## Mashy 3 years ago Candle swing experiment http://www.youtube.com/watch?v=4u7OwGkWqrk&feature=related this is a youtube link to the experiment, basically a candle is pivoted and allowed to rock with both sides lit, as time passes, it keeps rocking faster and faster i wanna know if its an example of conservation of angular momenta, cause m and r are both decreasing and to keep mvr constant v increases? anyone help me?

1. Ishaan94

Wow this is awesome!!

2. Ishaan94

Yeah, I think so. At beginning of the experiment it does have some initial angular velocity(though it's oscillating). Initial Angular momentum $$I_o \omega_o$$ and with time mass& its length are reducing but no external torque is acting on the candle which implies there must be no change in the candle's angular momentum. $$I_o \omega_o = I \omega$$ where $$I < I_o$$ as the Moment of Inertia of the rod depends upon its length and mass (which are reducing with time) causing its angular velocity to increase$$\frac{I_o \omega_o}{I} = \omega$$.

3. Mashy

ha thanks thanks.. I wanna do this demo and show students.. thats my aim now.. to do simple but POWERFUL demonstrations ;-)!

4. Ishaan94

btw I assumed the candle is pivoted about its center of mass and gravity is the only force acting (except some normal reaction to the candle of the needle which in my opinion could be ignored) but since the rotational axis is passing through its center of mass and so is gravity, the torque caused my gravity is zero. yeah awesome work mashy you must be a good teacher.

5. Ishaan94

..by* gravity..

6. Mashy

no no this is not my work.. i ll replicate this :P!!

7. JamesJ

There is definitely external torque on the system: from the flames themselves. And the angular velocity $$\omega$$ keeps on changing sign, so there's no way that angular momentum is being conserved in this system.

8. Mashy

@jamesJ.. crap.. then what is this experiment demonstrating?? :'(!!

9. JamesJ

The candle pivots around what is originally something close to the centre of mass. The left side dips down. The flame now melts more of the wax on the left side than the flame on the right, because the flame is close to vertical and when a side is down, more of the candle is close to the flame. When enough of the wax has dripped off, left side is no longer heavier (or more precisely, the torque from the right side approaches and then exceeds the torque from the left side). The candle rotates towards the right side. Then the pattern repeats itself on the right and the candle oscillates back to the left. And so on. So if this experiment demonstrates anything, it is of a system that keeps on oscillating its center of mass about the pivot. It's a good exercise to figure out what is going on here, but I wouldn't make this the first experiment in angular momentum. ===== If you want a demonstration that angular momentum is conserved, have a look at the "Skater's Delight" demonstration in this lecture, beginning around 18:00. http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-20/

10. Mashy

hey thanks james.. thanks a lot!!!!!!.. really your fan! man!!>.

11. Ishaan94

James wouldn't the flames burn nearly equal amount of wax with time and even if the side which is bent gets more burning or loss of wax I think the difference would be insignificant, the center of mass would shift only by few millimeters. And how would you describe the increase in candle's angular velocity? With mass and length of the rod decreasing with time the torque effect will only reduce $$\tau = r\times F_g$$. The torque does act but It must be insignificant. I think this experiment does show conservation of angular momentum as the major cause for candle's behavior.

12. Ishaan94

I might be wrong but I need to understand the correct cause.

13. JamesJ

Whatever else is going on, angular momentum is not conserved. It simply is not the case that it is constant when the candle keeps on changing the direction in which it is rotating.

14. Mashy

yes ishaan he is right.. and besides changing the direction.. the velocity is not at all constant.. angular momentum is not constant even for a split second there :D!!