now the boom experiences an extra 25 pounds besides the load, the crane body looses 25 pounds of weight. So that a 25 pound weight causes much more then 50 pounds of pressure/tension/force upon a cranes balance.

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    William McCormick
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  11. #10  
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    Quote Originally Posted by William McCormick



    Here you can see that certainly the two 25 pound weights are lifting or creating 50 pounds of lift. Meaning that there must be 50 pounds of tension from two 25 pound weights.
    William, if you and your friend each picked up one end of the 50 lb weight, how much weight do you think you would each be lifting?
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by William McCormick



    Here you can see that certainly the two 25 pound weights are lifting or creating 50 pounds of lift. Meaning that there must be 50 pounds of tension from two 25 pound weights.
    William, if you and your friend each picked up one end of the 50 lb weight, how much weight do you think you would each be lifting?

    We would each be lifting 25 pounds naturally.

    However, those two outer 25 pound weights in that one drawing, are lifting a fifty pound weight. And there is no dispute by you to that affect.

    That means that when I take that fifty pound weight away, and reconnect the rope between the two 25 pound weights, the rope will have 50 pounds trying to tear it apart. As it previously demonstrated it could lift the 50 pound weight before I removed the 50 pound weight and reconnected the rope.

    Here is the after effect after I take away the 50 pound weight, and reconnect the rope, not changing anything else.



    In other words the two ropes holding up the 50 pound weight, separately when recombined, reconnected. The 50 pound load they were hoisting, taken away. The ropes still have that same fifty pounds of pull. The rope along every inch has to support 50 pounds.

    I have seen guys go gaga over this. They cannot believe it. And it has to do with the number of years you are subjected to thinking about it a certain way. Younger kids tend to pick up on it quicker. I am often reminded by younger kids of things like this.

    The individual when it does happen is always honestly perplexed by it. But it can be funny. I had a few like this over the years myself. I just could not get them. They would slip like ell's right through my hands. Ha-ha. I had great friends, parents and teachers that always got me to see it.

    Sometimes I would wake up at night sweating about it. It was like a conspiracy or something. Ha-ha.




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    William McCormick
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  13. #12  
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    One problem William, tension isn't measured in the same units as weight. It's measured in the same units as acceleration.
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    Quote Originally Posted by MagiMaster
    One problem William, tension isn't measured in the same units as weight. It's measured in the same units as acceleration.

    Tension is tension. It is pounds of force. Like gravity places pounds of force upon objects. And positively accelerates them.


    ten·sion

    ten·sion (ten?sh?n) noun

    1. a. The act or process of stretching something tight. b. The condition of so being stretched; tautness.
    2. a. A force tending to stretch or elongate something. b. A measure of such a force: a tension on the cable of 50 pounds.
    3. The interplay of conflicting elements in a piece of literature, especially a poem.
    4. a. Mental, emotional, or nervous strain: working under great tension to make a deadline. b. Barely controlled hostility or a strained relationship between people or groups: the dangerous tension between opposing military powers. c. Uneasy suspense: a comic scene that relieved the tension of the drama.
    5. A balanced relation between strongly opposing elements: “the continuing, and essential, tension between two of the three branches of government, judicial and legislative” (Haynes Johnson).
    6. A device for regulating tautness, especially a device that controls the tautness of thread on a sewing machine or loom.
    7. Electricity. Voltage or potential; electromotive force.

    Excerpted from The American Heritage Dictionary of the English Language, Third Edition Copyright © 1992 by Houghton Mifflin Company. Electronic version licensed from Lernout & Hauspie Speech Products N.V., further reproduction and distribution restricted in accordance with the Copyright Law of the United States. All rights reserved.

