10
$\begingroup$

I have been thinking on the following problem.

Part I. Imagine we have a mouse enclosed in a rigid box. We placed the box on a frictionless surface (ice might be a good example). Can the box start moving because of something the mouse is doing inside? My answer to this question is: The center of mass of the combined system (mouse + box) will NOT move because there are no external forces and the internal forces satisfy at least the weak form of Newtons Third Law. So, Momentum is conserved for the system.

Part II. Now we put the mouse in a rigid wheel and we put the wheel on a frictionless surface or we suspend the wheel from its center on frictionless bearings. Can the wheel start rotating because of something the mouse is doing inside? My intuition will say YES it will move because I have seen this system in several pet stores and mice can actually rotate wheels. But I was trained in classical mechanics so, I know that the system must conserve angular momentum if there are no external torques and the internal forces satisfy the strong version of Newton's third law. So, How does it work in this case, I came with different ideas on how to reconcile the experiment with the theory but I don't like them very much. I want to get new insights.

Improve this question
$\endgroup$
1
  • $\begingroup$ Analogous, more intuitive experiment: Mouse on a manual treadmill. Observe how it only works if the treadmill is angled (otherwise the mouse will run off the treadmill due to lack of help from gravity). $\endgroup$
    – Superbest
    Commented Jul 2, 2015 at 21:39

2 Answers 2

Reset to default
24
$\begingroup$

In this case, gravity is still an external force. In a zero-g environment, the mouse would also begin to move around the inside of the wheel, opposite the rotation it causes in the wheel, which would keep the angular momentum at zero. This would happen because the only way for the mouse to exert a force on the wheel and rotate it is for it to push itself in the opposite angular direction, thus making both rotate about the central axis of the system.

However, in an environment with gravity, the mouse can make the wheel rotate by moving slightly away from the nadir of the wheel and allowing gravity to bring it back; wash, rinse, and repeat. In this way, it uses gravity as an external force to bring angular momentum into the system. The angular momentum added to the mouse-wheel system is drawn from Earth itself. However, the change in Earth's angular momentum would be so small in this case that it would be unmeasurable by any equipment we have. All in all, no laws are violated.

Improve this answer
$\endgroup$
3
  • $\begingroup$ I've deleted some off-topic comments. $\endgroup$
    – David Z
    Commented Jul 3, 2015 at 9:14
  • $\begingroup$ Possibly I am being overly pedantic; however I feel that it's worth mentioning that it's not gravity that is creating torque in this case. The Torque comes from the asymetric mass distibution around the point where the normal force is acting on the connection of the wheel. $\endgroup$
    – Taemyr
    Commented Jul 3, 2015 at 12:19
  • $\begingroup$ @Taemyr I strongly believe that technically correct is the best kind of correct, but even I have to admit that you are being overly pedantic. The energy and momentum input comes from gravity. Gravity generates (or rather couples to) the normal force that produces the torque, but I'd still attribute it to gravity. But yes, you are technically correct $\endgroup$
    – Jim
    Commented Jul 3, 2015 at 12:35
2
$\begingroup$

The angular momentum of the earth && mouse wheel system does not change. When the earth pulls on the mouse, the mouse pulls on the earth, so no net moment is seen any arbitrary point in the universe. enter image description here

Improve this answer
$\endgroup$
7
  • 7
    $\begingroup$ Why does the mouse look like a cow? Also, this doesn't seem to answer the question fully. It looks more like a response to another answer $\endgroup$
    – Jim
    Commented Jul 2, 2015 at 18:58
  • $\begingroup$ @JimsBond It is a transgenic mouse with cow DNA to pigment its fur. $\endgroup$
    – Mindwin Remember Monica
    Commented Jul 2, 2015 at 19:12
  • $\begingroup$ I think the body of the cow is supposed to be the wheel. The mouse itself consists only of the head and mouth of the cow. $\endgroup$
    – Dawood ibn Kareem
    Commented Jul 3, 2015 at 10:57
  • 1
    $\begingroup$ I'm actually more worried about how to feed a mouse that's roughly the size of Africa. $\endgroup$
    – Dawood ibn Kareem
    Commented Jul 3, 2015 at 17:29
  • 1
    $\begingroup$ I suppose what I was trying to say is that any system with internal forces will not be have a change in angular momentum. I drew a cow because I like cows. $\endgroup$
    – philn
    Commented Jul 4, 2015 at 14:28