A gravity racer turns a drop in height into motion. Its shape, wheels, alignment, mass, and course determine how much of that energy becomes forward speed and how much goes into wheel rotation, air movement, heat, and braking. This makes an unpowered vehicle a useful engineering example.
Nissan’s Flying Torpedo provides a historical starting point. The calculations here are invented teaching examples, separate from that car’s specifications or measured performance. They explain tradeoffs rather than supplying a construction plan.
What the Flying Torpedo’s award establishes
In its August 2005 announcement, preserved by The Auto Channel, Nissan Design America reported winning Best in Design at the Extreme Gravity Racing Series. The manufacturer described an enclosed, elongated body, covered wheels, a fiberglass shell, and a polycarbonate window.
Those features show the team’s interest in managing airflow and packaging an enclosed driver. The announcement establishes a design award and describes the concept. It supplies no drag coefficient, controlled comparison, or race-time dataset from which to rank its aerodynamic performance. A sleek appearance is a hypothesis worth testing.
Start with the vertical drop
Near Earth’s surface, a mass m descending through height h releases gravitational potential energy E = mgh. Translational kinetic energy is K = ½mv². Here, mass is in kilograms, height in meters, speed in meters per second, and energy in joules. We use g = 9.81 m/s². NASA’s archived energy lesson explains this exchange.
The ideal calculation uses vertical drop, not road length. A 30 m drop along a gentle winding road and a short steep road have the same ideal energy change. Their travel times and real losses differ. Neither calculated speed is an operating recommendation or a prediction for a real course.
Separate rolling losses from aerodynamic drag
Wheel and tire deformation, bearings, misalignment, surface roughness, and air resistance all consume part of the available energy. A useful first approximation for work against a constant resisting force is force × path length. For example, an assumed 10 N rolling resistance over 100 m consumes 1,000 J. Actual resistance varies with the vehicle and conditions.
The NASA drag equation is D = ½ρv²CdA. Air density is ρ, v is air-relative speed, and CdA is the drag coefficient multiplied by its matching reference area. Specify both the coefficient’s basis and area when comparing designs. A small coefficient paired with a large frontal area can still produce substantial drag.
Swipe the table sideways, or focus it and use the arrow keys.
| Air-relative speed | Drag force | Power against drag |
|---|---|---|
| 5 m/s | 4.5 N | 22.5 W |
| 10 m/s | 18.0 N | 180.0 W |
| 15 m/s | 40.5 N | 607.5 W |
This fictional table assumes still air, density 1.20 kg/m³, constant CdA = 0.30 m², and straight motion. Drag rises with speed squared; the power spent overcoming it, D × v, rises with speed cubed under these assumptions. Doubling speed from 5 to 10 m/s produces four times the drag and eight times the drag power.
The table describes forces at selected speeds. Calculating a finish speed requires tracking those forces along the course, including wind and changing speed. Halving the assumed CdA halves drag at the same speed, but the effect on a finishing time depends on the entire run.
Explain mass without prescribing ballast
Mass cancels from the ideal height-to-speed equation because both available gravitational energy and translational energy scale with it. A real car also has wheel inertia and resisting forces. At the same shape and speed, a given aerodynamic drag force produces less deceleration per kilogram in a heavier system. Rolling losses, handling, braking, structural loads, and rule limits complicate the comparison.
That observation supplies a question for analysis, not permission to add weight. Organized racing controls the vehicle, driver, equipment, and inspection requirements. For a different established program, the International Soap Box Derby rules hub provides its official rulebook and updates. Its requirements are specific to that program and should not be transferred to Nissan’s 2005 event or another race.
To assess a claimed improvement, ask for repeated timed runs with course, mass, wheels, alignment, weather, starting method, and timing method recorded. Change one permitted feature at a time and look at variation across runs. A single faster run leaves several possible explanations.
Download the gravity-racer calculation worksheet to reproduce the examples and change the assumed height or drag area. Keep classroom calculations separate from vehicle testing; any physical racing or testing belongs under the organizer’s approved procedures and inspection.
Researched and updated September 7, 2026. Recheck applicable model documentation, support resources, and local or event rules before acting.