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    William McCormick
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  15. #14  
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    Sorry, a mistake on my part. Tension isn't measured as acceleration. It's measured as force, which is mass times acceleration. Weight is a force too, so I guess they are measured in the same units. Still, they aren't the same thing.
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    Quote Originally Posted by William McCormick

    However, those two outer 25 pound weights in that one drawing, are lifting a fifty pound weight. And there is no dispute by you to that affect.
    They are lifting the 50 lb weight because there are two of them, each lifting 25, meaning each rope has 25 lb. of tension.
    That means that when I take that fifty pound weight away, and reconnect the rope between the two 25 pound weights, the rope will have 50 pounds trying to tear it apart.
    No, it doesn't mean that at all. Putting it on a drawing and saying it is so does not make it true. The spring scale will never measure more than 25. That means the tension is 25 lb.
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  17. #16  
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    Quote Originally Posted by MagiMaster
    Sorry, a mistake on my part. Tension isn't measured as acceleration. It's measured as force, which is mass times acceleration. Weight is a force too, so I guess they are measured in the same units. Still, they aren't the same thing.
    Actually, they are the same.
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  18. #17  
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    Oh. Well, nevermind then. It's weird. I'm good at math. I'm good at some of the theory of physics. But I'm terrible at the math of physics. I can't understand it. :P
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  19. #18 Physics 
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    Physics is the discipline devoted to understanding nature in a very general sense: the fundamental characteristic of physics is that it aims to gain knowledge, and hopefully understanding, of the general properties of the world around us.
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  20. #19  
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by William McCormick

    However, those two outer 25 pound weights in that one drawing, are lifting a fifty pound weight. And there is no dispute by you to that affect.
    They are lifting the 50 lb weight because there are two of them, each lifting 25, meaning each rope has 25 lb. of tension.
    That means that when I take that fifty pound weight away, and reconnect the rope between the two 25 pound weights, the rope will have 50 pounds trying to tear it apart.
    No, it doesn't mean that at all. Putting it on a drawing and saying it is so does not make it true. The spring scale will never measure more than 25. That means the tension is 25 lb.

    Harold a weight scale and a tension scale have to have two different markings. The tension scale will double the weight of on object hung from it.

    Most of our rope, and chain is calibrated by load rating, that is not tension rating. It is rated for load. Not for tension.

    But if individuals do not understand this coming out of school, you will see more mislabeled stuff due to this misunderstanding. I have seen it already. They are lowering standards on chain now. Doing away with the jostling safety factor, that was engineered into them.

    The force to lift the fifty pound weight, did not change when I took the weight away. There is fifty pounds of force created by two 25 pound weights.

    Most never figure out a scale. I know a couple times we would try to figure out a way, using standard stuff to show it, or make it easier to understand. If someone is not getting it.

    But you really have to just think about it until it makes sense.



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    William McCormick
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  21. #20  
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by William McCormick

    However, those two outer 25 pound weights in that one drawing, are lifting a fifty pound weight. And there is no dispute by you to that affect.
    They are lifting the 50 lb weight because there are two of them, each lifting 25, meaning each rope has 25 lb. of tension.
    That means that when I take that fifty pound weight away, and reconnect the rope between the two 25 pound weights, the rope will have 50 pounds trying to tear it apart.
    No, it doesn't mean that at all. Putting it on a drawing and saying it is so does not make it true. The spring scale will never measure more than 25. That means the tension is 25 lb.

    Harold here is something that you can do. I believe I heard someone else use this before. Because the fellow with the misunderstanding was a very collegiate type of individual. So he used all the other things that are pounded into the guy at college to get him to see this one. The college guy just memorized this and memorized it very well.

    In order for Mr. Bill in the image below, to lift those two 25 pound weights, just enough to move them. How many pounds of force will Mr. Bill's body have to apply to the rope to move them? Remember he is going to move 50 pounds of weight straight up.

    If he can move both weights up a micron with just 25.01 pounds of force, he is a perpetual motion machine. Because it will take at least 50.01 pounds of force to move two 25 pound weights straight up.

    One day you will look back at this and laugh. I know others that do all the time.

    My point is that Mr. Bill is the weak link in the rope. So if his body has to apply 50.01 pounds to the weights to move them. At a stall there is 50 pounds of pressure on his body. And if he lets the weights down just enough to move them he would have about 49.98 pounds of pressure upon his body.





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    William McCormick
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  22. #21  
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    Quote Originally Posted by William McCormick
    Harold a weight scale and a tension scale have to have two different markings. The tension scale will double the weight of on object hung from it.
    Where did you get that idea?

    Most of our rope, and chain is calibrated by load rating, that is not tension rating. It is rated for load. Not for tension.
    Where did you get that idea?

