A mousetrap car is a small vehicle whose only source of motive power is a mousetrap. Variations include the use of multiple traps, or very big rat traps, for added power. Mousetrap cars are often used in physics or other physical science classes to help students build problem-solving skills, develop spatial awareness, learn to budget time, and practice cooperative behavior. Design The general style for a mousetrap car varies. A number of commercial vendors offer plans, kits and complete cars for sale. In addition to mousetrap cars, contests have been created for mousetrap boats. and mousetrap airplanes. Spring power A mousetrap is powered by a helical torsion spring. Torsion springs obey an angular form of Hooke's law: where is the torque exerted by the spring in newton-meters, and is the angle of twist from its equilibrium position in radians. is a constant with units of newton-meters / radian, variously called the spring's torsion coefficient, torsion elastic modulus, or just spring constant, equal to the torque required to twist the spring through an angle of 1 radian. It is analogous to the spring constant of a linear spring. The energy of U, in joules, stored in a torsion spring is: When a mousetrap is assembled, the spring is initially twisted beyond its equilibrium position so that it applies significant torque to the bar when the trap is closed. Power transmission to axle This motion must be used to turn the car's axle or wheels. The most common solution is to attach a string to the mouse trap's arm and then wrap it around an axle. As the bar is released, it pulls on the string, causing the axle (and wheels) to turn. Tying the string directly to the mousetrap's bar, however, will not make good use of the energy stored in the spring. The distance between the opened and closed positions of the bar of a mousetrap is typically 10 cm, so this is how much string would be pulled. Wrapped around even a small diameter axle, this amount of string will not create enough revolutions to move the car as far as it might go. To get around this problem, most mousetrap cars add a lever to the bar so that the lever will pull a much greater length of string and cause the axle to turn many more revolutions. Friction of wheels Another reason to add a lever to