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I am having a partcular issue with several pairs of glasses and wish to understand the physics of it. The problem is that the right arm of the frame is too tight against my temple, and at the same time the left nosepad is too tight on my nose.

This seems rather unintuitive at first, as one might think if the right arm is too tight, then so should the right nosepad. My grasp of physics is poor but I think I can begin to see why this is not the case, and that the law that every action has an equal and opposite reaction might be applied.

Any help with this would be greatly appreciated. I have taken them to an optometrists who were puzzled.

enter image description here

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  • $\begingroup$ I don't really think we could tell you what the problem is here, head shape plays a large role in how things fit on the head. Without knowing much more detail it's really just a guess on our part. $\endgroup$
    – Triatticus
    Commented Jul 13, 2023 at 14:00
  • $\begingroup$ If the right arm is tight against your head then it torques the glasses and pulls the lens part towards the right, putting pressure on the left nose pad. Make a free body diagram. $\endgroup$
    – Jon Custer
    Commented Jul 13, 2023 at 14:11
  • $\begingroup$ @JonCuster Yes I think you are correct, thank you. I don't have the knowledge to do a diagram. I have read a bit online and put a starting picture in my OP. Please can you help me progress it $\endgroup$
    – Econ23
    Commented Jul 13, 2023 at 19:45

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Drawing on my legendary skills as an artist I propose the following interpretation:

Glasses

The diagram shows a top down view of a (bald!) person wearing glasses. If the right hand leg is too short the person's right ear will be pulling the glasses in the direction indicated by the blue arrow. This creates a clockwise torque, and in the absence of any opposing force the glasses would rotate clockwise around the persons head. Since the glasses do not rotate there must be an anticlockwise torque being applied by other parts of the persons anatomy. This could be applied by the person's left ear, but could also be applied by the left side of the person's nose as shown by the red arrow.

I suspect the actual distribution of forces would be more complicated that this as there will be friction between the glasses and the skin all round the head. However it does show how if the right leg is too short the force this causes will be balanced by force on the left side of your nose.

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    $\begingroup$ Thanks for this. If F nose was going in that direction, wouldn't the pain be on the right nosepad as the frame is being 'dragged' anticlockwise? I may be misintepreting because I don't think of anatomy parts 'applying' forces, rather the other way round. $\endgroup$
    – Econ23
    Commented Jul 14, 2023 at 8:08
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    $\begingroup$ @Econ23 we have to be careful to distinguish the force your body parts are exerting on the frame from the force the frame is exerting on your body parts. The two are equal and opposite (in accordance with Newton's 3rd law). The arrows I've drawn are the forces your body is exerting on the frame, so your right ear is pulling the frame clockwise and your nose is pulling it anticlockwise. That means your ear is being pulled anticlockwise (i.e. forwards) and your nose is being pulled clockwise i.e. the left side of the frame pushed to the right on your nose. $\endgroup$
    – John Rennie
    Commented Jul 14, 2023 at 8:19

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