Friction motors are simple mechanism to propel toy cars. The motor consists of a large flywheel

Flywheel

A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...

 which is connected to the drive wheels of the car via a very low gear ratio

Gear ratio

The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...

. The flywheel's axis is perpendicular to the direction the car is facing. When the car is pushed forward the drive wheels engage the flywheel. The flywheel stores the kinetic energy of the initial acceleration and propel the toy car after it is released. These toys were especially popular in the 70s and 80s. Some utilised a "zip cord" which would directly accelerate the flywheel when pulled from the body of the vehicle.