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Rotational Inertial and Wheels

By Doc Fizzix // Published Feb 02, 2005

An object at rest tends to stay at rest, an object in motion tends to stay in motion just as an object rotating about an axis tends to remain rotating about the same axis unless an external force or torque acts on the system. The property of an object to resist changes in its rotational state of motion is called rotational inertia and is a restatement of Newton's First Law of Motion.

Rotational inertia is the resistance an object has to changing its state of rotation. For an object that is not rotating we talk about its mass, the more mass an object has the more resistance the object will have to any change in it's state of motion. But with rotational inertia, we talk about the mass of the object with respect to the axis of rotation. The greater the distance between the bulk of an object's mass and its axis of rotation, the greater the rotational inertia. This principle is employed by a weight lifter when he or she twists a barbell with mass located on the ends. When the mass of the barbell is far from the axis of rotation, (i.e., its midpoint), the bar has considerable rotational inertia and is hard to turn or twist. If the mass is placed closer to the center of rotation, the bar twists with less effort. It follows that the greater the rotational inertia of an object, the harder it is to change the rotational state of that object. If rotating, it is difficult to stop; if at rest, it is difficult to rotate.

When it comes to mousetrap racers, the rotational inertia is a big factor that determines the performance of the vehicle. Wheels that have a lot of rotational inertia will require more pulling force and energy to get moving. With all speed-trap dragsters, you want to use wheels that have the least amount of rotational inertia as possible in order to achieve the maximum acceleration and top speed. With distance vehicle it turns out to be the same, you want wheels with the least amount of rotational inertia as possible but for a different reason.

With mousetrap distance travelers you want more energy going into the linear displacement of the vehicle. There have been many mousetrap vehicle builders that have gotten this concept wrong over the years. One would think that if you have a wheel with allot of rotational inertia and is turning that it will tend to keep turning, this would be right only if we were racing only the wheels, but we are trying to displace the whole car. With that said, every time a machine transform energy from one state to another the machine losses energy and become less efficient in the process. A mouse trap vehicle will travel further when more of the mousetrap's potential energy is transfer into the linear motion of the racer instead of the rotation motion of the wheels. By using DVD layers instead of a regular sized CD/DVD, a good distance mousetrap powered car will travel from 5-10 meter further because the DVD layers have only half the rotational inertia of a full sized DVD.

See the following related articles to get started:

• basic mousetrap vehicle propulsions (how to make it move)
• making a good distance traveler
• making a good speed-trap racer