    The force to lift the fifty pound weight, did not change when I took the weight away. There is fifty pounds of force created by two 25 pound weights.
    Wrong.
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by William McCormick
    Harold a weight scale and a tension scale have to have two different markings. The tension scale will double the weight of on object hung from it.
    Where did you get that idea?

    Most of our rope, and chain is calibrated by load rating, that is not tension rating. It is rated for load. Not for tension.
    Where did you get that idea?

    The force to lift the fifty pound weight, did not change when I took the weight away. There is fifty pounds of force created by two 25 pound weights.
    Wrong.

    To measure tension you have to double your actual scale measured values.

    How about this Harold. To move a fifty pound weight one inch it takes one twenty five pound weight moving two inches. With a block and tackle and or geared wench.

    Or two twenty five pound weights moving one inch each. Would you agree?

    So if our friend Mr. Bill contracts his arms by two inches over all. He lifts two 25 pound weights one inch each. The equivalent of a fifty pound weight one inch. Do you see that really what Mr. Bill did was to lift fifty pounds one inch.

    I mean it looks an awful lot like Mr. Bill is the only thing holding fifty pounds off the ground. Without Mr. Bill, 50 pounds will no longer be suspended against gravity. Do you agree with that?

    Do you see any other force holding the fifty pounds off the ground against gravity in that image?

    I mean a lot of individuals are walking around already with stuff they would not believe dangling over their heads. Trusses, are the most comical of all. You can trampoline on many roofs with trusses. If you drop an AC unit on a roof made with trusses in many cases, you can be inside, on the floor faster then you can say help.

    Probably due to this misunderstanding.




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    William McCormick
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  24. #23  
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    Quote Originally Posted by William McCormick
    How about this Harold. To move a fifty pound weight one inch it takes one twenty five pound weight moving two inches. With a block and tackle and or geared wench.

    Or two twenty five pound weights moving one inch each. Would you agree?
    Yes.

    So if our friend Mr. Bill contracts his arms by two inches over all. He lifts two 25 pound weights one inch each. The equivalent of a fifty pound weight one inch. Do you see that really what Mr. Bill did was to lift fifty pounds one inch.

    I mean it looks an awful lot like Mr. Bill is the only thing holding fifty pounds off the ground. Without Mr. Bill, 50 pounds will no longer be suspended against gravity. Do you agree with that?

    Do you see any other force holding the fifty pounds off the ground against gravity in that image?
    No, but that doesn't prove anything about the tension. If Mr. Bill put a 25 lb weight on a rope in each hand, he would be lifting 50 lb but the tension in each rope would be 25. Then if he tied the loose ends together, and picked up each rope separately the tension would still be 25. Then if he draped it over a pulley, the tension in each rope would still be 25.
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    Quote Originally Posted by Harold14370
    Quote Originally Posted by William McCormick
    How about this Harold. To move a fifty pound weight one inch it takes one twenty five pound weight moving two inches. With a block and tackle and or geared wench.

    Or two twenty five pound weights moving one inch each. Would you agree?
    Yes.

    So if our friend Mr. Bill contracts his arms by two inches over all. He lifts two 25 pound weights one inch each. The equivalent of a fifty pound weight one inch. Do you see that really what Mr. Bill did was to lift fifty pounds one inch.

    I mean it looks an awful lot like Mr. Bill is the only thing holding fifty pounds off the ground. Without Mr. Bill, 50 pounds will no longer be suspended against gravity. Do you agree with that?

    Do you see any other force holding the fifty pounds off the ground against gravity in that image?
    No, but that doesn't prove anything about the tension. If Mr. Bill put a 25 lb weight on a rope in each hand, he would be lifting 50 lb but the tension in each rope would be 25. Then if he tied the loose ends together, and picked up each rope separately the tension would still be 25. Then if he draped it over a pulley, the tension in each rope would still be 25.

    If Mr. Bill held a 25 pound weight in each hand. He would be lifting 50 pounds and there would be 50 pounds of tension on each rope, to hold the weights in the air.


    We just never get exercise in thinking about this properly. Tension on rope, chain, or cable, is twice the weight of a single weight lifted.

    This is why stuff is collapsing.

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    William McCormick
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    Harold, could you explain how 25 pounds of force can hold fifty pounds off the ground?

