# Is uniform circular motion perpetual?

Accelerated dielectrics become polarized, and, being accelerated, must radiate. Accelerate conductors radiate, too. Anything spinning is accelerated, and so must radiate.

jfizzix
Gold Member
In the context of newtonian physics, its angular momentum is conserved. In the context of general relativity, the angular momentum of the pencil plus the field(s) is conserved, though there can be transfer between the two.

And yes, because the pencil is made up of charged particles, there would be electromagnetic radiation too. Considering how weak gravity is, it would be interesting to see if the overall neutral pencil still loses more energy (in a given time) by electromagnetic radiation than gravitational radiation

mfb
Mentor
Accelerated dielectrics become polarized, and, being accelerated, must radiate. Accelerate conductors radiate, too. Anything spinning is accelerated, and so must radiate.
Not necessarily, see the ring as an example (a disk would do the same).

If both positive and negative charges are completely homogeneous in the pencil, you have charges, but no electromagnetic radiation.

Simon Bridge
Homework Helper
All this is splitting hairs compared with the fundamental misunderstanding illustrated, and dealt with, in the first few posts.

But I think I'll add this one to my puzzle set ... it reminds me of the discussion surrounding a question about how the G&T level in a glass changes as the ice melts.

@jd0g33: was any of that any use?
Haha ya. As soon as someone pointed out that the centripetal force was acting perpendicular to the direction I stopped checking the thread...

Whatever forces are acting, no energy is 'consumed' or transferred if there is no movement in the direction of those forces (i.e. the string / pencil gets no longer or shorter). Work done is Force times Distance moved in the direction of the force.
The distance covered is circular, so isn't there even some component of distance being covered in the direction of the centripetal force?

sophiecentaur
Gold Member
The distance covered is circular, so isn't there even some component of distance being covered in the direction of the centripetal force?
No. The Centripetal force is constantly at right angles to the direction of motion.

Simon Bridge