Here are some slides that we used when we were looking at moments. The two slides at the end are saying that if the two opposite moments are equal the see saw won't move. This means that two different weights can be put at different distances from the pivot and balance.
https://docs.google.com/leaf?id=0BzFUvVu_WNI_NTVjNGQ1ODEtYzNlZi00NDQ4LWJmMzktOGNiNDk1ODc0MjYx&hl=en&authkey=CNX8n74D
