Culture and Society

Why does a heavy pinewood derby car go faster than a light one?
Answered by Science Channel
  • Science Channel

    Science Channel

  1. When Cub Scouts pack school cafeterias, gyms and other gathering places for their annual pinewood derby races, they're in for good fun with healthy competition. Scouts learn to craft their wooden cars along with advice from their adult helpers. Building a pinewood derby car is a learning experience, but every Scout probably wants to win the local race. That means understanding a little about the laws of physics.

    The formula Force = Mass x Acceleration explains the divergence in speeds of differently weighted cars. It's Sir Isaac Newton's second law of motion, and it applies not just to pinewood derby cars, but to larger moving objects, such as roller coaster cars and g forces in fighter jets.  In this case -- a downhill trip on a pinewood derby track -- the acceleration is generated by gravity.

    As soon as you set a pinewood derby car at the top of the ramp, its initial energy is the potential energy it has between the start and end of the track, plus its kinetic energy, or the car's capacity to exert force over the track's distance [sources: Stanford, Pinewood Professor]. Much of the speed depends on the distance -- or height -- and angle of the ramp.

    But if all things are equal in the formula for the cars, such as air drag, ramp angle, and the like, there are only a few places where you can affect the speed by much. Aerodynamics are important; aerodynamic drag will impede your car's movement. That means that making your car aerodynamic helps, at least as opposed to leaving the car in its initial block shape. That's out anyway, not only if you want to make a respectable showing, but also to demonstrate you put some effort into your car. Still, you need more than aerodynamics for an edge, so don't forget the mass part of Newton's equation. Mass helps create force, or energy. It helps get your car out of the starting spot. That means that the more mass your car has, the higher the force that will pull your car along. Where the mass rests also is important. Some physicists say the mass should be near the back to push the car along, especially as you near the finish line.

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