# Opposite directions

## Main Question or Discussion Point

Can two objects that have no empty space between them move in opposite directoins?

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Doc Al
Mentor
What if they rotate?

If there is nothing that separates the rotating objects, are they not attached?

Doc Al
Mentor
They could be. But they can still rotate. Imagine two blocks stuck together. The system rotates about its center. So, one block goes left while the other goes right. (Momentarily.)

What are you getting at with this question? (I don't see any obvious physics here.)

Doc Al said:
They could be. But they can still rotate. Imagine two blocks stuck together. The system rotates about its center. So, one block goes left while the other goes right. (Momentarily.)

What are you getting at with this question? (I don't see any obvious physics here.)
Is there empty space between a marble and the glass table it moves across

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Doc Al
Mentor
Avgiu said:
Is there empty space between a marble and the glass table it moves across
I would say no, at least macroscopically.

That's a much better example (of two things with no space between them moving in opposite directions) than my rotating object example! So I guess you answered your own question. Another example would be two things sliding across each other.

In first response your talking about a baton type of thing where one end is going against one way and the other is going another way. But you spoke of things sliding across each other. How can a thing slide across another thing without there being empty space, and if there is no empty space doesnt that change most of modern day physics?

Doc Al
Mentor
I'm not sure what kind of answer you are looking for. It seems like you are hanging alot on the meaning of "empty space". You slide a block of wood across a table. Is there "empty space" between the wood and the table? Macroscopically, I'd say no; but microscopically, there's plenty of empty space. After all, most things are mainly "empty" space. The only real "contact" between the surfaces is at the surface irregularities. I'm not seeing the issue. (And I certainly don't see the impact on modern day physics!)

ZapperZ
Staff Emeritus