Spring designed for compression

[sid_galt]

Let's say, we have a spring designed for compression which is kept vertically on a table. Now if we press on the spring from the top, it will naturally compress.
My question is, what will happen if we compress the spring from the middle instead of from the top. Will the portion above the middle of the spring also be compressed due to the stress in the spring or will the transfer of force to the upper portion be much less when pressing the spring down from the middle in comparision to pressing the spring down from the top?

 

[Averagesupernova]

Nothing happens above where you press.

 

[Cyrus]

Think about your question for a second, what forces would cause the top portion of the spring to be under compression?

 

[sid_galt]

Think about your question for a second, what forces would cause the top portion of the spring to be under compression?

Well, if you press a spring, the angle of the turns relative to the horizontal changes. If you pick a specific point in the middle and press the spring there, then the relative angle between the turns below the middle and above the middle will change and increase. Since to mantain such an angle would require a constant upward force which doesn't exist, IMO, the portion above the middle would compress to a certain extent (to decrease the angle and thus the stress) between the upper and lower portion.

 

[Q_Goest]

Will the portion above the middle of the spring also be compressed due to the stress in the spring or will the transfer of force to the upper portion be much less when pressing the spring down from the middle in comparision to pressing the spring down from the top?

Hi Sid. The wire that a coiled spring is made of is typically viewed as being in torsion, so the equations for deflection and stress in a spring are derived from a long, cylindrical bar in torsion. A compression (or extension) spring is simply a coiled up bar to reduce space.

If you can imagine a long, cylindrical bar with one end fixed, and a torque applied to the bar somewhere in the middle, the portion of the bar between the fixed end and the applied torque will twist while the portion of the bar between the torque and the unfixed end will simply rotate. There will be no stress and no deflection created in the portion of the bar between the applied torque and the unfixed end. Similarly, any coils in a wound spring that aren't influenced by a load will not change dimension.

 

[sid_galt]

Thanks Q_Goest. That makes sense.