Mousetrap car

A mousetrap car designed for a distance competition

A mousetrap car is a small vehicle having the only form of motive power being from a mousetrap. Variations include the use of multiple traps, or larger rat traps, for added power.

Mousetrap cars are very 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.

Competition objectives

Mousetrap cars may be built by individuals for personal enjoyment, or may be built to enter into organized competitions.

Competitions will provide a set of rules that all competitors must adhere to. Typical rules will specify the type of mousetrap that can be used (for instance, in North America, Victor® brand snap traps are commonly specified), that the mousetrap provides the only source of motive power for the car, and the materials used and overall dimensions are often limited in some way.

Common competition objectives are:

• Greatest distance covered.
• Shortest time to cover a fixed distance from a standing start.
• Maximum velocity measured at a point on the track.
• Most accurate stopping.
• Ability to pull the most weight.

Design

A typical spring-loaded bar mousetrap, also known as a snap trap, used in mousetrap cars

The general style for a mousetrap car varies. Most people choose four-wheeled cars, but two- and three-wheeled cars also exist. There is one known example of a one wheeled mousetrap car designed and built by Philip Beltracchi. Some ways to increase your distance are replacing the string that pulls the axle with a rubber band. Larger wheels will increase the distance obtained using the same amount of energy. Use more string than necessary so the car doesn't start rewinding after the string has run out.

Spring Power

A mousetrap is powered by a helical torsion spring. Torsion springs obey an angular form of Hooke's law:

$\tau =-\kappa \theta \,$

where $\tau \,$ is the torque exerted by the spring in newton-meters, and $\theta \,$ is the angle of twist from its equilibrium position in radians. $\kappa \,$ 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 U, in joules, stored in a torsion spring is:

$U={\frac {1}{2}}\kappa \theta ^{2}$

When a mousetrap is assembled, the spring is initially twisted beyond its equilibribum position so that it applies significant torque to the bar when the trap is closed.

Power Transmission to Axle

The mousetrap bar travels through an arc of approximately 180 degrees. This motion must be used to turn the car's axle or wheels. The most common solution is to attache a string to the bar and 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 mousetraps 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 the mousetrap bar is to reduce the amount of torque applied to the wheels. If too much torque is applied to the wheels, the force between the wheels and the ground will exceed the maximum frictional force due to the coefficient of friction between the wheel and ground surfaces. When this happens, the wheels slip and energy stored in the spring is wasted. Using a long lever on the mousetrap bar reduces the tension in the string due to the spring's torque, and thus reduces the torque applied to the car's wheels.

In addition to reducing the torque applied to the wheels, the coefficient of friction may be improved by using higher friction materials, such as rubber, on the wheels.

Variations on Mousetrap Cars

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.

External Links

• Mousetrap Car Demonstration of a mousetrap powered car