Lean the plywood board against the table so it forms a ramp leading to the ground, wide enough to allow the soda cans to roll down it, side by side.
Draw a starting line across the plywood, near the top. Use the tape measure to ensure that the line is parallel to the end of the board by measuring its distance from the end on both sides.
Hold the empty and full soda cans in your hands on the starting line. Give the stopwatches to your assistant (or assistants). Have one of them tell you when to release the cans. Release them simultaneously.
Record the time it took for the empty and full cans each to reach the bottom of the ramp. While the full can got off to a faster start, the empty can should win.