    You mentioned a hanging scale, reading 25 pounds. Now if that scale reads 25 pounds and holds 50 pounds off the ground.

    It must be highly inaccurate, or it is made to measure a single side of the force applied to the scale. A scale is just telling you how much weight is hanging from it.

    Not what the tension in the scale, hooks and what ever it is held from, is. The scale is doing the right thing.

    But the physicists are not.



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    Here is another one to think about.

    You have a double acting cylinder, with two connecting rod journals, with its housing, mounted to a stationary block.

    The piston has a one inch bore equivalent. Meaning there is exactly one inch of piston surface for the oil to apply pressure to, in both directions. So if you put 25 pounds of pressure to the piston, you get 25 pounds measured on the scale.

    You attach one end of the piston rod to a scale and apply 25 pounds of fluid pressure, the piston comes out and the scale accurately records the 25 pounds of pressure from the piston rod. Now the block that the housing is mounted to also has an equal and opposite 25 pounds of force being applied to it. Naturally because the other end of the piston also has a one inch surface area for the oil to press against.

    And if I am not mistaken the formula to calculate piston pressure is, one inch of piston surface for each pound of pressure will give you that much pressure or force.

    Except we have two surfaces in the piston. So there are two opposing surface areas in the piston, both one inch in area. Now I know for sure that any one surface with 25 pounds applied to it creates 25 pounds of pressure.

    I am stating for the record that the other end of the piston has an equal 25 pounds of force acting in the opposite direction. Creating actually fifty pounds of tension upon the cylinder.



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  28. #27 AAAAAAAAAA 
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    hmmm... so if I am pulling a rope attached to a wall the tension (of the rope) is the same as it is when someone unhooks the rope from the wall and pulls back with an equal force... IINAP (I Am Not A Physicist) but that seems really weird to me...

    Is it Newton's 3rd law (ie.: the ground is really pushing me up when I stand on it and the wall is pulling back with equal force...?!?!?) that trips me up or is William more correct..??

    I guess it's hard for me to understand because when I stop pulling the wall doesn't fall down backwards like my friend would in the tug-of-war example.
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  29. #28 Re: AAAAAAAAAA 
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    Quote Originally Posted by New Here
    hmmm... so if I am pulling a rope attached to a wall the tension (of the rope) is the same as it is when someone unhooks the rope from the wall and pulls back with an equal force... IINAP (I Am Not A Physicist) but that seems really weird to me...

    Is it Newton's 3rd law (ie.: the ground is really pushing me up when I stand on it and the wall is pulling back with equal force...?!?!?) that trips me up or is William more correct..??

    I guess it's hard for me to understand because when I stop pulling the wall doesn't fall down backwards like my friend would in the tug-of-war example.
    That is really all it is. Just the way you look at it.

    It is funny but having been in construction and manufacturing. I have seen the walls come down when you pull on them. So you realize that they either exert an opposing force or they come down.

    The ground has to apply an equal force to your feet, or you would fall through the earth, or sink. As if you were on water.

    I believe Harold is saying that even if another guy does pull to match your pull, it is still only equal to your pull. To me that seems really strange.

    To me one twenty five pound weight will just fly downwards, unless you create an equal and opposite force. To me equal and opposite is another totally different force. That must be concatenated to the original force you are matching.

    If I use two hanging scales hung from a ceiling and place a 25 pound weight in each, Harold agrees that each scale is correct reading 25 pounds. For a total of 50 pounds of weight applied to the scales.

    But if I use two spring scales connected between the two pulleys and hang the two 25 pound weights, he will claim foul. In this case clearly the two twenty five pound weights add up to 50 pounds of pull.



    Yet when we hang both 25 pound weights from the ceiling they seem to work lovely. No foul.

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    William McCormick
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    So force and the resistance to force (not moving) is the same... got it!!! (3rd law it is then... lol)

    wall-------rubber band-------weight(25 lbs)

    weight(25 lbs)-------rubber band-------weight(25 lbs)

    So the rubber band would not stretch anymore in the 2nd example than the 1st? If it did does that imply more tension on the rope??? Maybe I am just sleepy and will understand it better in the morning... =)

    EDIT: I wish I had some rubber bands in the house right now...
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