MindShadowfax Posted December 10, 2012 Share Posted December 10, 2012 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! Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted December 10, 2012 Share Posted December 10, 2012 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. 1 Link to comment Share on other sites More sharing options...
MindShadowfax Posted December 10, 2012 Author Share Posted December 10, 2012 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 Link to comment Share on other sites More sharing options...
J.C.MacSwell Posted December 10, 2012 Share Posted December 10, 2012 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? Link to comment Share on other sites More sharing options...
swansont Posted December 10, 2012 Share Posted December 10, 2012 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. Link to comment Share on other sites More sharing options...
MindShadowfax Posted December 10, 2012 Author Share Posted December 10, 2012 It doesn't really explain how did they move it just say that they move :S Link to comment Share on other sites More sharing options...
swansont Posted December 11, 2012 Share Posted December 11, 2012 It doesn't really explain how did they move it just say that they move :S Then you can conclude that work was done. 1 Link to comment Share on other sites More sharing options...
MindShadowfax Posted December 11, 2012 Author Share Posted December 11, 2012 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 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now