[extropy-chat] unidirectional thrust

[Damien Broderick]

At 11:06 AM 3/14/2005 -0800, Hal wrote:

>If the gadget starts going forward and there's nothing else that starts
>going backward, by definition that violates conservation of momentum,
>right?  If this thing starts out stationary in empty space, and you turn
>it on and after a while it is moving, then its momentum has changed.
>Unless there's something else moving in the opposite direction, as with
>a rocket, you are violating conservation of momentum.

Every time metal starts spinning when you switch an electromagnet on and 
off, you are making something move that was stationary. I assume the equal 
and opposite force goes into the magnet which is bolted to the floor and 
hence doesn't move because the entire earth takes up the impact. (I do 
realize this is physics 001 territory.) For the same reason, I assume 
nobody would wish to be cushioning a rail gun against his shoulder when he 
switches it on. Or am I missing something elementary here?

Damien Broderick [still not a physicist]