# Flywheel -- Why does it reverse direction after spinning to a stop?

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1. Dec 1, 2014

Why does fly wheel return in opposite motion after it comes to rest due to friction?

2. Dec 1, 2014

### Bystander

Wind-up in the power train.

3. Dec 1, 2014

### CWatters

If most of the mass is in the rim and friction acts at the hub then I guess there might be stress in the spokes if it decelerates rapidly?

4. Dec 1, 2014

Rim? Hub? Spokes?

5. Dec 1, 2014

### Bystander

I was thinking more in accumulated shaft lengths, pedestals, frames, linkages.

6. Dec 1, 2014

### CWatters

My flywheel looks like a bicycle wheel with disc brake. What does yours look like?

7. Dec 1, 2014

### CWatters

Understood.

8. Dec 2, 2014

http://www.naugralabequipments.com/images/product/14692275775-250x250.jpg

9. Dec 2, 2014

The one in my Physics lab looks like this one.

10. Dec 2, 2014

### CWatters

Can you describe what happens? For example is the string and weight still attached when it stops and goes backwards?

11. Dec 2, 2014

String and weight falls and it continues to rotate until comes to rest and rotates in opposite direction.

12. Dec 2, 2014

### jerromyjon

What happens when you run out of string? I'm guessing the flywheel continues spinning and the string remains attached?

13. Dec 2, 2014

### Staff: Mentor

Is it winding the string and weight back up?

14. Dec 2, 2014

Nope!

15. Dec 2, 2014

It happens the same if there is no string and we just give is a slight push by hand.

16. Dec 2, 2014

### jerromyjon

Friction recoil in the axle. Can't seem to find any links about it.
I would imagine if it was very hard to turn, not from the weight itself but from the axle not wanting to turn, like a bicycle wheel with stiff bearings, it would be "springy" if you tried to turn it. When you do apply enough force to turn the wheel that extra force remains counter to the direction of rotation. As the energy left in the momentum dwindles below the friction threshold, the remaining energy is released in counter-rotation. Similar to if you apply less force to the wheel than it takes to turn the axle and release.

17. Dec 2, 2014

### Staff: Mentor

If it wasn't perfectly symmetrical it would want to turn until the heavy spot was downwards, and sometimes this will entail reversal of its rotation at the end, but not always. A handful of trials should discount this as the source.

So you have removed all of the string? Is it fixed to the shaft by a knot wedged into a slot, or how?

18. Dec 2, 2014

### jerromyjon

There also has to be some room in between the axle and the bushing where the axle can "walk up" the inner circumference of the bushing.

19. Dec 2, 2014

### DaveC426913

A flywheel with this trait would have - let's say ... undesirable side effects.

[EDIT] ohhhh. This is a high school flywheel. I was imagining a 10kg flywheel rotating at 30,000 rpms, developing a slight ... wibbly-wobble.

20. Dec 2, 2014

### Staff: Mentor

I was picturing the slot in the shaft for the string end would always like to end up uppermost, if all else were ideal.