Before lining up for takeoff, the motor glider pilot estimated that there would be sufficient time for his takeoff to be completed between a landing glider, and the tug aircraft that had just joined the circuit. The takeoff was delayed by the time taken to clear the landing glider from the strip. The tug aircraft, just prior to turning final, was observed to increase power and head towards the departing motor glider which had just commenced its takeoff run. It passed low over the motor glider and the tow rope struck the right aileron of that aircraft. The glider pilot had lined his aircraft up for takeoff on the strip the tug pilot intended to use for landing. The tug pilot stated that his intention was to teach the glider pilot a lesson not to attempt to takeoff in front of him again. In taking this action the tug pilot forgot that the tow rope was still attached and trailing his aircraft.