Let's say we have a spring placed vertically on a table. Assuming spring has negligible weight, we then place a 6N weight on top to compress it. From Newton's third law, the table will also exert a same magnitude and opposite force, 6N in this case. Does that mean the compressive force acting on the spring is doubled?
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Doubled from what?
If you exert a 6 N force on one spring end, then there must be a force from the other end as well. Otherwise the spring would simply move and you would not be able to apply those 6 N (in the case of a mass-less spring).
In short, the 6 N you exert from the top and the 6 N the table exerts from the bottom do both in union cause the compression. The compression distance is a function of the 6 N force from both sides.
So when you are asking whether the compression will be doubled now that 6 N are exerted from both sides, then the answer is no. Were it not pushed with 6 N from both sides but only from one side, then there would be no compression and there would be no force from only one side - it is an impossible scenario (for a mass-less spring).
Steeven
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I was thinking that since both sides of the spring are subjected to a force of 6N and these pair of forces both contribute to compression, therefore the compressive force involved can said to be doubled, which is 12N. I think your answer has clarified my doubt. But just to be sure, for example, if a string is said to be able to withstand a compressive force of 6N, it means both sides can be subjected to 6N, and not 3N on top and 3N that the table exerts on the spring. I hope that I got it right this time. – radastro Aug 24 '21 at 12:49
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@hao004 Yes, you are entirely correct. The term spring force is used for the force each either end. Not their total force. – Steeven Aug 24 '21 at 13:03
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If you just draw the free body diagram of the spring, you will see that there is a 6N force acting on it from the top , which is the weight placed on it.
And there will also be a 6N force acting on it from the bottom which is the normal reaction force.
These are the 2 forces acting on the spring , in the situation you have described. You can call it doubled if you want, but that seems more like a matter of semantics. The free body diagram is the complete and accurate representation of what is going on .
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No, 6N is the force going through the spring. This is a common misconception, but there is really only one force going through the spring. You are just measuring it at different points. In some sense, this force of 6N is shared by all point along the spring.
This means that if you cut the spring at an arbitrary location, there is going to be an equal and opposite fore of 6N acting on the two part on the split. The 6N acting towards the weight is going to balance the weight so that it does not move, and the 6N acting towards the ground is going to be reacted by the ground.
Similarly for the contact with the ground, there is a 6N force needed to balance the weight acting on the spring, but this is just a reaction to the applied load. And reaction forces do no work by definition.
JAlex
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