# Just a thought question

1. Aug 21, 2011

### quietrain

if i sit on my bed(those with rollers at the btm), wouldn't i form a close system with it?

so since i form a system with it, i cannot exert forces on it right? which means i can't move it unless i exert a force on a 3rd party like a wall of something.

but why is it if i roll around on the bed, it moves?

2. Aug 21, 2011

### cepheid

Staff Emeritus
If you roll in one direction, the force of friction between you and bed causes it to move in the other direction.

Isn't it that the centre of mass of the system doesn't move (because there are no external forces), but individual components within the system can exert forces on each other and move (and these forces are always in action-reaction pairs per the third law).

3. Aug 21, 2011

### DaveC426913

Note that your bed is not an ideal frictionless system. It will walk across the floor over time. But if you did this in zero-g, your system's CoM would not move.

4. Aug 21, 2011

### quietrain

but i thought individual components within the system cannot cause the system to move? is friction an internal force? since it is between me and the bed, not the floor?

5. Aug 21, 2011

### DaveC426913

Correct. Assuming the system is isolated.

It isn't.

Friction between the bed and the floor.

Move slow and the bed will stay stuck to the floor. Move fast and the bed will slide. Alternate the two and you can mosey your way across the room. (as anyone who's ever used a rolling office chair knows.)

6. Aug 21, 2011

### quietrain

i am a bit confused. if i roll to the right, wouldn't the entire system's com move to the right?

also, is this for the idealized case where there is no friction?

7. Aug 21, 2011

### DaveC426913

On a frictionless floor, if you roll to the right on the bed, the bed will move to the left (and then stop). The CoM of you/the bed will not have moved.

8. Aug 21, 2011

### quietrain

so since my rolling is internal, how can i produce an external force?

9. Aug 21, 2011

### quietrain

so if i can put in friction somehow, can this work?

10. Aug 21, 2011

### quietrain

? you mean the CoM of system when i am on the left = CoM of system when i am on the right?

or does it have to do with the bed moving?

11. Aug 21, 2011

### quietrain

oh i see what you mean.. you meant that the bed would balance off my movement perfect in a frictionless case

but in a friction case, it wouldn't? so the system would move as a result?

12. Aug 21, 2011

### cepheid

Staff Emeritus
"The System" is not moving unless if its centre of mass if moving. The internal forces are indeed cancelling each other out (just as the post you quoted asserted) so that there is no NET change in momentum (any momentum gained by you is countered by an opposite change in momentum of the bed). And I am talking about the ideal case in which there is no friction between the bed and the floor.

Two ice skaters on a frozen pond can push each other apart, but only an external force (provided by something other than the skaters) can cause a net change in their combined momentum.

13. Aug 21, 2011

### DaveC426913

By taking advantage of friction - such as with the floor.
1] Move rapidly to the left. The bed will move to the right. No change in CoM.
2] Now move slowly back to the right. The bed will not move (because you are not moving fast enough to overcome the static friction). CoM does move.
3] Repeat steps 1 and 2.

Also, you could do several other things.

1] Throw something off the bed, such as all the blankets. Thge CoM of you/the bed and the blankets will not move, but if you only care about hte bed's position and not the blanket's position, then you can consider that you've moved. This is how spaceships maneuver.

2] Use a fan to interact with the air.

14. Aug 21, 2011

### DaveC426913

No.

Man I hate those dumb cartoons.

15. Aug 21, 2011

### quietrain

ah i see...

with regards to the spaceship, if i throw my blanket to the right, you meant i would feel a tiny shift of my bed to the left? does the momentum law governs this? except it is not change in speed but a change in mass?

16. Aug 21, 2011

### DaveC426913

If there were no friction with the floor, yes, you would be using the blanket as a propellant, just like the Space Shuttle [strike]does[/strike] did.

17. Aug 21, 2011

### quietrain

:rofl:

oh i am starting to see it

the bed example is due to a shift of the bed to keep CoM constand while also ulitilizing the counter friction of the floor to prevent us from going back to square 1.

thats why the car don't work because theres no change in CoM? just a force internal only?

18. Aug 21, 2011

### quietrain

ah i see ,its clearer now :rofl:

19. Aug 21, 2011

### DaveC426913

I ... don't know anymore. I don't think I can describe it any better or any more than I already have. I'm not sure where your confusion lies.

What car? I don't know what you're talking about here.

20. Aug 21, 2011

### quietrain

ok, i am not good at expressing i guess

but i apply what i just learnt

the GREEN arrow is a machine that brings the RED rollers up and downwards(SLOW)
the RED rollers will roll down the ramp due to gravity(FAST)
this would work right? the system would move to the left with friction blocking the right(slow) movement.

so now if i made the ramp very long and with a lot of rollers, i would acheive a great distance but i need a machine right?