Centrifugal force, fictitious?

  • #1

Main Question or Discussion Point

If you've ever taken a bicycle wheel off, held it with both hands on either side of the axle and try to lean it side to side whilst the wheel is spinning it is very difficult, the wheel fights you and it's almost like the wheel wants to 'tip' about 90 degrees before or after (I can't remember)the direction you're persuading it.

Now, if you had two wheels attached to something, say a space ship, couldn't you get the two wheels to 'fight' each other at such an angle that it causes the spaceship to gain momentum in a particular direction?
 

Answers and Replies

  • #2
A.T.
Science Advisor
9,973
1,732
If you've ever taken a bicycle wheel off, held it with both hands on either side of the axle and try to lean it side to side whilst the wheel is spinning it is very difficult, the wheel fights you and it's almost like the wheel wants to 'tip' about 90 degrees before or after (I can't remember)the direction you're persuading it.
That is gyroscopic torque not centrifugal force:
http://en.wikipedia.org/wiki/Gyroscopic

200px-Gyroscope_wheel-text.png


Now, if you had two wheels attached to something, say a space ship, couldn't you get the two wheels to 'fight' each other at such an angle that it causes the spaceship to gain momentum in a particular direction?
No, you cannot gain linear momentum, without external forces. You also cannot gain angular momentum without external torques. But you can turn around without any net angular momentum:

https://www.youtube.com/watch?v=yGusK69XVlk
 
  • #3
Andrew Mason
Science Advisor
Homework Helper
7,566
330
If you've ever taken a bicycle wheel off, held it with both hands on either side of the axle and try to lean it side to side whilst the wheel is spinning it is very difficult, the wheel fights you and it's almost like the wheel wants to 'tip' about 90 degrees before or after (I can't remember)the direction you're persuading it.

Now, if you had two wheels attached to something, say a space ship, couldn't you get the two wheels to 'fight' each other at such an angle that it causes the spaceship to gain momentum in a particular direction?
You are mixing up angular momentum with "centrifugal force". The gyroscope behaviour of a spinning wheel is due to conservation of angular momentum. It is not due to a fictitious force.

AM
 
  • #4
Simon Bridge
Science Advisor
Homework Helper
17,847
1,644
... though you could use a gyroscope to change orientation of a space-craft, if you could figure out how to keep it spun-up.
 
  • #5
Interesting, yes I did mistake gyroscopic torque with centrifugal force......

So a space-craft could turn around using this force? Presumably around its centre of mass...... and that would mean that where ever the gyroscopic wheel was in the system, the system would pivot around on one point?
Even if the wheels where out on long, maneuverable arms so that the craft looked like a Y shape and the centre of mass was able to shift (artificially) around the ship?

I'm probably barking up the wrong tree but in my mind I see it working....
 
  • #6
1,233
17
At the end of the day it's just a blob of mass that is restricted by conservation of angular momentum. If part of that blob (a gyroscope) starts moving, the rest of the blob (spacecraft - gyroscope) will move in such a way as to keep the overall angular momentum constant.


There is no magical way it can turn around- it will just change orientation, like you would if you sat on a swivel chair and moved your arms from one side to another. You've changed the way you face, but you haven't changed your angular momentum; it's always zero.
 
  • #7
13
0
what if you have two pendulums, with the same center of oscillation, and you make them repel and attract each other (by means of electromagnetic force, lets say). The rotation will be not a complete circle, but an arc about let's say 90 (each describes a 45 arc) degrees. In this case .. still no linear momentum? .... nothing? .... please!?
 
  • #8
A.T.
Science Advisor
9,973
1,732
what if you have two pendulums, with the same center of oscillation, and you make them repel and attract each other (by means of electromagnetic force, lets say). The rotation will be not a complete circle, but an arc about let's say 90 (each describes a 45 arc) degrees. In this case .. still no linear momentum? .... nothing? .... please!?
no external force -> no change of net linear momentum
 
  • #9
13
0
no external force -> no change of net linear momentum
Thanks, i didn't get it how the conservation works in this example, but you're firm answer challenged my neuron and i think i got it. When repelled, the two spheres of pendulums tend to go in opposite directions, in a straight line. The pivot from the center of oscillation is however opposing the straight moving because of the rigid rod connection. This holding (lets call it centripetal force :) ) bends the path of the pendulums , which feels the restriction of going straight as a force, opposed to the centripetal, the centrifugal force. So what will happen is the spheres of pendulums will go in a curved path, and the pivot(center of rotation) will go in a straight path , in the direction of the line unifying the two spheres. So the moving of the spheres will drag the pivot, and the inertia of the pivot curves the path of the spheres. And that's it, the whole system stays still, it's just modifying shape (now the youtube clip above makes sense). and also how curved the path of the spheres is, depends on the mass ratio pivot/spheres. if pivot has infinite mass, the smallest circle described ( R= the rod), if mass is 0, the path of the spheres will bend no more. Pfuee, that was hard :).
 

Related Threads on Centrifugal force, fictitious?

  • Last Post
Replies
12
Views
816
  • Last Post
Replies
2
Views
398
  • Last Post
Replies
10
Views
1K
  • Last Post
Replies
4
Views
1K
  • Last Post
Replies
0
Views
2K
Replies
19
Views
670
  • Last Post
Replies
7
Views
3K
Replies
23
Views
3K
  • Last Post
Replies
22
Views
5K
  • Last Post
Replies
12
Views
1K
Top