Twins Paradox and Special Relativity

This post is a bit off topic to my blog, but there are some things in popular science that really bother me. One of the groups that I follow on Facebook posted a clip on Twins Paradox in Special Relativity. Again! There is no such thing as Twins Paradox in Special Relativity. In order to compare the ages of the twins, you need to return them to the same point.  You have to accelerate (decelerate) at least one of them, making his/her frame of reference a non-inertial one. This means we cannot longer use Special Relativity. Only General Relativity has non-inertial frames of reference. For detailed explanations one can look at the famous Landau & Lifshitz book.

To me, it was always a criterium of a good vs bad book on Special Relativity: if there is a chapter on Twins Paradox, I would never buy a book.


Equipotential color-coding

Last week on Twitter  posted a picture on equipotential color-coding for electric circuits.


I liked it, and a short discussion followed.   directed us to PASCO CASTLE kit where this idea is described. Please, be aware that their description has a physical error: using hydrodynamic analogy, they claim that all points of the same color have the same static pressure. This is not true. Just drop their explanation completely and use equal levels of gravitational potential energy instead.

Today I tried this method in my classes for review of electric current. It did work well, and many students liked the idea.  I added energy diagram that also clarified some details and, hopefully, will help me, when it comes to atomic energy levels.

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Overall, I believe that this method works for establishing in students’ heads universal energy/ law of conservation of energy idea.

Sink or swim

Sink or swim – a topic as old as our civilization. Everything about it is well known since Archimedes.

What can be added to a lesson on this subject to make it interesting and unusual? I decided on everyday vegetables and fruit. Carrots? Sink. Radishes? Sink. Onions? Swim. Apples? Swim. Lemons? Swim.


Ok, let’s peel this lemon.


Now it sinks! Why? Let your kids explain it.

Side note: a peeled lemon always sinks. If you try the same experiment with an orange or tangerine, the result is unpredictable and depends on the variety, size etc of the orange. If you want predictable results, use a lemon.