Hi everyone, this Thursday I have my Physics test and doing some exercises to practice I've noticed that in "rotational exercises" even though External Forces and Torques are = 0 you loose kinetic energy, for example when you have a rotating disc and then you put another one on the top of it, you loose angular speed because you gain mass but I am not sure of is how to explain the lost of energy. I'm guessing it is about the internal forces but I'd like someone who really knows to tell me the why,

 

I'm not sure if I made myself clear with the question thank you guys!

 

This sounds equivalent to an inelastic collision. The lost kinetic energy is transferred to thermal energy due to friction between the two discs as one slows down and one speeds up until they match speed.

So the friction is the cause.. I wasn't sure about that. And for example if I have 2 kids on a merry-go-round with certain angular velocity w₀ and then they move to the center, they gain speed and also they have more energy, where did that energy come from? If you can explain that with forces, i'm just a beginner here

So the friction is the cause.. I wasn't sure about that. And for example if I have 2 kids on a merry-go-round with certain angular velocity w₀ and then they move to the center, they gain speed and also they have more energy, where did that energy come from? If you can explain that with forces, i'm just a beginner here

What forces and energy was used to move the kids toward the center?

So the friction is the cause.. I wasn't sure about that. And for example if I have 2 kids on a merry-go-round with certain angular velocity w₀ and then they move to the center, they gain speed and also they have more energy, where did that energy come from? If you can explain that with forces, i'm just a beginner here

 

It depends on how they move to the center. If they are moving with the merry-go-round, i.e. at the same angular peed. then their linear speed goes down, as does their kinetic energy. If you are arguing a situation where they speed up owing to conservation of angular momentum, then work must be done. i.e. J.C. Maxwell's questions must be answered.

It doesn't really explain how did they move it just say that they move :S

It doesn't really explain how did they move it just say that they move :S

 

Then you can conclude that work was done.

 

Then you can conclude that work was done.

 

Ok thanks, I had the exam today, I think I did great, I'll tell you next week