Changing the RPM of a frictionless spinning wheel in a box

Click For Summary

Discussion Overview

The discussion revolves around the behavior of a frictionless spinning wheel within a vacuum box, specifically focusing on whether the RPM of the wheel can be changed through external manipulation of the box. The scope includes conceptual reasoning and mechanical principles related to rotational motion.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the wheel will continue to rotate as if in space and that the RPM cannot be changed without breaking the box or its contents.
  • Another participant questions the attachment of the wheel axle to the box and the role of gravity in the scenario.
  • A participant draws an analogy with a bike wheel, proposing that turning the handlebars does not change the RPM of the wheel if there is no friction.
  • Another response supports the idea that in an ideal case with no axial friction, the RPM would remain unchanged, suggesting that conservation principles may apply.
  • One participant emphasizes the necessity of assuming that the wheel is balanced for the discussion to hold.

Areas of Agreement / Disagreement

Participants express differing views on the influence of external manipulation on the RPM of the wheel. While some agree that RPM remains constant under ideal conditions, others raise questions about the assumptions involved, indicating that the discussion remains unresolved.

Contextual Notes

Participants note the importance of assumptions such as the absence of friction and the balance of the wheel, which may affect the conclusions drawn from the discussion.

SpaceThoughts
Messages
16
Reaction score
1
Imagine a spinning wheel built into a hand size vacuum box. There is no friction between the axe bearings of the wheel and the box. Let's say that the wheel rotates with 60 RPM. Am I right if I assume:

1. The wheel continues to rotate, approximately as if in space.
2. It is not possible to change the 60 RPM in any way, as long as the box and the content is not broken. No matter how we twist and turn the box from the outside.
 
  • Like
Likes   Reactions: Delta2
Physics news on Phys.org
How is the wheel axle attached to the interior of the box? How does gravity play in all of this?

Also, you can certainly design things to be able to add rotational energy to the spinning wheel, such as using a rotor from an electric motor as your spinning wheel...
 
Let me ask the same question in a less confusing way:

I lift the front wheel of my bike off the ground and make it spin. I turn the handlebars right-left-right-left.
I feel the resistance of moving the handlebar back and forth. The faster the wheel spins the more resistance.

Will turning the handlebar back and forth change the RPM of the bike wheel, or would the wheel continue to rotate with the same RPM if there were no friction?
From my experience I think the RPM would not change, but I have not found a clear answer to this question.
 
SpaceThoughts said:
Will turning the handlebar back and forth change the RPM of the bike wheel, or would the wheel continue to rotate with the same RPM if there were no friction?
For the ideal case with no axial friction and, say, a perfectly rigid wheel your experience is correct. To see this you could perhaps ask yourself what mechanical quantity usually is conserved in cases of no friction and see if this helps you conclude something. This will also explain why the wheel, in addition to the axis bearings, has to be idealized (i.e. have no internal friction).
 
One needs the assumption that the wheel is balanced.
 

Similar threads

  • · Replies 60 ·
3
Replies
60
Views
6K
  • · Replies 8 ·
Replies
8
Views
3K
  • · Replies 22 ·
Replies
22
Views
5K
  • · Replies 6 ·
Replies
6
Views
788
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
5K
  • · Replies 8 ·
Replies
8
Views
3K
Replies
19
Views
3K