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25 April 2014

A spring that's both stretchable and compressible

At the recent AP Physics consultant powwow, I heard great ideas from about twenty really good physics teachers.  At one point we discussed all the many investigations that can be done with just a class set of springs and masses.  But which springs do you want?

The PASCO equal length spring set is great, if expensive.  It requires using heavy (500 g or more) masses; but the advantages are the equal length, durability, and the clearly stated spring constants.  The PASCO dynamics track spring set is my preference for most student experimental work, due to its utility with smaller (~100-200 g) masses.

The main disadvantage of these spring sets is that they do not compress -- they only stretch.  In their resting state, the coils touch.  It's often tough to demonstrate or show experimentally that Hooke's law (F = kx) is valid for both stretching and compressing.

A fellow consultant -- I forget who, but it might well have been Miami's own David Jones -- suggested purchasing springs from McMaster-Carr.  In particular, he suggested the spring pictured above.  It has a spring constant of 1.9 pounds per inch; a moment's googling makes that 330 N/m, or much stiffer than most of the other classroom springs I own.  But I've always wanted a compressible spring, and this one fits the bill.  I don't doubt that one could scour the McMaster-Carr catalog to find a compressible spring with a smaller spring constant -- if you find one you like, please let me know.

The best part about this spring is the price -- $6.50 for a pack of 12, plus shipping.  I bought a pack for each of our physics teachers as a gift.  Most people want chocolates, flowers, or the like as gifts... me, I'm happy with a bag o' springs.

3 comments:

  1. For my home school physics students, I bought http://www.amazon.com/Neiko-Steel-Spring-Shop-Assortment/dp/B000K7M36W which has 200 springs for about $8, about 10 each of 20 different types. The compression springs may be too fragile for high school classes. I had the students measure the spring constants of the 11 extension springs that had loops on the ends, which varied from 4 N/m to 1300N/m. I didn't have them measure the compression springs, because the only decent force measurement we had used a hook, not a platform.

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    Replies
    1. correction: that should have been 57N/m to 1300N/m (my bad for trying to read and respond to blogs in the middle of the night).

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  2. I found the spring measurements the students made at
    http://gasstationwithoutpumps.wordpress.com/2011/10/29/physics-update/

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