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Torsional locomotion

Davide Bigoni's picture

Can a torque induce longitudinal motion of an elastic rod?

See the explanation and an example of use of the 'torsional gun' at

If you're having trouble playing videos on YouTube, click here to watch it.


More information about my research activity can be found in
More information about our experiments can be found in


Dibakar Datta's picture

Dear Prof. Bigoni,

I really enjoyed reading your book : Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability. Your work is truly inspiring. 

Thank you.

Davide Bigoni's picture

Dear Dibakar Datta,

thank you for your nice comment!
I am really glad that you have enjoyed reading my book.




Tianxiang's picture

Prof. Bigoni,

Thanks for posting this vedio. This is interesting. I wonder if you can help explain the following that I find it difficult to understand.

(1) When you consider the energy of the system, do you think the roller constraint does work on the rod? When the rod slides axially crossing the constrain, I feel the constaint plays a role of untwisting the rod.

(2) From the energy equation you wrote down, potential energy is -(M^2/2D)*L. There is a negative sign there. So the smaller the L, the larger the potential energy because it is less negative. I am wondering why the system favors sliding across the constrain to reduce L and increase its potential energy?  



Tianxiang Su

Tianxiang's picture

Ah, in the video the force is pointing to the right increasing L and reducing the potential energy. I understand question (2). 

Tianxiang's picture

I misunstood the direction of the force P just now. Sorry about that. Let me re-write my question here:

(1) I am wondering when the rod slides across the roller constraint, does the constraint does work on the rod? The constaint applies a torque on the rod and there is a relative rotational motion between the rod and the constraint, so the constaint does work?


Davide Bigoni's picture

Dear Tianxiang,

Thanks for your interest in my work. No, the constraint does not work! The constraint is perfectly smooth and bilateral, so it cannot work.

Best wishes,



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