Let's Make Robots!

calculating

how to calculate motor strength?

Is there a common formula for finding required motor strength?

If I know the total bot weight and wheel diameter, could I find out what kind of motor I need. I know that gearing will come into play, but I dont know anything about these physics thingys so I choose to phrase the question as vaguely as possible.. 

Like, what kind of specs would a motor for a scooter (with me on top of it), doing 35km/Hrs require

or,  an offroader weighting 10K, doing 3km/Hrs with 10cm big wheels in rough terrain?