Period for real-life spring experiment

[doppelganger007]

Hi,

I have a question from a lab experiment for an online course. The question states "In a real spring experiment, you wouldn't expect the period predicted by the formula for the period of a harmonic oscillator. What is the reason for this, and the would the actual value be larger or smaller than the predicted one?"

My partner and I initially thought it was air resistance, but we finally concluded that air resistance would not have an affect on the period of the spring. The spring in question is one that is hanging downward with a mass attached to it. Any suggestions? Thanks in advance.

 

[OlderDan]

Air could have a very small effect on the frequency, making the oscillator behave as a damped oscillator. Some damping could also come from heat generated by flexing the spring. An additional consideration might be the mass of the spring itself. The effects of damping are modeled as a velocity dependent resistance to the motion.

http://hyperphysics.phy-astr.gsu.edu/Hbase/oscda.html

 

[doppelganger007]

Thanks OlderDan, now I have another related problem. It asks about why a second grader on a swing who tries to maintain a constant period keeps slowing down. Isn't it then because of damping, as with the spring? And also, what could then be done to "combat" the deceleration and thus maintain a constant period?

 

[OlderDan]

Thanks OlderDan, now I have another related problem. It asks about why a second grader on a swing who tries to maintain a constant period keeps slowing down. Isn't it then because of damping, as with the spring? And also, what could then be done to "combat" the deceleration and thus maintain a constant period?

For a swing, that cool breeze you feel is in fact always slowing you down. There is also some friction in the pivot points. A second grader might appreciate the help of a friend doing what?

 

[andrevdh]

Real springs do have mass. Also in a real experiment one would expect the up and down motion to include swinging from side to side. How this would actually influence the period is beyond me.

 

[OlderDan]

Real springs do have mass. Also in a real experiment one would expect the up and down motion to include swinging from side to side. How this would actually influence the period is beyond me.

Even without the swinging motion, there is a twisting effect. The spring winds and unwinds as it stretches and relaxes. This looks like a nice discussion of the situation, and includes a brief discussion of other two-degree-of freedom systems.

http://online.redwoods.cc.ca.us/instruct/darnold/deproj/sp04/stevemisay/Project1.pdf

Adding your swinging would make it 3-degrees if it stayed in a plane and another if it does not. Messy stuff when all is said and done, but there are simple examples of coupled oscillators where the energy migrates from one mode to another. Two pendula connected by a light spring for example.