# Mousetrap car

A mousetrap car designed for a distance competition

A mousetrap car is a small vehicle having only one form of motive power being from 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. [1][2] Most people choose four-wheeled cars, but three-wheeled and two-wheeled cars also exist. One way to increase distance is to replace the string that pulls the axle with a rubber band.[citation needed] Larger wheels will increase the distance obtained using the same amount of energy. Using more string than needed may cause the string to rewind around the axle after the string runs 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 of 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 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 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 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 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.

## Variations

A number of commercial vendors offer plans, kits and complete cars for sale.[3][4][5][6]

In addition to mousetrap cars, contests have been created for mousetrap boats[7] and mousetrap airplanes.[8]

## References

1. ^ Wired.com Sunday Afternoon Project: Building a Mouse Trap Car By Dave Banks
2. ^ Instructables.com, Mouse Trap car
3. ^ Docfizzix.com, Mouse Trap Powered Vehicles
4. ^ Escience.ca, Efson Science Mousetrap Car Kit
5. ^ Kelvin.com, Kelvin company Mousetrap Car Kit
6. ^ Kidder.ca, Kidder company catalog page
7. ^ Lakeviewjhs.net, Mousetrap Boat Contest
8. ^ AB.ca, 2001–2002 Parkland Science Olympics