Wednesday, March 14, 2007

Arms Length

The topic for today comes from the 'silly teaching examples' file. Others may wish to use it.

For the average human being, both arms are the same length, right? Actually, not quite. It all depends. This is something that can easily be studied at home in a bored moment.

First, hold out both arms, straight in front of you. This is best done with the palm face down. Unless you have had some trauma, or other misfortune, both arms should be appear to be of equal length. Now, there are ways of messing with this, by pushing each shoulder forward or backward. That is not the point here. If you find that you get the wrong result, or you are tempted to roll your shoulders, then check the lengths of your arms, while standing with your shoulders flush against a wall.

So, after the initial examination, it would appear that, for most people, arms are of equal length. We have now set a base line. It is at this point, that we try something a little unusual. The next thing to do is to leave one arm sticking out and turn it so that the palm is face up.

The next step is to bend your elbow such that your lower arm is at ninety degrees to the upper arm. You need to keep doing this, up and down, for about thirty seconds. Doing this should not be too tiring.

Once you have completed flexing the one arm for the required time, try checking the lengths of your arms again. What you should find is that the arm you have been flexing is now shorter than the arm that you have not flexed. Thus, it is not always the case that our arms of equal length! It all depends upon what we have been up to.

The reason this effect occurs is because of the contraction of the muscles in the arm that we have been flexing. Fear not, fairly rapidly, your arms will return to their previous length. The point here is that even amazingly obvious truths are less straightforward than we might initially imagine. This should give us pause when we are asked to believe complex propositions, which are less than entirely obvious.

The CP


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