Physical Principle:
The toy must be balance on the guide wire in order for to work properly.This toy is based on the principles of equilibria. The toy also relies on the principles of motion and velocity. The rate at which the motor turns a pulley/gear is related to how the wire walker traverses along the wire.

Description:
The Wire Walker is a motorized toy intended to travel across a metal wire strung between two points. The arrangement of gears, pulleys, and drive belts give the power necessary to rotate the guide pulley located at the top of the device. The mechanical components (gears, pulleys, and belt driven discs) can be rearranged to provide experimental changes in velocity. The holes in the back of the toy enable the gear shafts to be mounted in a variety of positions.

The Wire Walker comes with two additional features, the Joust Stick and the Carry Hook. The joust stick enables the toy to change direction when it hits a wall. The impact of the stick causes a reverse switch to engage, thus reversing the current through the motor and sending the toy back along the wire. The joust stick could also be used against another wire walker competing on the same wire. The carry hook provides the child with a method of delivering messages across a wire or carrying other toys. Both features are intended to increase the fun of the toy.

Cost, Durability and Safety:
To be reasonable, the toy would cost about $15. Most of the components are made out of plastic and the motor is fairly inexpensive. The toy is fairly durable under normal use. However, it would probably be likely to break if it were to fall of the wire. That is why the design has a curved part along the top of the toy. If the guide pulley were to fall off the wire, the safety hook would hold the onto the wire.
The toy is rather safe. Safety should be taken underneath the path of the wire just in case it were to fall completely off the guide. The toy is intended for children age 9+.