In the opposite direction is the reaction. The car's wheels reduce friction and provide some stability to the car's motion. A well-designed and constructed car will travel several meters in a straight line across a smooth floor. Encourage students to design their own cars. Cars can be made long or short, wide or narrow, or even trapezoidal.
Wheels can be large or small.
Hold car distance trials on the floor. Have students measure and chart the distances each car travels. Average multiple runs for individual cars to identify the best cars.