mousetrap cars and vehicles

Decreasing Rotational Inertia

By Doc Fizzix // Published March 20, 2007

Here, a student is removing mass from the inside of a CD that will be used as a wheel using a Dremel tool. By decreasing the rotational inertial of your mouse trap car's wheels, your vehicle will be able to accelerate at a greater rate and convert more potential energy into the linear kinetic energy of the car rather then storing it into the wheels. This student is actual making this wheel for a long distance car. The less rotational inertia in the wheels the smaller the pulling force that will be required to move the vehicle; therefore, the vehicle can have a longer lever arm which will translates into more string around the axle and a longer pulling distance.

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.

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