Hovercraft Fan Runs: Planning Tests of Net Force and Mass

Build Notes: The Library Maker Lab Hovercraft

1Every winter, teen members of the Westfield Public Library maker lab build a small hovercraft. Its deck is a sheet of foam board that floats on a thin cushion of air blown out underneath by a quiet fan. Three small propeller motors on the back push the craft along the floor. Mentor Mr. Kwame Lindahl challenged Sunniva Ortega and Dariusz Mbeki to plan runs that show how the net force on the craft and the craft's mass affect how quickly its speed changes. Sunniva pointed out that a hovercraft suits these tests well: it floats instead of rolling on wheels, so very little friction acts on it. Dariusz wrote the team's question at the top of the plan: how do the net force and the mass of the craft each affect how much its speed changes in 2.0 s?

2First, the team clamped each propeller motor to a digital force gauge. Each one pushed with 0.10 N. All three motors stay mounted on the craft for every run, even when some of them are switched off, and any motor can be turned around so that it pushes backward instead of forward. With all three motors on board, the craft has a mass of 0.40 kg. To add mass, the team tapes steel washers to the deck and checks the total mass on a balance.

3Next, the team ran a friction check. With the air cushion on and every propeller motor off, Sunniva gave the craft a gentle shove along the smooth meeting-room floor. The video showed it gliding at 100 cm/s, and 2.0 s later it was still gliding at 98 cm/s. The team decided that friction on the floating craft was small enough to ignore, so the motors' pushes alone set the net force.

4In each run, the craft starts at rest on a tape line, and all the motors in use are switched on together. A phone above the lane films the craft beside a meter stick, and the team finds the craft's speed 2.0 s after the start. Each run is repeated three times and the speeds are averaged. The lane is padded at the far end, the craft never moves faster than a walking pace, and a spotter catches it after every run.