Let's Make Robots!

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Technical info on speakers

Hello guys. I've made a simple audio board by following the instructions provided by the "Arduino Waveshield" homepage. Only changed one part (the audio amp) but it works flawlessly.

Now, i'm using a 1W audio amplifier, and i have an 8ohm 0.5W speaker. I'm not too satisfied with the volume, as i'd like it louder than it currently is. Options are:

Windshield wiper motors, have you tried'em?

Have any of you experimented with windshield wiper motors? I know they've been used in budget robotwars fighters. I just need to get an idea what kind of strength they have (with the original gearbox).
Will it be able to pull 20 Kg vwith a 20 cm diameter wheel mounted directly to the output shaft? what abot 50 Kg? -or a 100?

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?