The Weight-
Powered
Vehicle (WPV)
HOW FAR CAN A CAR GO
ON ONE FOOT-POUND
OF ENERGY?
BY RICHARD B. GRAEBER
After my son won his high school science class mousetrap-powered vehicle (MPV) competition, I asked him if he knew what a foot-pound of energy
was. He gave a vague answer that didn’t really nail it. Admittedly, a foot-pound
(ft-lb) is an abstract concept — where do you get a pound of feet from anyway?
Construction
My son and I built the car from pinewood we picked
up at the hardware store. We also bought a wooden
yardstick, and we found lead sinkers and monofilament fishing line at a sporting goods store. The local
hobby shop supplied us with wheels, a small pulley,
aluminum tubing for axles, and polypropylene tubing
lubricated with teflon powder for bearings. We used
these parts, along with a few other odds and ends,
to cobble together our experimental vehicle.
Photograph by Ed Troxell
To illustrate what a foot-pound of energy is and
what it can do, I designed a vehicle that derives its
propulsion from a falling weight (rather than an
unwinding spring as used in an MPV).
This small car, which I call a weight-powered
vehicle (WPV), demonstrates the conversion of
potential energy into kinetic energy in one of its
most rudimentary forms, leading to a better understanding of energy conversion and loss.
74 Make: Volume 20
