n the case of the dirt bike, the wheels were in charge of moving the bike forward as it would on land while the ski kept the bike from sinking. Arthur Schmidt, a physics professor at Northwestern University, explained in an email how the wheels were able to keep the bike moving forward:
"The motor bike has to supply its own motivation through its rear wheel," he wrote. "A dirt bike is designed to run through mud, a fluid much like water only with dirt added. That too is not a far reach to apply to water surfing."
The most IMPORTANT factor, Schmidt said, is keeping the bike moving:
"The critical thing is to be able to develop enough force with the wheel treading water to oppose the frictional drag of the ski through water to maintain a speed sufficient to lift the ski and bike against gravity," Schmidt wrote.
Remember those force diagrams you used to be asked to draw in physics class? That's what's happening here.