Violating the law of conservation of energy

  • Thread starter deep838
  • Start date
  • #1
117
0

Main Question or Discussion Point

I know that's impossible, so please help me!!!

Let there be a body rotating about its axis, with a moment of inertia I and an angular velcity w.
The angular momentum of the system is L = Iw.
Now, if the radius of the body change, its moment of inertia will also change. Let the new moment of inertia be I`, such that, I`= kI.
But the angular momentum of the system will remain constant.
So, the angular velcity must change. So, w`=w/k.
But, initially, the energy of the system was purely kinetic and was given by
E=1/2 Iw2.
The new energy is given by, I`= 1/2 I`w`2 = 1/2 kI (w/k)2
So, E`= E/k.

So the new energy of the system is either more {0<k<1} of less {k>1} than the previous energy!

What am I doing wrong?
 

Answers and Replies

  • #2
1,254
105
You need to do work to change the radius. Imagine whirling a ball on a string around. If takes work to pull in the ball.
 
  • #3
117
0
ok...got it... so silly of me to even ask this.
 
  • #4
CWatters
Science Advisor
Homework Helper
Gold Member
10,528
2,294
The only silly question is the one you don't ask.
 
  • #5
117
0
hmm... who said that?
 

Related Threads for: Violating the law of conservation of energy

Replies
25
Views
2K
Replies
3
Views
2K
  • Last Post
Replies
4
Views
3K
Replies
22
Views
3K
  • Last Post
Replies
7
Views
2K
  • Last Post
Replies
5
Views
2K
Replies
2
Views
884
Replies
10
Views
6K
Top