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Consider the following figures.

  • The top one shows the construction of a spring where its left end is attached to the wall and its right end is stretched by a force.

  • The bottom one is supposed to be the free diagram of the spring. Is it correct?

enter image description here

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  • $\begingroup$ Yes, if it is a massless spring. $\endgroup$ Jan 8, 2014 at 17:23

2 Answers 2

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You have to understand the fact that the point on the wall where the spring is fixed is at rest. Hence, the so called free diagram applies there with the other force as the normal from the wall. Whereas, when you pull the rest of the spring, along with the normal you would have a force linearly proportional to the length that has been stretched.

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  • $\begingroup$ So the net force on the spring is directed to the right? If so why doesn't the spring get accelerated to the right? $\endgroup$ Jan 8, 2014 at 12:40
  • $\begingroup$ Understand that, if you pull the spring, it will exert a $kx$ force away from your hand, where x is the elongation. As long as $F_(applied)$ is greater than $kx$ at that moment, it will accelerate to the right. However, if the spring is massless, you will constantly be in a state of quasiequillibrium with your force equalling $kx$ $\endgroup$ Jan 8, 2014 at 12:48
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consider a very little length of the string. (dl ) 2 forces act on it. they have the same magnitude and the opposite position. (consider that otherwise the part would be accelerating.) so, if we draw a diagram for every little part and then want to summarize the diagram by considering the hole string one body, then your diagram is completely right.it is helpful noting that the diagram shows the forces applied "on" the sting not "by" the string.

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