Suppose an agent elongates a spring by $x$. Then the restoring force of the spring is $-kx$. But what is the force applied by the agent? Is it equal to $kx$? If so, then why? It should only happen when it is at rest for the forces to equate or cancel.
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$\begingroup$ Hint: study Newton's 3rd law, and believe it. $\endgroup$– David WhiteCommented Aug 5, 2021 at 0:34
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$\begingroup$ Hello? Did we help? $\endgroup$– Al BrownCommented Aug 7, 2021 at 9:24
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$\begingroup$ @ACB Do you really look back at a user's past questions to see if they have accepted answers in the past before making an answer on a new post? While we are answering questions for the OP here, keep in mind that answers can also be useful to others who did not make the question. Unless your only goal is to gain reputation, I would think you would answer a question regardless of the OP's tendency to accept answers on their questions. $\endgroup$– BioPhysicistCommented Aug 7, 2021 at 11:48
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1$\begingroup$ @AlBrown Oftentimes people will ask questions and then leave once they get their answer. They aren't concerned about the inner workings of the site. Best just to move on from it. Your answers can still be useful to other users in the future even after the OP has "abandoned" the post. $\endgroup$– BioPhysicistCommented Aug 7, 2021 at 11:50
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2 Answers
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From wikipedia
…the restoring force is a force which acts to bring a body to its equilibrium position. The restoring force is a function only of position of the mass or particle, and it is always directed back toward the equilibrium position of the system.
So it is better to say $kx$ is equal to the force applied by the agent, rather that saying the force applied by the agent is equal to $kx$. (Seems as the same, no? But there is a difference) : )
It should only happen when it is at rest for the forces to equate or cancel.
Yes, the wikipedia article above says so.
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$\begingroup$ Well of course it’s hard not to think of it as a game with points. Lol. Why have points then? I’ll check they’re behavior on occasion. But yeah I see is a bit petty. Quite human tho. Have a good one acb. $\endgroup$– Al BrownCommented Aug 8, 2021 at 7:12
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The forces are equal if there is no acceleration, not if there is no velocity. This means that the end (for example if it has a mass on it) is not changing its speed, because the net force is zero.
F=ma means things don’t change speed unless a net force acts on them. Like a rolling car without friction or wind resistance would maintain its speed.
So we know the end is not changing speed, not accelerating.
