If a lens fits on a Nikon D3100 will it fit on a Nikon D7000?

If a lens fits on a Nikon D3100 will it fit on a Nikon D7000?

Yes, Nikon make their lens can fit all Nikon cameras. Even the old one.

Of course. That is what a camera system is all about.

Since going from a D3100 to a D7000 is an upgrade, every lens you have for your D3100 will fit the new camera. As a bonus, since the D7000 has a built-in autofocus motor you'll now be able to use any Nikon autofocus lens, not just AF-S models.

Yes.

Yes.

http://www.kenrockwell.com/...s.htm#dslr

Yes, every lens which works on a D3100 will work on a D7000, but not the other way round.
The D7000 has a focus motor built in, so some lenses which will not autofocus on the D3100 will work fine on the D7000, like the 50mm f1.8 "D" lens for example.